Copyright: Copyright © 1997 Stephen Michael Schimpf
Citation: Copyright © 1997 Stephen Michael Schimpf


Overview

CyberSky is a colorful, educational, and entertaining program that transforms your personal computer into your personal planetarium, allowing you to learn about astronomy by exploring the sky in the past, present, and future!

CyberSky can truly be thought of as a personal planetarium, because it duplicates the features of a traditional planetarium on your personal computer. CyberSky displays the stars, constellations, deep sky objects, sun, moon, and planets as visible from any location on the earth at a given time. Information about celestial objects is available by simply clicking on them with the right mouse button.

CyberSky provides an excellent way for you to learn the names and locations of the constellations, the names of the stars, and to become familiar with the motions of the sun, moon, and planets across the sky. CyberSky provides a superb, innovative, and motivating learning environment. It places the sky under your control.

If you've already learned these basics, CyberSky is a great way to conveniently check out "what's up" tonight without leaving your desk. Or, use it to find out what you'll be able to see next weekend, or next month. Some people use CyberSky to learn to identify the stars and constellations they'll be able to see for the first time when they take a vacation to another hemisphere. And, don't forget the past! What did the sky look like on the day you were born?

One of CyberSky's most powerful and educational features is its ability to animate the display, allowing you to see "movies" of the changing appearance of the sky over time. Many of the most basic and interesting astronomical phenomena can only be observed over days or months of time, or by observing the sky in the distant past or future. CyberSky compresses these time spans into minutes, thereby allowing you to learn things about astronomy that previously would have required days or weeks of stargazing or a trip to a planetarium to learn.


Features

CyberSky includes exciting features that make the program far more educational and entertaining than the static sky charts printed in astronomy books and magazines!

General

Displays the sky in Planetarium view, which shows the entire sky as projected on the ceiling of a planetarium, or in Horizon view, which shows a more detailed view along one horizon.

Displays the sky in full screen mode to make maximum use of your monitor's display area.

Performs accurate calculations. CyberSky calculates the effects of precession, nutation, proper motion, perturbations due to solar system objects, finite light travel time, and atmospheric refraction.

Displays the sky at any time from 4,000 BC to 4,000 AD. View the sky as it will appear next week, next year when you go on your dream vacation, or as it appeared to the Egyptians at the time the great pyramids were being built.

Allows you to set the time by entering local time, universal time, or a Julian date.

Allows you to quickly set the time to midnight, sunrise, noon, or sunset.

Allows you to quickly set the time to when the sun, moon, or a planet rises, transits, or sets.

Shows "movies" of the changing appearance of the sky over time using animation. Allows you to move backwards or forwards in time at various speeds.

Allows you to quickly step backwards or forwards by one hour, one day, or one animation step.

Prints sky charts on any type of printer, including in color on the popular color inkjet printers. Allows you to preview a sky chart before you print it.

Allows you to customize all colors and all fonts to your liking. Screen and printer colors and fonts can be customized independently.

Location database

Displays the sky from any one of 1,157 locations included in the program's database of terrestrial locations. Latitudes, longitudes, and time zones are all provided for you.

Allows you to search the location database for any sequence of characters to make finding locations easier.

Allows you to edit the locations provided with CyberSky, add new locations to the database, or delete existing locations from the database.

Sky

Displays the sky in several shades of blue as well as in black depending on whether it is daytime, twilight, or nighttime. This feature can be turned off.

Displays the times of astronomical, nautical, and civil dawn and dusk.

Stars

Displays the positions of 8,403 stars to magnitude 6.5. The shareware version only displays 1,629 stars to magnitude 5.0.

Allows you to adjust the magnitude limit of stars in the sky display.

Displays stars in white and several shades of gray, or in colors based on their spectral types.

Labels stars in the sky display by proper names ("Betelgeuse", "Rigel") or Bayer letters ("α", "β").

Displays detailed information about any star in the display when you click on it with the right mouse button. This information includes the star's proper name, Bayer and Flamsteed designations, equatorial and horizontal coordinates, distance, magnitude, constellation, rising, transit, and setting times, spectral type, and luminosity class.

Deep sky objects

Displays 99 of the easiest to see deep sky objects. The shareware version only includes 36 deep sky objects. Deep sky objects include galaxies, globular clusters, planetary nebulae, and open clusters.

Labels deep sky objects in the sky display by name ("Great Orion Nebula"), Messier catalog number ("M 42"), or New General Catalogue number ("NGC 1976").

Displays detailed information about any object in the display when you click on it with the right mouse button. This information includes the object's name, Messier catalog and New General Catalogue numbers, equatorial and horizontal coordinates, distance, magnitude, constellation, rising, transit, and setting times, and object type and class.

Constellations

Displays constellation lines for all 88 internationally recognized constellations. Zodiacal and nonzodiacal constellation lines can be displayed independently of each other.

Displays the internationally recognized boundaries of all constellations. Constellation boundaries are unavailable in the shareware version.

Labels constellations in the sky display by their abbreviations ("Aql", "Cyg"), Latin names ("Aquila", "Cygnus"), English names ("Eagle", "Swan"), or German names ("Adler", "Schwan").

Solar system objects

Displays the positions of the sun, moon, and all planets in the solar system.

Displays detailed information about the sun, moon, or a planet when you click on it with the right mouse button. This information includes the object's equatorial, ecliptic, and horizontal coordinates, distance, elongation, angular diameter, magnitude, constellation, rising, transit, and setting times, disk, and illumination.

Displays information about the sun, moon, and planets even when they are not visible in the sky display.

Displays information about all planets in one place, making it easy to compare them to each other.

Reference grids, lines, and points

Displays reference grids including the equatorial and horizontal coordinate systems. Grid lines can be labeled.

Displays reference lines including the meridian, ecliptic, celestial equator, and galactic equator.

Displays reference points including the north and south ecliptic poles, the north and south celestial poles, and the north and south galactic poles. Points can be labeled.

Help system

Includes context-sensitive help for all menu commands, toolbar buttons, and dialog boxes.

Includes a tour of the sky as displayed by CyberSky, diagrams, tables, and an extensive glossary of astronomical terms.

Includes a series of exercises that suggest ways to use CyberSky to learn more about astronomy. You will learn about topics such a precession, proper motion, and the retrograde motion of the planets not by reading a description in a book but by watching these effects in the sky as simulated by CyberSky.

Includes a constellation reference. This reference includes information about the names of the constellations, their positions, the times they are best seen, tables of named stars, and the histories and mythologies related to each constellation. This constellation reference is unavailable in the shareware version.

Includes a solar system object reference. This reference includes overviews of the sun, moon, and all planets, descriptions of each object's interior, surface, and atmosphere, information about rings and moons, and tables of orbital and physical properties. This solar system object reference is unavailable in the shareware version.


What's new

CyberSky 2.0d, released in June 1998, is a minor upgrade only and does not include any new features or significant modifications. I released this latest version of CyberSky to correct some mistakes in the program's Help file, including a rather embarrassing one that made it difficult to access the topics in the Explorations section.

CyberSky 2.0c, released in March 1998, is a minor upgrade only and does not include any new features or significant modifications. I released this latest version of CyberSky to update my addresses in both the registered and shareware versions, and to include new ordering instructions in the shareware version.

March 1998: I arranged for customers in Australia and New Zealand to order from my local distributor, Shareware-Australia. In addition to accepting payment by Visa, MasterCard, and American Express, Shareware-Australia can accept cheques and money orders drawn on local banks, as well as Bankcard.

October 1997: I obtained my own domain and therefore can be permanently found at www.cybersky.com. I also obtained permanent e-mail addresses: sales@cybersky.com for all questions related to purchasing my software, and support@cybersky.com for all questions related to installing and using my software.


Getting started

In order for CyberSky to produce accurate sky displays and sky charts, the program must know your location and whether daylight saving time is currently in effect.

The first time you start CyberSky the program will display the sky as visible from Kabol, Afghanistan. The reason for this is simply that CyberSky uses the first location in its location database unless you tell it to use another location. Unless you happen to live in Kabol, you need to specify another location so that CyberSky can produce accurate sky displays.

To specify your location

1	On the Options menu, click Location. This will display the Location dialog box.

2	To specify the location on the earth from which you want to view the sky, click the down arrow in the Name box, and then click the name of the location. If you can't find your location in the list, select the closest location you can find.

3	Click OK to remove the Location dialog box.

Daylight saving time is adopted in some areas for spring and summer use to prolong sunlight into evening hours. One hour is added to standard time to obtain daylight saving time.

In the United States, daylight saving time is currently used from 2:00 AM on the first Sunday in April to 2:00 AM on the last Sunday in October. The states of Arizona and Hawaii, as well as some counties in the Midwest, do not observe daylight saving time.

To specify whether daylight saving time is in effect

1	On the Time menu, click Local Time.

2	To specify whether daylight saving time is in effect, click the Daylight Saving Time option.

3	Click OK to remove the Local Time dialog box.


Learning to use CyberSky

CyberSky has many educational and entertaining features. You are probably already familiar with some of them. The rest may be easily discovered, but others may not be.

Exploring CyberSky and CyberSky Help

To get the most out of CyberSky you should spend some time exploring the program as well as reading this Help file.

Read the Features topic of this Help file to learn what CyberSky can do.

Read the topics in the Basic Tour of CyberSky section to learn about CyberSky's sky display, the objects you'll see there, and how the sky display relates to the real sky.

Explore CyberSky's menu commands, toolbar buttons, and dialog boxes. Don't be afraid to experiment!

Giving CyberSky commands

The primary way to give CyberSky a command is to select one from a menu. Some menu commands perform an action immediately, while others display a dialog box that require you to specify what you'd like the program to do. You can always use the menu to perform commands, except when CyberSky's animation feature is active. During animation the only command you can perform is Stop.

You should be aware that some commands can be given much faster by clicking a button on the toolbar, or by pressing one or more keyboard keys. See the Shortcuts section of this Help file for a list of keyboard and toolbar shortcuts.

Getting help when you need it

Help for menu commands is available in the program's status bar along the bottom of the window. When you move the pointer over a menu command you'll see a short description of what the command does in the status bar. When you move the pointer over a toolbar button you'll see a small ToolTip window appear containing the name of the button, and you'll see a short description of what the button does in the status bar.

Help for menu commands, toolbar buttons, and other parts of CyberSky's window is also available using the program's context-sensitive Help feature. To use this feature, first click the ? button in the toolbar. The pointer will change into an arrow with a question mark next to it. Then click the command, button, or area of the screen you are interested in to display Help.

Help is also available for CyberSky's dialog boxes. Each dialog box has a Help button that displays information about the dialog box and about each of the controls or groups of controls found in it.

CyberSky and its Help file use many astronomical terms that you may be unfamiliar with. Be sure to read the Glossary topic of this Help file to learn the definitions of these terms.


Sky

Celestial sphere

When discussing the appearance and motion of the sky, it is useful to imagine that the sky is a great, hollow sphere surrounding the earth, and that the celestial bodies we see are somehow attached to the inner surface of this sphere. This imaginary sphere, called the celestial sphere, appears to rotate around the earth from east to west, although we know it is really the earth that is moving from west to east.

Planetarium view

The first time you start CyberSky it will display a view of the sky called Planetarium view, because it shows the entire sky at once just as a planetarium does. Imagine lying on your back in a field located far from city lights. The circle you see on your computer's screen represents the entire visible sky. The center of the circle corresponds to the zenith, the point directly above you. The edge of the circle corresponds to a horizon unobstructed by trees, buildings, or mountains. The top of the circle corresponds to the northern horizon, the bottom to the southern horizon, the left edge to the eastern horizon, and the right edge to the western horizon.

CyberSky allows you to label the edges of the sky display with abbreviations such as "N" and "S" to indicate the positions of the cardinal points. The color of these labels as well as the font used to display them can be customized.

Horizon view

Although Planetarium view is very useful there are times when you'll want to see the sky along one horizon only. CyberSky allows you to switch between Planetarium view and Horizon view.

Cardinal directions

You may be confused by the fact that east and west appear to be switched on the screen. The reason for this is because you are looking up at the sky rather than down at the surface of the earth.

Imagine yourself floating in the air high above the earth with your head pointing north and your feet pointing south. You are looking down at the land below. In this position you will find east to your right and west to your left. This is the orientation of the cardinal directions you are familiar with from maps of the earth's surface.

Now, suppose you want to take a look at the sky. In order to do this you'll have to roll over to look away from the earth and into the sky. In this position your head will still point north and your feet will still point south, but you'll see that east and west have switched sides, with east now at your left and west at your right. This is the orientation of the cardinal directions used in sky charts, star atlases, and by CyberSky.

Reference points and lines

The points on the celestial sphere directly above and below the earth's north and south poles are called the north and south celestial poles. As the celestial sphere rotates from east to west everything in the sky appears to rotate around these two points. Observers in the northern hemisphere can find the north celestial pole by looking for the star Polaris which is nearby. Unfortunately, there are no bright stars near the south celestial pole to help observers in the southern hemisphere find this point.

CyberSky allows you to display the positions of a number of reference points. The colors of the points and their labels can be customized, as can the font used to display the labels.

Halfway between the celestial poles is found a great circle known as the celestial equator. The celestial equator is the projection of the earth's equator onto the celestial sphere. It divides the sky into a northern half and a southern half.

The point directly above you is called the zenith. The line that passes the north point on the horizon, the zenith, and the south point on the horizon is called the meridian.

Animation

As the earth turns on its axis the sky appears to turn in the opposite direction. Over time the sky will turn from east to west and the stars, sun, moon, and planets will appear to rise in the east, move across the sky, and set in the west.

CyberSky can do far more than just create a static sky chart on your computer screen. It has the ability to set the simulated sky in motion, making it a true planetarium program. The ability to view the changing appearance of the sky over time is very educational. CyberSky allows you to animate the display with time moving backwards or forwards, or to single step backwards or forwards by one animation frame.


Stars

Overview

The first time you start CyberSky it will display all stars that are magnitude 4.0 or brighter. The shareware version of CyberSky can display all stars that are magnitude 5.0 or brighter. That's over 1,600 stars and includes all the stars you're likely to see under typical conditions in urban areas. The registered version of CyberSky can display over five times that many stars, to magnitude 6.5, and that includes all stars visible without optical aid under the very best observing conditions.

CyberSky allows you to hide the stars, and the number of displayed stars can be changed by changing the limiting magnitude for the sky display. The stars are normally displayed in white and several shades of gray. The colors used to display stars can be customized to your liking, but it may be best not to use this option.

Names of the stars

Most of the brightest stars were given proper names, such as "Betelgeuse" and "Rigel", a long time ago. Unlike the constellations, which were given Latin names, the proper names of stars are generally derived from Arabic. For example, the name "Rigel" comes from the Arabic phrase "Rijl Jauzah al Yusra" meaning "left leg of the giant".

Although CyberSky can display the proper names of hundreds of stars, only the two or three dozen brightest stars are still referred to by their proper names today. There are far too many stars in the sky for each one to have its own proper name. Another system of stellar nomenclature was developed by Johann Bayer in 1603. He assigned the Greek letter α (alpha) to the brightest star in a constellation, β (beta) to the second-brightest star, γ (gamma) to the third brightest, and so on. This allowed Bayer to name up to two dozen stars in each constellation. In 1725 John Flamsteed devised a similar system in which he assigned numbers to stars in each constellation based on right ascension rather than on brightness.

The result is that a bright star may be referred to by its proper name ("Betelgeuse"), or by its Bayer designation ("α Orionis"), or by its Flamsteed number ("58 Orionis"). CyberSky can display proper names and Bayer letters in the sky display, and can display all three forms in the Star Data dialog box.

Distances to the stars

By simply looking up at the stars you have no way of knowing how near or how far away any of them are. It is natural to assume that a bright star must be nearby whereas a faint star must be much further away, but this is not necessarily the case.

For example, take a look at Procyon and Betelgeuse, two bright stars in the constellations Canis Minor and Orion, respectively. Procyon is only slightly brighter than Betelgeuse. Yet, it turns out that Procyon is about 3.5 parsecs (11.3 light years) from us and Betelgeuse is about 200 parsecs (650 light years) from us. Imagine how bright Betelgeuse must be to shine as bright as Procyon even though it is so much farther away. Betelgeuse is in fact about 1,700 times as bright as Procyon.

Looking back in time

A star's distance from us not only affects how bright it will appear to us, but also how far back in time we are looking when we see it. Stars are so far away from us that it takes their light years to reach us. In the case of Betelgeuse, you are seeing the star as it was 650 years ago. The most distant object you can see using just your eyes is a collection of stars known as the Andromeda galaxy. In a dark location far away from city lights you should be able to see this galaxy as a faint smudge just to the west of the star ν Andromedae. If you do see the Andromeda galaxy, you'll be seeing light that has been traveling across space for more than 2 million years to reach your eyes!

Colors of the stars

If you take a careful look at stars in the sky you'll see that they're not all white. Sure, many appear to have a pure white color, but many others appear to have a touch of blue in their light, while others appear to have a touch of yellow, or orange, or even red. These colors you see are indications of the surface temperatures of these stars. The blue stars are the hottest, and the red stars are the coolest. Stars are classified into spectral classes, named O, B, A, F, G, K, and M, based on the characteristics of their light, which are in turn dependent on their surface temperatures. CyberSky can display stars in colors based on their spectral types.


Constellations

Overview

Constellations are not physical objects. They are arbitrary groupings of stars, invented to break the sky into smaller, more manageable pieces, thereby making it possible for the human mind to quickly identify the stars visible at a given time. Easy identification of stars was important to ancient sailors, because stars played a key role in navigation.

Many constellations are very old, but in ancient times constellations had no fixed limits. A constellation consisted of a group of stars that defined an outline resembling a mythological person, an animal, or a monster. The Greek astronomer Ptolemy listed only 48 constellations. The names of these ancient Greek constellations were eventually Latinized, and it is by these Latin names that we know these constellations today. In more recent times more constellations were added for the areas of the southern sky that became visible to European explorers.

CyberSky can label the constellations using their internationally recognized abbreviations ("UMa"), their Latin names ("Ursa Major"), or their equivalent English names or translations ("Big Bear"). The colors used to display constellation lines, boundaries, and labels can all be customized, as can the font used to display constellation labels.

Stars

The stars in a given constellation usually have no connection to each other, but merely lie in the same direction as viewed from the earth. Some of the stars in a constellation are relatively near the solar system, and others are tens or hundreds of times farther away. Seen from another direction, our constellations would be unrecognizable, because the stars we recognize as making up the constellations would change both their brightness and their positions. If human beings one day establish a colony on a planet orbiting another star, they will need to invent a new set of constellations.

Although the stars found in a given constellation do move across space, their apparent motion is so slow that it doesn't affect the appearance of a constellation over a human lifetime, or in fact many human lifetimes. The constellations we see in our sky today appeared the same to people who lived hundreds or thousands of years ago. But, if we were to travel 25,000 or 50,000 years into the past or into the future, we would find that many of our constellations would look distorted, or in some cases would be unrecognizable.

Constellation boundaries

Early in this century 88 constellations were officially recognized by international agreement. What was decided upon was the precise boundaries separating all constellations. Just as any city on the earth falls within the boundaries of some state, province, or country, all celestial bodies fall within the boundaries of a constellation.

Constellation lines

CyberSky can display lines to help you identify the constellations. Although CyberSky uses a set of constellation lines that are intended to give the impression of the object for which the constellation is named, the fact is that these lines are simple stick figures. It often takes a great deal of imagination to see these objects in the sky.

The lines used by CyberSky to draw the constellations are not in any way official or sanctioned by an international body. Different authors use different sets of lines, some more attractive and suggestive of the objects after which the constellations are named than others. Although constellations are often thought of as these "pictures" in the sky, the only official definition of the constellations are their boundaries. Strictly speaking, constellations are simply areas of the sky.

Zodiacal constellations

Twelve of the 88 constellations are special in the sense that the ecliptic, and therefore the sun, passes through them. These twelve constellations are called the constellations of the zodiac. CyberSky allows you to display the constellations of the zodiac independently of the nonzodiacal constellations.

Because of an effect known as the precession of the equinoxes, the ecliptic now passes through a thirteenth constellation, Ophiuchus, but this constellations is usually not considered to be a constellation of the zodiac.

Asterisms

Several of the groups of stars you may think of as constellations are really asterism. The best example is the Big Dipper, which is itself not a constellation but is part of the constellation Ursa Major, the Big Bear. Another example is the Summer Triangle, made up of the brightest stars in the constellations Aquila, Cygnus, and Lyra, named Altair, Deneb, and Vega.


Deep sky objects

Overview

Deep sky objects are objects such as galaxies, nebulae, and star clusters. With only a handful of exceptions, deep sky objects are not generally visible without optical aid such as binoculars or a small telescope.

The shareware version of CyberSky can display deep sky objects that are magnitude 5.0 or brighter. That's over 30 objects. The registered version of CyberSky can display over three times that many objects, to magnitude 6.5.

Deep sky objects appear fainter than their published magnitude values would indicate. Unlike a star, whose light appears to be concentrated in a point, a deep sky object appears spread out over a large area. Deep sky objects are therefore said to have a low surface brightness.

Some deep sky objects have names associated with them, and most have been assigned catalog numbers. You can label deep sky objects with their names ("Great Orion Nebula"), their Messier catalog numbers ("M 42"), or their New General Catalogue numbers ("NGC 1976").

The colors used to display deep sky object symbols and the color and font used to displays labels can be customized to your liking.

Galaxies

A galaxy is represented by a red oval. The best example is the Great Andromeda Galaxy.

Globular clusters

A globular cluster is represented by a yellow circle with a cross through it. The best example is the Great Hercules Cluster.

Nebulae

A nebula is represented by a green square. The best example is the Great Orion Nebula.

Open clusters

An open cluster is represented by a yellow circle of dots. The best example is the cluster in Taurus we call the Pleiades.


Sun

Overview

The sun is a typical star. In fact, if it were placed as far away from the earth as many typical stars are it would be nothing more than an inconspicuous point of light in the sky. But, because the sun is the center of our solar system and is much closer to us than any other star, the sun's appearance in our sky is unique.

The sun is represented in the sky display by a yellow disk. Note that that sun is not drawn to scale. Solar system objects such as the sun can be hidden, and they can be labeled with their names. The color used to display the sun can be customized to your liking, but it may be best to not use this option.

Motion

The sun's apparent path across the sky, which reflects the revolution of the earth around the sun, is called the ecliptic. As the earth moves along its orbit during the course of a year, we look at the sun from different places in our orbit and we see the sun projected against different stars in the background. These stars are part of areas of the sky known as constellations, and the twelve constellations through which the sun moves are known as the constellations of the zodiac. Ten of the twelve constellations of the zodiac are named for animals, a fact which gave the zodiac its name because "zoo" is the Latin root for "animal".

Daytime, twilight, and nighttime

It is, by definition, daytime when the sun is above the horizon. The sun's light is scattered by the earth's atmosphere, resulting in our sky appearing blue. Our atmosphere scatters some light even when the sun is below the horizon, resulting in the period we call twilight. Civil twilight is defined as the time when the sun is less than 6 degrees below the horizon. Nautical twilight is the time when the sun is between 6 and 12 degrees below the horizon. Astronomical twilight is the time when the sun is between 12 and 18 degrees below the horizon. After the sun has moved more than 18 degrees below the horizon it is nighttime and the sky is as dark as it will get.

CyberSky displays the sky in different colors, from blue, to shades of dark blue, to black, depending on whether it is daytime, twilight, or nighttime. It is possible to turn this feature off. The colors used to display the sky during daytime, twilight, and nighttime can be customized to your liking, but it may be best to not to use this option.

Solar eclipses

A solar eclipse can only occur when the moon is new. The moon's orbit is inclined about five degrees with respect to the ecliptic and intersects the ecliptic at two points known as the nodes of the orbit. If the moon's orbit was not inclined to the ecliptic, we would observe solar eclipses every month because the moon would pass over the sun's disk every month. But, because the moon's orbit is inclined by five degrees, most of the time the moon passes above or below the sun. An eclipse occurs only when the moon and the sun are both near one of the nodes of the moon's orbit.

The moon casts a shadow, and when this shadow sweeps across the surface of the earth people in the shadow see an eclipse. Observers in the area swept across by the umbra, the darkest part of the shadow, see a total solar eclipse. The solar disk is completely blocked in this case, allowing view of the sun's chromosphere, corona, and prominences. The moon's umbra is only about 270 kilometers (168 miles) across, and moves so quickly that a total solar eclipse typically lasts only 3 or 4 minutes. The area surrounding the umbra is called the penumbra, and observers in the area swept across by the penumbra see only a partial eclipse. An annular eclipse occurs when the moon is farther than normal from the earth, and is therefore too small to completely cover the sun. When the moon passes in front of the sun observers see a ring (annulus) of sun surrounding the moon's disk.

It is a remarkable coincidence of nature that the sun and moon are nearly the same size in the sky. The sun is about 400 times larger than the moon, but it is also about 400 times farther way from the earth. It is a fact that the moon is slowly moving away from the earth, so sometime in the distant future total solar eclipses will no longer be possible.

CyberSky does not (yet!) offer any solar eclipse prediction or display. Solar eclipses are rare enough and spectacular enough that local broadcast and print media will provide information about any solar eclipses visible in your area. Astronomy magazines are also a good source of information. The next three solar eclipses will occur in March 1997, February 1998, and August 1999.


Moon

Overview

The moon is our planet's only natural satellite. As far as moons go it is really nothing special. The moons of Jupiter and Saturn are much larger and more interesting worlds. But, thanks to its proximity to the earth, our moon bathes our nighttime landscape with its light and is easily the most prominent celestial object other than the sun.

The moon is represented in the sky display by a disk, drawn in two colors to match the moon's phase. Note that that moon is not drawn to scale. Solar system objects such as the moon can be hidden, and they can be labeled with their names. The colors used to display the light and dark parts of the lunar disk can be customized to your liking, but it may be best to not use this option.

In ancient times the moon was used as the basis for calendars, and this explains why our month is about as long as the moon's synodic period, that is, the length of time it takes the moon's phases to repeat themselves. It is also no coincidence that we divide our months into weeks lasting seven days, because it takes about one week for the moon to go from new to first quarter, from first quarter to full, from full to last quarter, and from last quarter to new.

One of the moon's characteristics we take for granted is that we see the same side of the moon all the time. The "man in the moon" is always visible. The reason for this is that the moon rotates once on its axis each time it revolves around the earth.

Motion

The moon moves rapidly across the sky, an average of 13 degrees per day eastward. During the course of one evening it is possible to notice the motion of the moon against the background stars. Because of the moon's eastward motion, it rises about 50 minutes later each day.

Although the moon is an interesting and beautiful object, its reappearance in our sky each month can be bothersome to those interested in astronomy. The moon's light increases the brightness of the sky, causing faint stars and deep sky objects to appear "washed out" and hard to see.

Lunar eclipses

A lunar eclipse can only occur when the moon is full. Rather than the earth passing into the moon's shadow, which is what happens when a solar eclipse occurs, during a lunar eclipse it is the moon that passes into the earth's shadow. If the moon passes into the umbra of the earth's shadow, an observer sees a total lunar eclipse, and if the moon only passes into the penumbra of the earth's shadow an observer sees a partial lunar eclipse. Total lunar eclipses typically last an hour and a half.

CyberSky does not (yet!) offer any lunar eclipse prediction or display. Local broadcast and print media will provide information about any lunar eclipses visible in your area. Astronomy magazines are also a good source of information. The next total lunar eclipse will occur in September 1997. The next partial lunar eclipses will occur in March 1997 and July 1999.


Planets

Overview

Planets, like the moon, generate no light of their own. We see them only because they reflect sunlight. Planets that are closer to the sun move faster across the sky than planets that are farther away. The planets appear to move slowly eastward with respect to the stars, a fact which gave them their name, as "planet" means "wanderer" in Greek. Occasionally a planet will slow down in its prograde or eastward motion and temporarily move westward. This motion is called retrograde motion.

The eight planets of the solar system visible from the earth are represented by their traditional symbols. The first time you start CyberSky only the planets visible without optical aid are included in the sky display. These five planets are Mercury, Venus, Mars, Jupiter, and Saturn. The planets Uranus, Neptune, and Pluto all require optical aid to be seen.

Solar system objects such as the planets can be hidden, and they can be labeled with their names. The colors used to display the planets can be customized to your liking.

The planets do not move at random over the sky but stay close to the line called the ecliptic. The ecliptic is the line along which the sun moves, and is simply the projection of the earth's orbit onto the celestial sphere. The reason all the planets move close to this line is that their orbits lie nearly in the same plane as the orbit of the earth. In fact, you'll find all planets other than Pluto within about 7 degrees of the ecliptic. Pluto's orbit is more highly inclined to the plane of the ecliptic, and so you'll find that planet as much as 17 degrees away.

Inferior planets

The two inferior planets, Mercury and Venus, are always close to the sun. In fact, Mercury can never be more than about 28 degrees from the sun, and Venus can never be more than 48 degrees from the sun. These angular distances are known as the greatest elongations of these planets. The consequence of this is that unlike the other planets, Mercury and Venus can never be seen at midnight. They always rise or set within a few hours of the sun.

When either planet is west of the sun, it rises before the sun and is known as a "morning star". Either planet is described as an "evening star" when it is east of the sun and sets after the sun.

Mercury is the most difficult of the naked-eye planets to observe, because it is usually lost in the sun's glare. It can only be seen under favorable conditions at dusk or dawn when it is low on the horizon and our view of it is poor. Mercury is not very interesting when observed through a small telescope. Venus is the brightest point of light in the sky, and is fun to see for that fact alone. Through a small telescope Venus shows phases much like the moon's but its cloud cover prevents any surface features from being seen.

Superior planets

Mars is the closest superior planet to the earth, and therefore moves more quickly across the background of stars than do Jupiter and Saturn. Superior planets such as Mars are best seen when they are at opposition, that is, when the earth is between them and the sun. Because Mars's orbital period is slightly less than twice as long as the earth's, Mars does not reach opposition once every year. Oppositions occur about two years and two months apart. This means that Mars is in a favorable position for observation from the earth only half as often as are the other superior planets.

Jupiter appears as a very bright yellow "star" in the sky and is easily differentiated from a real star. Saturn, on the other hand, is faint enough to be confused with a bright star. Jupiter and Saturn move much more slowly than Mars and both reach opposition about once a year as the earth moves between them and the sun. Jupiter, with its colored bands and four bright moons, and Saturn, with its spectacular rings, are arguably the most interesting planets to observe through a small telescope.

Uranus, Neptune, and Pluto require the aid of binoculars or a telescope to be seen. If you own a telescope you may be able to observe Uranus as a small greenish disk. Neptune's bluish disk is even smaller and significantly fainter. Pluto is far too faint to observe unless you have a large telescope, good seeing conditions, and detailed star charts. Uranus, Neptune, and Pluto move slowly across the background of stars. In fact, Pluto has completed only about a quarter of its orbit since its discovery in 1930, because it has an orbital period of over 248 years.


Grids

Overview

CyberSky can display coordinate system grid lines. These grids include the equatorial coordinate system and the horizontal coordinate system. Grids can be displayed with or without labels. The colors used to display grid lines and the color and font used to displays labels can be customized to your liking.

Equatorial coordinate system

The equatorial coordinate system is equivalent to the system of longitude and latitude used on the surface of the earth. The coordinates are called right ascension and declination.

Right ascension is a measure of the angular distance of a point from the vernal equinox, measured eastward. Right ascension is usually given in hours rather than degrees, where one hour is equal to 15 degrees. An object at the vernal equinox has a right ascension of 0 hours, and one on the opposite side of the sky has a right ascension of 12 hours.

Declination is a measure of the angular distance of a point from the celestial equator, measured northward. An object at the celestial equator has a declination of 0 degrees, and one at the north celestial pole has a declination of 90 degrees.

The equatorial coordinate system appears fixed with respect to the stars, making it possible to specify coordinates that do not depend on the observer's time or exact location on the earth. An object's right ascension and declination do change over time due to precession, but this change is slow.

Horizontal coordinate system

The horizontal coordinate system is also known as the altitude-azimuth system because the coordinates are called altitude and azimuth.

Azimuth is a measure of the angular distance of a point from the north point on the horizon, measured eastward. An object that is due north has an azimuth of 0 degrees, one that is due east has an azimuth of 90 degrees, one that is due south has an azimuth of 180 degrees, and one that is due west has an azimuth of 270 degrees.

Altitude is a measure of the angular distance of a point above the horizon. An object at the horizon has an altitude of 0 degrees, and one at the zenith has an altitude of 90 degrees.

An important point to keep in mind about the horizontal coordinate system is that a celestial body's position changes very rapidly over the course of a day. A position given in terms of altitude and azimuth is only valid for a short time, because the celestial body is moving across the sky with the motion of the celestial sphere.


Lines

Overview

CyberSky can display a number of important reference lines. These lines include the meridian, ecliptic, celestial equator, and galactic equator. The colors used to display these lines can be customized to your liking.

Meridian

The line that passes through the north point on the horizon, the zenith, and the south point on the horizon is called the meridian. It divides the sky into an eastern half and a western half. A celestial object reaches its highest point in the sky when it crosses the meridian. You can quickly bring the sun, the moon, or one of the naked-eye planets to the meridian.

You probably expect the sun to cross the meridian at noon, but this is generally not the case. The reasons for this are the location from which you are viewing the sky is not located in the center of its time zone, the effect called the equation of time, and the adoption of daylight saving time.

Ecliptic

The sun's apparent path across the sky, which reflects the revolution of the earth around the sun, is called the ecliptic. The sun, moon, and all planets are always found close to the ecliptic. The ecliptic also passes through all zodiacal constellations.

Celestial equator

The celestial equator is the projection of the earth's equator onto the celestial sphere. It divides the sky into a northern half and a southern half. Objects south of the celestial equator have a negative declination, and cannot be seen from the north pole. Objects north of the celestial equator have a positive declination, and cannot be seen from the south pole.

Galactic equator

The galactic equator is the projection of our galaxy's equator onto the celestial sphere. If you display all the stars CyberSky can display you'll notice a definite concentration of stars along this line.

Our galaxy, the Milky Way galaxy, is a collection of hundreds of billions of stars distributed in a thin disk. When we look at the sky perpendicular to the plane of this disk, we see relatively few stars. But, when we look at the sky parallel to the plane we see many more stars. This concentration of stars forms a band around the sky that has been called the Milky Way since ancient times.


Points

Overview

CyberSky can display and label a number of important reference points. These points include the north and south ecliptic poles, celestial poles, and galactic poles. If you choose to show these reference points, they will appear as small plus (+) signs in the sky.

The ecliptic poles, celestial poles, and galactic poles are displayed using the same colors used to display the ecliptic, celestial equator, and galactic equator. The colors used to display these points, as well as the font used to label them, can be customized to your liking.

Ecliptic poles

If you choose to show labels in the form of abbreviations, they will take the form of "NEP" and "SEP" for the north and south ecliptic poles.

Celestial poles

The north and south celestial poles are the most useful reference points to display. Observers in the northern hemisphere will be able to find the north celestial pole relatively easily because Polaris, a second magnitude star, lies nearby. There are no bright stars near the south celestial pole, so observers in the southern hemisphere will find the ability to display the south celestial pole especially useful.

If you choose to show labels in the form of abbreviations, they will take the form of "NCP" and "SCP" for the north and south celestial poles.

Galactic poles

If you choose to show labels in the form of abbreviations, they will take the form of "NGP" and "SGP" for the north and south galactic poles.


Legend

Overview

CyberSky's legend appears on the right side of the program's window. Although the information that appears in the legend is very useful, there are times when you'll want to hide the legend. For example, when you are viewing the sky in Horizon view, you may want to hide the legend to provide more screen space for the sky display. The colors and font used to display the legend can be customized to your liking.

Be careful when customizing the legend font. If you make it too big, the legend text may not fit on the screen. Remember that fonts are customized independently for the screen and for the printer. If you customize the legend font used when printing a sky chart, it's a good idea to use CyberSky's print preview feature to make sure the legend text fits on a sky chart.

Symbols

The top part of the legend contains an explanation of the symbols used to represent the sun, moon, planets, deep sky objects, grids, and lines in the sky display. The symbols don't change, except for the symbol used to represent the moon. This symbol always reflects the moon's current phase. Therefore, looking at the moon's symbol in the legend quickly tells you its current phase even when the moon isn't visible in the sky display.

Data

The legend also displays data about the sky display. This data includes the local time, universal time, local sidereal time, Julian date, location from which you are viewing the sky, and magnitude of the faintest stars included in the sky display.


Explorations, general

The exercises in this section deal with general aspects of astronomy. Each exercise starts with a short introduction, lists the steps you need to perform, and discusses what you'll see in the sky. You may find it useful to look up some of the terms used in the Glossary section of this Help file.

Which topic are you interested in?

Apparent motion of the sky at the north pole

Apparent motion of the sky at the equator

Apparent motion of the sky at the south pole


Apparent motion of the sky at the north pole

In this exercise you will watch the apparent motion of the sky at the north pole.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "North Pole" using the Location command on the Options menu.

3	Show the ecliptic and celestial equator using the Lines command on the Sky menu.

4	Set the animation step size to 30 minutes using the Step Size command on the Animation menu.

5	On the Animation menu, click Play Forwards.

You should now see the stars moving in the sky. Watch their motions for a minute or so. Notice anything interesting? The stars are moving counterclockwise in circles around the sky. We see that at the north pole all stars are circumpolar, that is, they never rise or set.

Where is the North Star? The star at the center of the sky is the North Star, also known as Polaris or more formally as α Ursae Minoris. Remember that the center of the sky corresponds to the zenith, the point directly above your head. So, if you were at the north pole and looked straight up, you would see the North Star.

To continue this exercise

6	On the Animation menu, click Stop.

7	Set the animation step size to seven solar days using the Step Size command on the Animation menu.

8	On the Animation menu, click Play Forwards.

The celestial equator lies 90 degrees from the north celestial pole, and since the north celestial pole is directly overhead at the north pole the celestial equator coincides with the horizon. We see that an observer at the north pole can only see half of the sky, that part which is north of the celestial equator.

The ecliptic is the line along which the sun moves during the course of a year. Because the earth's axis is tilted about 23.5 degrees with respect to its orbit around the sun, the celestial equator and ecliptic are tilted about 23.5 degrees with respect to each other. The celestial equator and ecliptic cross at exactly two points, meaning that part of the ecliptic is north of the celestial equator and another part is south of the celestial equator.

After watching the sky for a minute or two you should see the consequence of this fact. The sun spends half a year below the celestial equator, and therefore below the horizon! This illustrates why the north pole experiences six months of day and six months of night. Because the north pole and south pole are at opposite ends of the earth, when it is day at the north pole it is night at the south pole, and vice versa.


Apparent motion of the sky at the equator

In this exercise you will watch the apparent motion of the sky at the equator.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "Equator" using the Location command on the Options menu.

3	Show the ecliptic and celestial equator using the Lines command on the Sky menu.

4	Set the animation step size to 30 minutes using the Step Size command on the Animation menu.

5	On the Animation menu, click Play Forwards.

You should now see the stars moving in the sky. Watch their motions for a minute or so. Notice anything interesting? The stars are moving from east to west across the sky. We see that at the equator there are no circumpolar stars, that is, stars that never rise or set.

The celestial equator lies 90 degrees from both the north and south celestial poles. Here at the equator the celestial equator crosses the sky from due east to due west and passes through the zenith, the point over your head. This makes sense because the celestial equator is the projection of the earth's equator onto the celestial sphere, and you are watching the sky from the earth's equator. We see that an observer at the equator can theoretically see the entire sky over the course of one day. Of course, this isn’t actually possible because of the sun.

The ecliptic is the line along which the sun moves during the course of a year. Because the earth's axis is tilted about 23.5 degrees with respect to its orbit around the sun, the celestial equator and ecliptic are tilted about 23.5 degrees with respect to each other. The celestial equator and ecliptic cross at exactly two points, meaning that part of the ecliptic is north of the celestial equator and another part is south of the celestial equator.


Apparent motion of the sky at the south pole

In this exercise you will watch the apparent motion of the sky at the south pole.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "South Pole" using the Location command on the Options menu.

3	Show the ecliptic and celestial equator using the Lines command on the Sky menu.

4	Set the animation step size to 30 minutes using the Step Size command on the Animation menu.

5	On the Animation menu, click Play Forwards.

You should now see the stars moving in the sky. Watch their motions for a minute or so. Notice anything interesting? The stars are moving clockwise in circles around the sky. We see that at the south pole all stars are circumpolar, that is, they never rise or set.

Where is the North Star? Try as you may, you won't be able to find the North Star or any of the constellations that surround the north celestial pole. These stars never rise south of the Equator.

So...is there a South Star? No, there isn't. There is a south celestial pole, but there are no bright stars near enough for one of them to have been named the South Star.

To continue this exercise

6	On the Animation menu, click Stop.

7	Set the animation step size to seven solar days using the Step Size command on the Animation menu.

8	On the Animation menu, click Play Forwards.

The celestial equator lies 90 degrees from the south celestial pole, and since the south celestial pole is directly overhead at the south pole the celestial equator coincides with the horizon. We see that an observer at the south pole can only see half of the sky, that part which is south of the celestial equator.

The ecliptic is the line along which the sun moves during the course of a year. Because the earth's axis is tilted about 23.5 degrees with respect to its orbit around the sun, the celestial equator and ecliptic are tilted about 23.5 degrees with respect to each other. The celestial equator and ecliptic cross at exactly two points, meaning that part of the ecliptic is north of the celestial equator and another part is south of the celestial equator.

After watching the sky for a minute or two you should see the consequence of this fact. The sun spends half a year above the celestial equator, and therefore below the horizon! This illustrates why the south pole experiences six months of day and six months of night. Because the south pole and north pole are at opposite ends of the earth, when it is day at the south pole it is night at the north pole, and vice versa.


Explorations, time

The exercises in this section deal with time. Each exercise starts with a short introduction, lists the steps you need to perform, and discusses what you'll see in the sky. You may find it useful to look up some of the terms used in the Glossary section of this Help file.

Which topic are you interested in?

Solar and sidereal time


Solar and sidereal time

In this exercise you will learn about the difference between solar and sidereal time.

The earth rotates once on its axis per day, and this rapid motion makes it difficult to see the more subtle motions of the sun with respect to the stars. We will therefore eliminate the earth's rotational motion.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "Equator" using the Location command on the Options menu.

3	Set the time to 12:00:00 using the Local Time command on the Time menu. Be sure that the Daylight Saving Time option is not checked!

4	Set the animation step size to one solar day using the Step Size command on the Animation menu.

5	On the Animation menu, click Play Forwards.

By setting the animation step size to one solar day you have created a situation where you are seeing the sky at the same time every day. If you look at the Local Time in the legend you'll see that the time is not changing but the date is advancing one day per animation frame. The important thing to notice about what's happening in the sky is that the sun does not move in an east to west or west to east direction, but the stars are moving slowly from east to west day after day.

To continue this exercise

6	On the Animation menu, click Stop.

7	Set the animation step size to one sidereal day using the Step Size command on the Animation menu.

8	On the Animation menu, click Play Forwards.

If you look at the Local Time in the legend you'll see that the date is advancing one day per animation frame but the time is about four minutes earlier at each step. The important thing to notice about what's happening in the sky is that the stars are stationary in the sky, but the sun is moving slowly from west to east day after day.

Why is there a difference between solar time, time measured with respect to the position of the sun, and sidereal time, time measured with respect to the position of the stars? It takes 23 hours, 56 minutes, and 4 seconds for the earth to rotate once with respect to the stars, which is the definition of a sidereal day. If the sun and a star are observed to lie on the meridian at a given time, the star will be observed to lie on the meridian again in 23 hours, 56 minutes, and 4 seconds. Not so in the case of the sun!

The reason is that during one sidereal day the earth has also moved about one degree in its orbit about the sun. Therefore, the sun has changed direction with respect to the background stars. The earth must rotate another degree, which takes 3 minutes and 56 seconds, in order to bring the sun back to the meridian. It therefore takes 24 hours for the earth to rotate once with respect to the sun, which is the definition of a solar day.


Explorations, stars

The exercises in this section deal with stars. Each exercise starts with a short introduction, lists the steps you need to perform, and discusses what you'll see in the sky. You may find it useful to look up some of the terms used in the Glossary section of this Help file.

Which topic are you interested in?

Dog star

Precession

Proper motion


Dog star

The brightest star in the sky is Sirius, known more formally as α Canis Majoris. Sirius is a brilliant white star of magnitude -1.46 and is a dazzling sight on a clear, frosty winter night.

The star's proper name is probably derived directly from the Greek word for "sparkling" or "scorching". Sirius has also been called the "Dog Star" because it is the brightest star in the constellation Canis Major, the Big Dog. Sirius was revered by the ancient Egyptians as the "Nile Star" or "Star of Isis". A number of Egyptian temples were oriented toward the rising of this star.

To perform this exercise

1	On the View menu, click Horizon.

2	On the View menu, click East Horizon.

3	Set the location to "Egypt, Aswan" using the Location command on the Options menu.

4	Set the date to June 30, 3,000 BC using the Local Time command on the Time menu.

5	Set the time to sunrise using the Sunrise command on the Time menu.

6	Set the animation step size to 1 minute using the Step Size command on the Animation menu.

You are now looking at the sky as visible from southern Egypt just before dawn. The bright star you see just above the horizon on the right side of the display is Sirius. You can click on the star with the right mouse button to make sure it's really Sirius.

To continue this exercise

7	Advance the time by using the Next Step command on the Animation menu. Every time you use the Next Step command the time will advance by one minute. After two or three steps you'll see the sun rise on the left side of the display.

Ancient peoples did not have clocks to keep time for them. Accurate timekeeping required careful observation of the skies. The date on which Sirius rose just before the sun was very important to the Egyptians, as it coincided with the rising of the Nile river on which Egyptian civilization depended. The fact that this event also occurred at the beginning of summer explains why the hottest part of the year is referred to as the "dog days".

If you attempted to perform this exercise without setting the time to 3,000 BC, it probably didn't work. Thanks to an effect called precession, Sirius currently rises just before the sun about a month later than it did in 3,000 BC.


Precession

In this exercise you will observe the effect of precession.

The earth is not a perfect sphere, but has an equatorial diameter that is slightly larger than its polar diameter. The sun, moon, and other planets of the solar system exert a gravitational attraction on the equatorial bulge of the earth. It is this force that is responsible for precession.

The positions of the celestial poles actually move across the sky, tracing out circles with a diameter of about 47 degrees on the celestial sphere every 25,800 years. Therefore, the pole star observed thousands of years ago was not the star we see near the north celestial pole today.

The star we call the North Star was not near the celestial pole thousands of years in the past, and will not be near the pole thousands of years in the future. Currently the north pole of the earth's axis of rotation points toward the star α Ursae Minoris, better known as Polaris or simply the North Star.

Precession does not alter the constellation outlines or change their boundaries with respect to the stars, but causes a slow drift of the entire celestial coordinate system on the celestial sphere. This drift is called precession, or precession of the equinoxes.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "North Pole" using the Location command on the Options menu.

3	Show the ecliptic and celestial equator using the Lines command on the Sky menu.

4	Set the time to 2,700 BC using the Local Time command on the Time menu.

5	Set the animation step size to 30 minutes using the Step Size command on the Animation menu.

6	On the Animation menu, click Play Forwards.

Watch the motion of the stars in the display. You will find that the stars no longer move around α Ursae Minoris. The star we call the North Star was not at the north celestial pole in 2,700 BC.

Try to identify the star that is closest to the north celestial pole. Remember that at the North Pole the north celestial pole is directly overhead, in other words at the center of the display. Stop animation and then click on the star closest to the center of the display with the right mouse button. The Star Data dialog box will appear, telling you that the pole star back when the Egyptians were building the great pyramids was α Draconis, also known as Thuban!

About 12,000 years from now Vega, the brightest star in the constellation Lyra, will be the North Star. CyberSky does not allow you to set the time to 14,000 AD. CyberSky performs very accurate calculations, but the formulas used are simply not valid this far into the future.


Proper motion

In this exercise you will observe the effect of proper motion.

To perform this exercise

1	On the View menu, click Planetarium.

2	Set the location to "North Pole" using the Location command on the Options menu.

3	Show the constellations using the Constellations command on the Sky menu.

The star at the center of the display is Polaris, the North Star. It is located at the end of the handle of the Little Dipper, part of the constellation Ursa Minor. Not far from the Little Dipper you should be able to find the Big Dipper, part of the constellation Ursa Major.

Follow the curve of the handle of the Big Dipper and you will be guided toward a star called Arcturus, the brightest star in the constellation Bootes. When you have found Arcturus, take note of its location with respect to the lines of the constellation Bootes. Arcturus lies at the point where two lines meet.

To continue this exercise

4	Set the time to 4,000 BC using the Local Time command on the Time menu.

When the display is redrawn you will find that everything has moved. If you have performed the previous exercise, Precession, you will know that this is due to the precession of the equinoxes. Polaris is no longer at the center of the display. Nevertheless, you should be able to find the Little Dipper, Big Dipper, and Bootes in the new display.

When you have found Bootes in the display, you will see that Arcturus is no longer located where it was with respect to the constellation lines.

When you change the date and time, CyberSky calculates the effects of precession on the stars and constellation lines, and then calculates the effect of proper motion on the stars. This allows you to see that stars do move across the sky with respect to other stars and references like constellation lines.

All stars move through space, just as our sun does, but those stars that are close to our solar system appear to move much faster than those stars that are farther away. Even the closest stars outside the solar system are very far away, and detecting proper motion takes large telescopes and very sensitive equipment. This exercise shows that the effects of proper motion do accumulate over thousands of years so that you can notice it.

Proper motion was first discovered by the English astronomer Halley when he noticed that Arcturus was not at the same location it was shown at in star charts made 2,000 years earlier by the Greek scholar Hipparchus.


Explorations, sun

The exercises in this section deal with the sun. Each exercise starts with a short introduction, lists the steps you need to perform, and discusses what you'll see in the sky. You may find it useful to look up some of the terms used in the Glossary section of this Help file.

Which topic are you interested in?

Following the sun through the seasons

Analemma


Following the sun through the seasons

In this exercise you will be follow the sun's apparent motion through the seasons.

The earth rotates once on its axis per day, and this rapid motion makes it difficult to see the more subtle motions of the sun with respect to the stars. We will therefore eliminate the earth's rotational motion.

To perform this exercise

1	On the View menu, click Planetarium.

2	Show the ecliptic and celestial equator using the Lines command on the Sky menu.

3	Set the time to 12:00:00 using the Local Time command on the Time menu. Be sure that the Daylight Saving Time option is not checked!

4	Set the animation step size to one solar day using the Step Size command on the Animation menu.

5	On the Animation menu, click Play Forwards.

By setting the animation step size to one solar day you have created a situation where you are seeing the sky at the same time every day. If you look at the Local Time in the legend you'll see that the time is not changing but the date is advancing one day per animation frame.

Because the earth's axis of rotation is inclined 23.5 degrees to its orbital plane, the sun's daily motion across the sky changes as the earth moves around the sun over the course of one year. The important thing to notice in this exercise is the location of the sun with respect to the ecliptic and celestial equator at certain times of the year. As you watch the motion of the sun over months and years of time, you should notice the following events.

The sun crosses the celestial equator from south to north near the end of March. This point is called the vernal equinox, and it coincides with the beginning of spring in the northern hemisphere and fall in the southern hemisphere. This point is called an equinox because on the date the sun is on the equinox the length of day and night are equal.

The sun reaches the northernmost point of the ecliptic near the end of June. This point is called the summer solstice, and it coincides with the beginning of summer in the northern hemisphere and winter in the southern hemisphere. This point is called a solstice because on the date the sun is on the solstice it stops its motion north or south of the celestial equator and start moving back toward the celestial equator again. At any given location in the northern hemisphere the sun appears the highest in the sky on this day.

The sun crosses the celestial equator from north to south near the end of September. This point is called the autumnal equinox, and it coincides with the beginning of fall in the northern hemisphere and spring in the southern hemisphere.

The sun reaches the southernmost point of the ecliptic near the end of December. This point is called the winter solstice, and it coincides with the beginning of winter in the northern hemisphere and summer in the southern hemisphere. At any given location in the northern hemisphere the sun appears the lowest in the sky on this day.

It is these changing positions of the sun in the sky over the course of one year that is responsible for the seasons. This may come as a surprise to you. Many people believe that the reason for the seasons is that the earth moves closer or farther away from the sun. It is true that the earth's orbit around the sun is not a perfect circle, and that the earth is closer to the sun at one time of the year than it is half a year later. But, the difference in distance amounts to a change of only about three percent, and it is a fact that the earth is closest to the sun in early January when the weather is coldest in the northern hemisphere. Furthermore, if the earth's distance from the sun were responsible for the seasons, the northern and southern hemispheres would experience the same seasons at the same time, but they don't.

The reason for the seasons we experience has to do with how much sunlight is absorbed by the ground. At the summer solstice the sun is higher in the sky than it is during the rest of the year, and so more than an average amount of sunlight is absorbed by a given area of ground. The sun's rays hit the ground at a higher angle and are therefore more concentrated than they are during the winter. Also, the sun is above the horizon for a longer time during the summer, so the ground has more time to absorb the sun's light.

At the winter solstice the sun is lower in the sky than it is during the rest of the year, and so less than an average amount of sunlight is absorbed by a given area of ground. The sun's rays hit the ground at a lower angle and are therefore less concentrated than they are during the summer. And, the sun is above the horizon for a shorter time during the winter, so the ground has less time to absorb the sun's light.


Analemma

On a globe or a map of the earth you will sometimes see an elongated figure-eight drawn on the surface of the earth. This figure, called an analemma, represents the position of the sun over the course of one year. The vertical part of the figure is due to the sun's changing declination, and the horizontal part is due to the equation of time, the difference between true solar time and mean solar time.

In this exercise you will be able to see a similar figure traced out by the motion of the sun in the sky over the course of one year.

To perform this exercise

1	On the View menu, click Horizon.

2	On the View menu, click South Horizon.

3	Set the location to the "USA, CA, Los Angeles" using the Location command on the Options menu.

4	Set the time to 12:00:00 using the Local Time command on the Time menu. Be sure that the Daylight Saving Time option is not checked!

5	Set the animation step size to seven solar days using the Step Size command on the Animation menu.

6	On the Animation menu, click Play Forwards.

Watch the motion of the sun in the sky. Each animation step shows the sun at noon. Over the course of one year you will see the sun move farther and farther south, reach a minimum declination, move farther and farther north, reach a maximum declination, and then move south again. Notice that the sun is not exactly on the meridian at noon, but is sometimes east and sometimes west of the meridian at noon. This is what causes the sun to trace out a figure-eight rather than simply move north and south along a vertical line.

The position of the sun at the points farthest north and south on the analemma correspond to the summer and winter solstices. The point at the center of the figure-eight corresponds to the vernal and autumnal equinoxes.


Explorations, planets

The exercises in this section deal with the planets. Each exercise starts with a short introduction, lists the steps you need to perform, and discusses what you'll see in the sky. You may find it useful to look up some of the terms used in the Glossary section of this Help file.

Which topic are you interested in?

Planetary alignment

Retrograde motion of an inferior planet

Retrograde motion of a superior planet


Planetary alignment

In this exercise you will watch an interesting planetary alignment.

To perform this exercise

1	On the View menu, click Horizon.

2	On the View menu, click South Horizon.

3	Set the location to "USA, CA, Los Angeles" using the Location command on the Options menu.

4	Set the date to May 2, 2000 and the time to 12:00:00 using the Local Time command on the Time menu.

You should see an interesting alignment of the sun, moon, and all five planets visible without optical aid. Farthest to the east you'll see Mars, all by itself in the constellation Taurus. Next you'll see Saturn, Jupiter, the sun, Mercury, and then Venus all within the constellation Aries. Finally, farthest to the west you'll see the moon all by itself in the constellation Pisces.


Retrograde motion of an inferior planet

In this exercise you will observe the retrograde motion of the planet Mercury.

The orbit of Mercury around the sun lies inside the earth's orbit. The earth moves slower in its orbit than does Mercury, and at some part of their motion around the sun Mercury will move past the earth.

To perform this exercise

1	On the View menu, click Horizon.

2	On the View menu, click South Horizon.

3	Set the location to "USA, CA, Los Angeles" using the Location command on the Options menu.

4	Set the date to May 1, 1995 and the time to 14:00:00 using the Local Time command on the Time menu. Be sure that the Daylight Saving Time option is not checked!

5	Show the ecliptic using the Lines command on the Sky menu. This will make it easier to follow Mercury.

6	Set the animation step size to one sidereal day using the Step Size command on the Animation menu. This will allow you to see the motion of the planet with respect to the stars while keeping the stars fixed in the sky.

Find Mercury in the display. Look at the symbol in the legend if you need help identifying Mercury. You should find Mercury near the top of the display between the moon and the sun. Notice that Mercury is very close to the ecliptic.

To continue this exercise

7	On the Animation menu, click Play Forwards.

Watch Mercury move along the ecliptic. By the last week of May you will see Mercury slow down in its direct motion. Over the next few weeks you will see Mercury moving backwards along the ecliptic. By the middle of June Mercury will again be slowing down over the Hyades asterism, the V-shaped group of stars in the constellation Taurus. By July it will again be moving forward along the ecliptic.


Retrograde motion of a superior planet

In this exercise you will observe the retrograde motion of the planet Mars.

The orbit of Mars around the sun lies outside the earth's orbit. The earth moves faster in its orbit than does Mars, and at some part of their motion around the sun the earth will move past Mars. This happens when Mars is near opposition.

Imagine driving in a car, and passing another car. The other car will appear to be moving backwards in relation to your car. Similarly, viewed against the background stars, Mars will appear to move backwards in relation to its prograde or direct motion.

To perform this exercise

1	On the View menu, click Horizon.

2	On the View menu, click South Horizon.

3	Set the location to "USA, CA, Los Angeles" using the Location command on the Options menu.

4	Set the date to November 1, 1996 and the time to 9:00:00 using the Local Time command on the Time menu. Be sure that the Daylight Saving Time option is not checked!

5	Show the ecliptic using the Lines command on the Sky menu. This will make it easier to follow Mars.

6	Set the animation step size to one sidereal day using the Step Size command on the Animation menu. This will allow you to see the motion of the planet with respect to the stars while keeping the stars fixed in the sky.

Find Mars in the display. Look at the symbol in the legend if you need help identifying Mars. You should find Mars near the top of the display between the constellations Leo and Cancer. Notice that Mars is very close to the ecliptic.

To continue this exercise

7	On the Animation menu, click Play Forwards.

Watch Mars move along the ecliptic. By February you will see Mars slow down in its direct motion. Over the next three months you will see Mars moving backwards along the ecliptic in the constellation Virgo. By April Mars will again be slowing down, and by May it will again be moving forward along the ecliptic.


Ordering information

CyberSky is not free software. You are licensed to evaluate the shareware version of CyberSky for a period of 30 days after your initial use. Continued use after your 30-day evaluation period requires that you purchase the registered version of the software.

Use of the shareware version after your 30-day evaluation period, or for any purpose other than evaluation, is in violation of US and international copyright laws and deprives the developer of the software of income he has earned through hard work.

Shareware version: The shareware version of CyberSky allows you to "try before you buy" and to determine whether it works correctly on your computer and with your printer. Constellation boundaries have been disabled, the number of stars and deep sky objects that can be displayed has been reduced, and some reference material has been removed to further conserve space.

Registered version: The registered version of CyberSky is the complete, fully-functional version of the software. The registered version includes the constellation boundaries disabled in the shareware version, all available stars and deep sky objects, and all reference material, but does not include the annoying registration reminder screen displayed when you run the shareware version.

Ordering benefits

Ordering benefits

Ordering instructions

Ordering from CompuServe

Ordering from Public Software Library

Ordering from Shareware-Australia

Ordering from Stephen Michael Schimpf


Ordering benefits

I hope you find the shareware version of CyberSky entertaining and educational and that you'll purchase the registered version. Below is a list of benefits you'll receive by ordering.

Floppy disks: You'll receive 3.5-inch floppy disks containing the latest registered versions of CyberSky for Windows 3.1 and CyberSky for Windows 95.

Faster startup: The registered version does not include the annoying registration reminder screen displayed for 20 seconds when you run the shareware version.

More stars: The registered version includes over five times more stars than the shareware version.

Constellation boundaries: The registered version allows you to display constellation boundaries, so you'll know exactly which areas of the sky belong to which constellations.

More deep sky objects: The registered version includes about three times more deep sky objects than the shareware version.

Constellation reference: The registered version's Help file contains a complete constellation reference. You'll get information about the names of the constellations, their positions, the times they are best seen, tables of named stars, and the histories and mythologies related to each constellation.

Solar system object reference: The registered version's Help file also contains a complete solar system object reference. You'll get overviews of the sun, the moon, and all planets, descriptions of each object's interior, surface, and atmosphere, information about rings and moons, and tables of orbital and physical properties.

Product support: You'll receive priority support for the lifetime of the product. I will do my best to resolve problems to your satisfaction, or will refund the price within 60 days of your purchase.

Discount eligibility: You'll become eligible to purchase all future versions of CyberSky at substantially discounted prices.

Writing software such as CyberSky requires years of programming experience, a substantial investment in development tools, and a huge amount of time. Your order means a lot to me and helps support a small business based on honesty and trust.


Ordering from CompuServe

CyberSky can be ordered from CompuServe's Shareware Registration

service, provided you have a CompuServe account. The registered

version of CyberSky costs US$ 28, which includes shipping and

handling. Your order will be conveniently billed directly to your

CompuServe account.

CompuServe's Shareware Registration service can only be used to

order a CyberSky Single License, which allows you to run the

software on one computer at a time. If you would like to order a

CyberSky Site License or a CyberSky Upgrade License, please use

another ordering method.

To order online from CompuServe

1  Connect to CompuServe.

2  Go to the Shareware Registration service (CIS:SWREG).

3  Search for Registration ID 6128 or for Title CyberSky.

4  Follow the instructions to place your order.

Questions about the status of your order or about the product must

be mailed to Stephen Michael Schimpf, 2444 West 230th Street,

Torrance, CA 90501-5242, or e-mailed to sales@cybersky.com.

CompuServe will notify me within one business day of your order and

I will ship your package to you within two business days of

receiving this notification.


Ordering from Public Software Library

CyberSky can be ordered from Public Software Library. Payment can

be made by Visa, MasterCard, American Express, or Discover.

Order online at       http://www.cybersky.com/

Fax this form to      713-524-6398

E-mail your order to  14084@pslweb.com

Mail this form to     Public Software Library

                      PO Box 35705

                      Houston, TX 77235-5705

Phone your order to   800-242-4775, ext. 14084   (8 AM to 6 PM CST

                      713-524-6394, ext. 14084    Monday to Friday)

Questions about the status of your order or about the product must

be mailed to Stephen Michael Schimpf, 2444 West 230th Street,

Torrance, CA 90501-5242, or e-mailed to sales@cybersky.com.

Name ______________________________________________________________

Address ___________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

E-mail ____________________________________________________________

CyberSky Single License          _____  x  US$   28  =  US$ _______

CyberSky Site License

    For  3 Computers             _____  x  US$   53  =  US$ _______

    For  5 Computers             _____  x  US$   78  =  US$ _______

    For 10 Computers             _____  x  US$  103  =  US$ _______

    For 20 Computers             _____  x  US$  128  =  US$ _______

    For 30 Computers             _____  x  US$  153  =  US$ _______

CyberSky Upgrade License         _____  x  US$   15  =  US$ _______

Shipping and handling is FREE!             Total        US$ _______

Card # ___________________________________ Expiration date ________

Signature _________________________________________________________


Ordering from Shareware-Australia

CyberSky can be ordered in Australia and New Zealand from my local

distributor, Shareware-Australia. Payment can be made by cheque,

money order, Bankcard, Visa, MasterCard, or American Express.

Fax this form to      (02) 9565-2362

Phone your order to   (02) 9565-2339

Mail this form to     Shareware-Australia

                      PO Box 1075

                      Maroubra, NSW 2035

Order online at       http://www.shareware-australia.com/

E-mail your order to  sales@shareware-australia.com

Name ______________________________________________________________

Address ___________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

E-mail ____________________________________________________________

CyberSky Single License          _____  x  AU$   45  =  AU$ _______

CyberSky Site License

    For  3 Computers             _____  x  AU$   90  =  AU$ _______

    For  5 Computers             _____  x  AU$  130  =  AU$ _______

    For 10 Computers             _____  x  AU$  170  =  AU$ _______

    For 20 Computers             _____  x  AU$  210  =  AU$ _______

    For 30 Computers             _____  x  AU$  255  =  AU$ _______

CyberSky Upgrade License         _____  x  AU$   25  =  AU$ _______

Shipping and handling is FREE!             Total        AU$ _______

[ ] Payment by cheque or money order enclosed

[ ] Payment by credit card authorized below

Card # ___________________________________ Expiry date ____________

Signature _________________________________________________________


Ordering from Stephen Michael Schimpf

CyberSky can be ordered from Stephen Michael Schimpf, the program's

developer. Payment can be made by check drawn on a US bank and

payable in US dollars, or international postal money order payable

in US dollars, or cash in US dollars sent by registered mail or at

your own risk. Please make your check or money order payable to

Stephen Michael Schimpf. Please print and mail this form to

                      Stephen Michael Schimpf

                      2444 West 230th Street

                      Torrance, CA 90501-5242

Credit-card orders: Please order from Public Software Library or

from Shareware-Australia if you would like to pay by credit card.

Name ______________________________________________________________

Address ___________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

E-mail ____________________________________________________________

CyberSky Single License          _____  x  US$   28  =  US$ _______

CyberSky Site License

    For  3 Computers             _____  x  US$   53  =  US$ _______

    For  5 Computers             _____  x  US$   78  =  US$ _______

    For 10 Computers             _____  x  US$  103  =  US$ _______

    For 20 Computers             _____  x  US$  128  =  US$ _______

    For 30 Computers             _____  x  US$  153  =  US$ _______

CyberSky Upgrade License         _____  x  US$   15  =  US$ _______

Shipping and handling is FREE!             Total        US$ _______

[ ] Payment by check, money order, or cash enclosed

[ ] Payment by international postal money order; receipt enclosed


Product support information

Product support is available to all users, but priority support will be given to registered users.

Searching for help

CyberSky includes an extensive Help system that describes the software and its operation. Details about the program's options and the objects displayed are accessible from various dialog boxes and by searching this Help file. Before contacting me for product support, please search this Help file to see if it contains the answer to your question.

Reporting a problem

If you encounter a problem while using CyberSky, please report it as soon as possible so that it can be corrected in another version of the software. Please provide relevant information about the hardware and software you are using, describe the problem and any messages you may have seen, and list the steps required to reproduce the problem.

Contacting me

Please contact me by mail or e-mail, or visit my Web site to check for upgrade announcements.

Stephen Michael Schimpf	www.cybersky.com

2444 West 230th Street	sales@cybersky.com

Torrance, CA 90501-5242	support@cybersky.com

I welcome any comments or suggestions you may have.


License agreement

You should carefully read the following terms and conditions before using this software. Unless you have a different license agreement signed by Stephen Michael Schimpf, your use of this software indicates your acceptance of this license agreement and disclaimer of warranty.

License agreement

CyberSky is not free software. You are licensed to evaluate the shareware version of CyberSky for a period of 30 days after your initial use. Continued use after your 30-day evaluation period requires that you purchase the registered version of the software.

Use of the shareware version after your 30-day evaluation period, or for any purpose other than evaluation, is in violation of US and international copyright laws and deprives the developer of the software of income he has earned through hard work.

Governing law

This agreement is governed by the laws of the State of California.

Disclaimer of warranty

CYBERSKY IS SUPPLIED "AS IS" WITHOUT WARRANTY OF ANY KIND. TO THE MAXIMUM EXTENT PERMITTED BY APPLICABLE LAW, STEPHEN MICHAEL SCHIMPF DISCLAIMS ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. STEPHEN MICHAEL SCHIMPF'S LIABILITY IS LIMITED EXCLUSIVELY TO PRODUCT REPLACEMENT OR REFUND OF PURCHASE PRICE.


Trademarks

Microsoft and Windows are registered trademarks of Microsoft Corporation.

CompuServe is a registered trademark of CompuServe, Inc.

PSL is a registered trademark of Public Software Library, Inc.

Trademarks of other companies mentioned in this documentation appear for identification purposes only and are the property of their respective companies.


Print a sky chart

CyberSky can print a sky chart on any Windows printer.

To print a sky chart

1	On the File menu, click Print. This will display the Print dialog box.

2	Specify the printer and printing options.

3	Click OK to remove the Print dialog box.

Tips

CyberSky normally prints sky charts in landscape orientation. If you select another printer, the page orientation may change to portrait orientation. Be sure to click Landscape to restore landscape orientation.

CyberSky always prints the legend, even if the legend is not displayed on the screen.


Edit, add, or delete a location

CyberSky includes 1,157 locations in its location database. You can edit the information associated with any of these locations, add locations to the database, or delete locations from the database.

To edit a location in the database

1	On the Options menu, click Location. This will display the Location dialog box.

2	Specify the location you want to edit by selecting it in the Name list.

3	Click Edit. This will display the Edit Location dialog box.

4	Change the location's name, latitude, longitude, and time zone.

5	Click OK to remove the Edit Location dialog box.

6	Click OK to remove the Location dialog box.

To add a location to the database

1	On the Options menu, click Location. This will display the Location dialog box.

2	Click Add. This will display the Edit Location dialog box.

3	Specify the location's name, latitude, longitude, and time zone.

4	Click OK to remove the Edit Location dialog box.

5	Click OK to remove the Location dialog box.

To delete a location from the database

1	On the Options menu, click Location. This will display the Location dialog box.

2	Specify the location you want to delete by selecting it in the Name list.

3	Click Delete.

4	Click Yes to confirm that you really want to delete this location.

5	Click OK to remove the Location dialog box.

Tip

Latitude, longitude, and time zone information for each of the cities in CyberSky's location database was obtained from a reliable source. Nevertheless, mistakes are possible and therefore you are given the chance to edit this information.


Customize screen and printer colors

CyberSky allows you to customize all colors used by the program.

To customize screen and printer colors

1	On the Options menu, click Colors. This will display the Colors dialog box.

2	To specify whether you want to customize a screen or printer color, choose the Screen or Printer option in the Colors box.

3	Select the item who's color you want to customize in the Items lists.

4	Click Edit. This will display the Color Browser dialog box.

5	To change the color used to display the selected item, select a color in the Colors box.

6	Click OK to remove the Color Browser dialog box.

7	Click OK to remove the Colors dialog box.

CyberSky allows you to restore an item's default color.

To restore an item's default color

1	On the Options menu, click Colors. This will display the Colors dialog box.

2	To specify whether you want to restore a screen or printer color, choose the Screen or Printer option in the Colors box.

3	Select the item who's default color you want to restore in the Items lists.

4	Click Reset.

5	Click OK to remove the Colors dialog box.

CyberSky allows you to restore the default colors of all items.

To restore the default colors of all items

1	On the Options menu, click Colors. This will display the Colors dialog box.

2	To specify whether you want to restore all screen or all printer colors, choose the Screen or Printer option in the Colors box.

3	Click Reset All.

4	Click OK to remove the Colors dialog box.

Tips

Screen and printer colors are independent of each other. Changing an item's screen color has no effect on its printer color, and vice versa.

Any changes made to the color settings will be automatically saved when you exit CyberSky.


Customize screen and printer fonts

CyberSky allows you to customize all fonts used by the program.

To customize screen and printer fonts

1	On the Options menu, click Fonts. This will display the Fonts dialog box.

2	To specify whether you want to customize a screen or printer font, choose the Screen or Printer option in the Fonts box.

3	Select the item who's font you want to customize in the Items lists.

4	Click Edit. This will display the (Item) Font dialog box.

5	To change the font used to display the selected item, select a font, font style, and size.

6	Click OK to remove the (Item) Font dialog box.

7	Click OK to remove the Fonts dialog box.

CyberSky allows you to restore an item's default font.

To restore an item's default font

1	On the Options menu, click Fonts. This will display the Fonts dialog box.

2	To specify whether you want to restore a screen or printer font, choose the Screen or Printer option in the Fonts box.

3	Select the item who's default font you want to restore in the Items lists.

4	Click Reset.

5	Click OK to remove the Fonts dialog box.

CyberSky allows you to restore the default fonts of all items.

To restore the default fonts of all items

1	On the Options menu, click Fonts. This will display the Fonts dialog box.

2	To specify whether you want to restore all screen or all printer fonts, choose the Screen or Printer option in the Fonts box.

3	Click Reset All.

4	Click OK to remove the Fonts dialog box.

Tips

Screen and printer fonts are independent of each other. Changing an item's screen font has no effect on its printer font, and vice versa.

Any changes made to the font settings will be automatically saved when you exit CyberSky.


Change the number of stars visible in the sky

CyberSky can display a simulated sky that includes stars as faint as magnitude 5.0 in the shareware version and 6.5 in the registered version. Stars of magnitude 6.0 are the faintest stars you can see without optical aid in the best seeing conditions far aware from city lights.

To change the number of stars visible in the sky

1	On the Sky menu, click Stars. This will display the Stars dialog box.

2	Move the scroll bar in the Limit box left to display fewer stars, or right to display more stars.

3	Click OK to remove the Stars dialog box.

Tips

The more stars are visible in the sky, the longer it will take for CyberSky to update the display each time it needs to do so.

The magnitude of the faintest stars currently visible in the sky is displayed in the legend.


Change the time

CyberSky can display the sky for any time between 4,000 BC and 4,000 AD. When CyberSky starts it reads your computer's time setting and displays the sky for that time. You can view the sky at another time by changing the local or universal time, or the Julian date.

To change the local time

1	On the Time menu, click Local Time. This will display the Local Time dialog box.

2	Specify the local time.

3	Check the Daylight Saving Time option if daylight saving time is in effect.

4	Click OK to remove the Local Time dialog box.

To change the universal time

1	On the Time menu, click Universal Time. This will display the Universal Time dialog box.

2	Specify the universal time.

3	Click OK to remove the Universal Time dialog box.

To change the Julian date

1	On the Time menu, click Julian Date. This will display the Julian Date dialog box.

2	Specify the Julian date.

3	Click OK to remove the Julian Date dialog box.

Tips

Use toolbar buttons to quickly change the time by one hour or one day.

October 4, 1582 AD and earlier dates are on the Julian calendar. October 15, 1582 AD and later dates are on the Gregorian calendar.

CyberSky's limits of 4,000 BC to 4,000 AD correspond to Julian dates 260,423.5 and 3,182,029.5, respectively.

Changing the time in CyberSky does not affect your computer's time setting, and vice versa.


Display information about an object

CyberSky can display detailed information about objects in the sky. Such information is available for stars, deep sky objects, the sun, the moon, and all planets.

To display information about an object

1	Move the pointer over the object.

2	Click the right mouse button.

3	Click OK to remove the dialog box after you have finished viewing the information.

Tips

If you want to display information about the sun, the moon, or a planet, and the object you are interested in is not visible in the display, use one of the commands on the Reports menu to display this information.

If more than one object is located where you moved the pointer, dialog boxes will be displayed one after the other containing information about all objects.

When you click on the OK button to remove the dialog box from the screen, the pointer will automatically be moved back to the object on which it started.


Display the times at which an object rises, transits, and sets

CyberSky can display the times at which an object rises, transits, and sets. Such times are available for stars, deep sky objects, the sun, the moon, and all planets.

To display the times at which an object rises, transits, and sets

1	Move the pointer over the object.

2	Click the right mouse button.

3	Click OK to remove the dialog box after you have finished viewing the information.

Tips

If you want to display the times at which the sun, the moon, or a planet rises, transits, and sets, and the object you are interested in is not visible in the display, use one of the commands on the Reports menu to display this information.

If more than one object is located where you moved the pointer, dialog boxes will be displayed one after the other containing information about all objects.

When you click on the OK button to remove the dialog box from the screen, the pointer will automatically be moved back to the object on which it started.


Print command
Print command (File menu)

Prints a sky chart using the current print settings.


Print Preview command
Print Preview command (File menu)

Shows a sky chart as it will look when printed.


Print Setup command
Print Setup command (File menu)

Changes the printer and printing options.


Exit command
Exit command (File menu)

Exits the CyberSky application.


Planetarium command
Planetarium command (View menu)

Shows the sky in planetarium view.


Horizon command
Horizon command (View menu)

Shows the sky in horizon view.


North Horizon command
North Horizon command (View menu)

Sets the orientation to north, 0 degrees azimuth.


South Horizon command
South Horizon command (View menu)

Sets the orientation to south, 180 degrees azimuth.


East Horizon command
East Horizon command (View menu)

Sets the orientation to east, 90 degrees azimuth.


West Horizon command
West Horizon command (View menu)

Sets the orientation to west, 270 degrees azimuth.


Legend command
Legend command (View menu)

Shows or hides the legend.


Full Screen command
Full Screen command (View menu)

Toggles full screen mode on or off.


Toolbar command
Toolbar command (View menu)

Shows or hides the toolbar.


Status Bar command
Status Bar command (View menu)

Shows or hides the status bar.


Location command
Location command (Options menu)

Changes the location from which the sky is being viewed.


Sky command
Sky command (Options menu)

Changes sky options.


Orientation command
Orientation command (Options menu)

Changes the orientation of the sky.


Colors command
Colors command (Options menu)

Changes screen and printer colors.


Fonts command
Fonts command (Options menu)

Changes screen and printer fonts.


Stars command
Stars command (Sky menu)

Changes which stars and star labels are included in the sky.


Constellations command
Constellations command (Sky menu)

Changes which constellation lines, boundaries, and labels are included in the sky.


Deep Sky Objects command
Deep Sky Objects command (Sky menu)

Changes which deep sky objects and object labels are included in the sky.


Solar System Objects command
Solar System Objects command (Sky menu)

Changes which solar system objects and object labels are included in the sky.


Grids command
Grids command (Sky menu)

Changes which grids and grid labels are included in the sky.


Lines command
Lines command (Sky menu)

Changes which lines are included in the sky.


Points command
Points command (Sky menu)

Changes which points and point labels are included in the sky.


Local Time command
Local Time command (Time menu)

Changes the local time at which the sky is being viewed.


Universal Time command
Universal Time command (Time menu)

Changes the universal time at which the sky is being viewed.


Julian Date command
Julian Date command (Time menu)

Changes the Julian date at which the sky is being viewed.


Computer Time command
Computer Time command (Time menu)

Sets the local time to your computer's system time.


Midnight command
Midnight command (Time menu)

Sets the local time to midnight.


Sunrise command
Sunrise command (Time menu)

Sets the local time to sunrise.


Noon command
Noon command (Time menu)

Sets the local time to noon.


Sunset command
Sunset command (Time menu)

Sets the local time to sunset.


Event command
Event command (Time menu)

Sets the local time to that of a rising, transit, or setting event.


Sun command
Sun command (Reports menu)

Shows a report containing information about the sun.

Tip

Use the Sun command to view information about the sun even when it is not visible in the sky.


Moon command
Moon command (Reports menu)

Shows a report containing information about the moon.

Tip

Use the Moon command to view information about the moon even when it is not visible in the sky.


Planets command
Planets command (Reports menu)

Shows a report containing information about the planets.

Tip

Use the Planets command to view information about a planet even when it is not visible in the sky.


Play Backwards command
Play Backwards command (Animation menu)

Starts animation with time moving backwards.


Previous Step command
Previous Step command (Animation menu)

Shows the previous animation step.


Stop command
Stop command (Animation menu)

Stops animation.


Next Step command
Next Step command (Animation menu)

Shows the next animation step.


Play Forwards command
Play Forwards command (Animation menu)

Starts animation with time moving forwards.


Step Size command
Step Size command (Animation menu)

Changes the animation step size.


Help Topics command
Help Topics command (Help menu)

Displays Help topics.


About CyberSky command
About CyberSky command (Help menu)

Displays the application's name, version number, and copyright notice.


Proper Names command
Proper Names command (Toolbar)

Shows or hides proper names.


Constellation Lines command
Constellation Lines command (Toolbar)

Shows or hides constellation lines.


Constellation Boundaries command
Constellation Boundaries command (Toolbar)

Shows or hides constellation boundaries.


Equatorial Grid command
Equatorial Grid command (Toolbar)

Shows or hides the equatorial coordinate system grid.


Horizontal Grid command
Horizontal Grid command (Toolbar)

Shows or hides the horizontal coordinate system grid.


One Day Earlier command
One Day Earlier command (Toolbar)

Shows the sky as it appeared one day earlier.


One Hour Earlier command
One Hour Earlier command (Toolbar)

Shows the sky as it appeared one hour earlier.


One Hour Later command
One Hour Later command (Toolbar)

Shows the sky as it will appear one hour later.


One Day Later command
One Day Later command (Toolbar)

Shows the sky as it will appear one day later.


Keyboard shortcuts

Press this	To do this

Ctrl+P	Print a sky chart

Alt+F4	Quit the CyberSky application

Ctrl+Q	Toggle full screen mode on or off

Ctrl+L	Change the local time at which the sky is being viewed

Ctrl+U	Change the universal time at which the sky is being viewed

Ctrl+J	Change the Julian date at which the sky is being viewed

Ctrl+M	Set the local time to midnight

Ctrl+R	Set the local time to sunrise

Ctrl+N	Set the local time to noon

Ctrl+S	Set the local time to sunset

Ctrl+E	Set the local time to that of a rising, transit, or setting event

Ctrl+B	Start animation with time moving backwards

Esc	Stop animation

Ctrl+F	Start animation with time moving forwards

F1	Display Help topics


Toolbar shortcuts

Click this	To do this

Print a sky chart

Show a sky chart as it will look when printed

Set the orientation to north, 0 degrees azimuth

Set the orientation to south, 180 degrees azimuth

Set the orientation to east, 90 degrees azimuth

Set the orientation to west, 270 degrees azimuth

Show or hide proper names

Show or hide constellation lines

Show or hide constellation boundaries

Show or hide the equatorial coordinate system grid

Show or hide the horizontal coordinate system grid

Change the local time at which the sky is being viewed

Show the sky as it appeared one day earlier

Show the sky as it appeared one hour earlier

Show the sky as it will appear one hour later

Show the sky as it will appear one day later

Start animation with time moving backwards

Show the previous animation step

Stop animation

Show the next animation step

Start animation with time moving forwards

Display Help topics

Display Help for clicked on buttons, menus, and windows


About CyberSky dialog box

Use the About CyberSky dialog box to view the name, version number, and copyright notice for your copy of CyberSky.

OK

To close this dialog box, click OK.

Total physical memory

Displays the total amount of RAM in your system.

Available physical memory

Displays the amount of RAM that is currently available for use in your system.


Color Browser dialog box

Use the Color Browser dialog box to preview and select a color used to display an item on the screen or on a printer.

Color

To change the color used to display the selected item, select a color in the Colors box.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Colors dialog box

Use the Colors dialog box to customize the colors used to display items on the screen and on a printer.

Colors

To specify whether you want to change screen or printer colors, choose the Screen or Printer option in the Colors box.

Sample

Shows the color used to display the item selected in the Items list.

Items

To preview, edit, or reset the color used to display an item, select the item in the Items list.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Edit

To edit the color used to display the item selected in the Items list, click Edit.

Reset

To reset the color used to display the item selected in the Items list to the color used the first time you ran CyberSky, click Reset.

Reset All

To reset the colors used to display all items in the Items list to the colors used the first time you ran CyberSky, click Reset All.

Help

To display help for this dialog box, click Help.


Constellations dialog box

Use the Constellations dialog box to show or hide constellation lines, boundaries, and labels.

Constellation lines

To show or hide all constellation lines, click Constellation lines.

Zodiacal

To show or hide zodiacal constellations, click Zodiacal.

Nonzodiacal

To show or hide nonzodiacal constellations, click Nonzodiacal.

Constellation boundaries

To show or hide constellation boundaries, click Constellation Boundaries.

Don't show any labels

To hide labels, click Don't Show Any Labels.

Abbreviations

To show constellation abbreviations such as "Aql", click Abbreviations.

Latin names

To show constellation names in Latin, such as "Aquila", click Latin Names.

English names

To show constellation names in English, such as "Eagle", click English Names.

German names

To show constellation names in German, such as "Adler", click German Names.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Deep Sky Objects dialog box

Use the Deep Sky Objects dialog box to show or hide deep sky objects and object labels.

Deep sky objects

To show or hide all types of deep sky objects, click Deep sky objects.

Galaxies

To show or hide galaxies, click Galaxies.

Globular clusters

To show or hide globular clusters, click Globular clusters.

Nebulae

To show or hide nebulae, click Nebulae.

Open clusters

To show or hide open clusters, click Open clusters.

Don't show any labels

To hide labels, click Don't Show Any Labels.

Names

To show deep sky object names such as "Great Orion Nebula", click Names.

Messier numbers

To show deep sky object Messier numbers such as "M 42", click Messier numbers.

New General Catalogue numbers

To show deep sky object New General Catalogue numbers such as "NGC 1976", click New General Catalogue numbers.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Deep Sky Object Data dialog box

Use the Deep Sky Object Data dialog box to display detailed information about any deep sky object in the display when you click on it with the right mouse button. This information includes the object's name, Messier catalog and New General Catalogue numbers, equatorial and horizontal coordinates, distance, magnitude, constellation, rising, transit, and setting times, and object type and class.

galaxy

globular cluster

nebula

open cluster

Messier catalog

New General Catalogue

equatorial coordinate system

right ascension

declination

horizontal coordinate system

azimuth

altitude

distance

magnitude

constellation

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Edit Location dialog box

Use the Edit Location dialog box to edit the longitude, latitude, and time zone settings associated with a location in the location database.

Name

To specify the name of the location, enter a name up to 38 characters in length.

Latitude

To specify the latitude of the location, enter a latitude in degrees, minutes, and seconds, then click the down arrow and select "N" to specify a north latitude or "S" to specify a south latitude.

Longitude

To specify the longitude of the location, enter a longitude in degrees, minutes, and seconds, then click the down arrow and select "E" to specify an east longitude or "W" to specify a west longitude.

Time zone

To specify the time zone of the location, enter a time in hours and minutes, then click the down arrow and select "Ahead of " to specify a time zone ahead of universal time or "Behind" to specify a time zone behind universal time.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Event dialog box

Use the Event dialog box to quickly set the time to when the sun, moon, or a planet rises, transits, or sets.

Object

To specify an object, click one of the objects in the Object box.

Event

To specify an event, click one of the events in the Event box.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Font dialog box

Use the Font dialog box to preview and select a font used to display an item on the screen or on a printer.

Font

To change the font, select a font name.

Font style

To change the font style of the selected font, select a style in the Font style box.

Size

To change the size of the selected font, select a size in the Size box. The sizes available depend on the printer and the selected font.

Sample

Shows how the font will look with the selected options.

Script

To specify a script, click the down arrow and select a script in the Script list.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Fonts dialog box

Use the Fonts dialog box to customize the fonts used to display items on the screen and on a printer.

Fonts

To specify whether you want to change screen or printer fonts, choose the Screen or Printer option in the Fonts box.

Sample

Shows the font used to display the item selected in the Items list.

Items

To preview, edit, or reset the font used to display an item, select the item in the Items list.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Edit

To edit the font used to display the item selected in the Items list, click Edit.

Reset

To reset the font used to display the item selected in the Items list to the font used the first time you ran CyberSky, click Reset.

Reset All

To reset the fonts used to display all items in the Items list to the fonts used the first time you ran CyberSky, click Reset All.

Help

To display help for this dialog box, click Help.


Grids dialog box

Use the Grids dialog box to show or hide coordinate system grid lines and grid labels.

Grids

To show or hide all grids, click Grids.

Equatorial

To show or hide the equatorial coordinate system grid, click Equatorial.

Horizontal

To show or hide the horizontal coordinate system grid, click Horizontal.

Don't show any labels

To hide labels, click Don't Show Any Labels

Equatorial and horizontal

To show or hide both equatorial and horizontal grid labels, click Equatorial and Horizontal.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Julian Date dialog box

Use the Julian Date dialog box to change the time by entering a Julian date.

Julian Date

To specify a Julian date, enter a number into the Julian date box. CyberSky's limits of 4,000 BC to 4,000 AD correspond to Julian dates 260,423.5 and 3,182,029.5, respectively.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Lines dialog box

Use the Lines dialog box to show or hide reference lines.

Lines

To show or hide all lines, click Lines.

Meridian

To show or hide the meridian, click Meridian.

Ecliptic

To show or hide the ecliptic, click Ecliptic.

Celestial equator

To show or hide the celestial equator, click Celestial Equator.

Galactic equator

To show or hide the galactic equator, click Galactic Equator.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Local Time dialog box

Use the Local Time dialog box to change the local date and time.

Date

Specify the local date. October 4, 1582 AD and earlier dates are on the Julian calendar. October 15, 1582 AD and later dates are on the Gregorian calendar.

Time

Specify the local time.

Daylight saving time

To specify whether daylight saving time is in effect, click the Daylight Saving Time option.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Location dialog box

Use the Location dialog box to specify the location on the earth from which you want to view the sky.

Name

To specify the location on the earth from which you want to view the sky, click the down arrow, and then click the name of the location.

Location name containing text

To specify what text to search for, enter it in the box.

Find Next

To search for the next location containing the text in the box to the left, click Find Next.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Edit

To edit the longitude, latitude, and time zone for the location selected in the Name list to the right, click Edit.

Add

To add a location to the location database, click Add.

Delete

To delete a location from the location database, click Delete. Use this option with caution! Once you've deleted a location, the only way to restore it is to reinstall CyberSky.

Help

To display help for this dialog box, click Help.


Moon Data dialog box

Use the Moon Data dialog box to display detailed information about the moon when you click on it with the right mouse button. This information includes the moon's equatorial, ecliptic, and horizontal coordinates, distance, elongation, angular diameter, magnitude, constellation, rising, transit, and setting times, disk, and illumination.

moon

equatorial coordinate system

right ascension

declination

ecliptic coordinate system

ecliptic longitude

ecliptic latitude

horizontal coordinate system

azimuth

altitude

distance

elongation

angular diameter

magnitude

constellation

disk

illumination

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Moon Report dialog box

Use the Moon Report dialog box to display detailed information about the moon, even when it is not visible in the sky display. This information includes the moon's equatorial coordinates, constellation, distance, angular diameter, magnitude, rising, transit, and setting times, disk, and illumination.

moon

right ascension

declination

constellation

distance

angular diameter

magnitude

disk

illumination

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Orientation dialog box

Use the Orientation dialog box to specify which horizon is positioned at the bottom of the display.

Horizon

To specify which horizon is positioned at the bottom of the display, click one of the available cardinal points.

Don't show any labels

To hide orientation labels, click Don't Show Any Labels.

Abbreviations

To show abbreviations for the cardinal points, click Abbreviations.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Planet Data dialog box

Use the Planet Data dialog box to display detailed information about a planet when you click on it with the right mouse button. This information includes the planet's equatorial, ecliptic, and horizontal coordinates, distance, elongation, angular diameter, magnitude, constellation, rising, transit, and setting times, disk, and illumination.

planet

equatorial coordinate system

right ascension

declination

ecliptic coordinate system

ecliptic longitude

ecliptic latitude

horizontal coordinate system

azimuth

altitude

distance

elongation

angular diameter

magnitude

constellation

disk

illumination

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Planets Report dialog box

Use the Planets Report dialog box to display detailed information about all eight planets at once. This information includes the equatorial coordinates, constellation, distance, angular diameter, magnitude, rising, transit, and setting times for each planet.

planet

right ascension

declination

constellation

distance

angular diameter

magnitude

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Points dialog box

Use the Points dialog box to show or hide reference points.

Poles

To show or hide all poles, click Poles.

Ecliptic

To show or hide the north and south ecliptic poles, click Ecliptic.

Celestial

To show or hide the north and south celestial poles, click Celestial.

Galactic

To show or hide the north and south galactic poles, click Galactic.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Print dialog box

Use the Print dialog box to specify printing options such as the printing quality and the number of copies you want printed.


Print Setup dialog box

Use the Print Setup dialog box to specify printing options such as which printer you want to use, the page orientation, and the paper size and source.


Sky dialog box

Use the Sky dialog box to specify sky options.

Use daytime color during daytime

To specify whether you want the "Sky, Daytime" color to be used during daytime, click this option.

Use twilight colors during twilight

To specify whether you want the "Sky, Twilight, Civil", "Sky, Twilight, Nautical", and "Sky, Twilight, Astronomical" colors to be used during civil, nautical, and astronomical twilight, click this option.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Solar System Objects dialog box

Use the Solar System Objects dialog box to show or hide solar system objects and object labels.

Solar system objects

To show or hide all solar system objects, click Solar System Objects.

Sun

To show or hide the sun, click Sun.

Moon

To show or hide the moon, click Moon.

Planets

To show or hide all planets, click Planets.

Visible without optical aid

To show or hide all planets visible without optical aid, click Visible Without Optical Aid.

Mercury

To show or hide Mercury, click Mercury.

Venus

To show or hide Venus, click Venus.

Mars

To show or hide Mars, click Mars.

Jupiter

To show or hide Jupiter, click Jupiter.

Saturn

To show or hide Saturn, click Saturn.

Visible with optical aid

To show or hide all planets visible with optical aid, click Visible With Optical Aid.

Uranus

To show or hide Uranus, click Uranus.

Neptune

To show or hide Neptune, click Neptune.

Pluto

To show or hide Pluto, click Pluto.

Don't show any labels

To hide labels, click Don't Show Any Labels.

Names

To show or hide solar system object names such as "Mars", click Names.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Star Data dialog box

Use the Star Data dialog box to display detailed information about any star in the display when you click on it with the right mouse button. This information includes the star's proper name, Bayer and Flamsteed designations, equatorial and horizontal coordinates, distance, magnitude, constellation, rising, transit, and setting times, spectral type, and luminosity class.

star

proper name

Bayer letter

Flamsteed number

equatorial coordinate system

right ascension

declination

horizontal coordinate system

azimuth

altitude

distance

magnitude

constellation

spectral types

luminosity classes

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Stars dialog box

Use the Stars dialog box to show or hide stars and star labels.

Stars

To show or hide stars, click Stars.

Limit

Move the slider left to include fewer stars, or right to include more stars.

Colors based on spectral types

To display stars in colors based on their spectral types, click Colors Based on Spectral Types.

Don't show any labels

To hide star labels, click Don't Show Any Labels.

Proper names

To show proper names such as "Antares", click Proper Names.

Bayer letters

To show Bayer letters such as "α", click Bayer Letters.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Step Size dialog box

Use the Step Size dialog box to specify the step size between animation frames.

Step size

To specify the step size, click one of the options in the Step size box.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Sun Data dialog box

Use the Sun Data dialog box to display detailed information about the sun when you click on it with the right mouse button. This information includes the sun's equatorial, ecliptic, and horizontal coordinates, distance, angular diameter, magnitude, constellation, rising, transit, and setting times, spectral type, and luminosity class.

sun

equatorial coordinate system

right ascension

declination

ecliptic coordinate system

ecliptic longitude

ecliptic latitude

horizontal coordinate system

azimuth

altitude

distance

angular diameter

magnitude

constellation

spectral types

luminosity classes

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Sun Report dialog box

Use the Sun Report dialog box to display detailed information about the sun, even when it is not visible in the sky display. This information includes the sun's equatorial coordinates, constellation, distance, angular diameter, magnitude, and the times at which astronomical, nautical, and civil twilight begin and end.

sun

right ascension

declination

constellation

distance

angular diameter

magnitude

astronomical twilight

nautical twilight

civil twilight

OK

To close this dialog box, click OK.

Help

To display help for this dialog box, click Help.


Universal Time dialog box

Use the Universal Time dialog box to change the universal date and time.

Date

Specify the universal date. October 4, 1582 AD and earlier dates are on the Julian calendar. October 15, 1582 AD and later dates are on the Gregorian calendar.

Time

Specify the universal time.

OK

To close this dialog box and save any changes you've made, click OK.

Cancel

To close this dialog box without saving any changes you've made, click Cancel.

Help

To display help for this dialog box, click Help.


Constellations
Constellations listed alphabetically

Click the first letter of the constellation you want to look up.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

This list contains all 88 constellations arranged alphabetically.

A

Andromeda

Antlia

Apus

Aquarius

Aquila

Ara

Aries

Auriga

B

Bootes

C

Caelum

Camelopardalis

Cancer

Canes Venatici

Canis Major

Canis Minor

Capricornus

Carina

Cassiopeia

Centaurus

Cepheus

Cetus

Chamaeleon

Circinus

Columba

Coma Berenices

Corona Australis

Corona Borealis

Corvus

Crater

Crux

Cygnus

D

Delphinus

Dorado

Draco

E

Equuleus

Eridanus

F

Fornax

G

Gemini

Grus

H

Hercules

Horologium

Hydra

Hydrus

I

Indus

L

Lacerta

Leo

Leo Minor

Lepus

Libra

Lupus

Lynx

Lyra

M

Mensa

Microscopium

Monoceros

Musca

N

Norma

O

Octans

Ophiuchus

Orion

P

Pavo

Pegasus

Perseus

Phoenix

Pictor

Pisces

Piscis Austrinus

Puppis

Pyxis

R

Reticulum

S

Sagitta

Sagittarius

Scorpius

Sculptor

Scutum

Serpens

Sextans

T

Taurus

Telescopium

Triangulum

Triangulum Australe

Tucana

U

Ursa Major

Ursa Minor

V

Vela

Virgo

Volans

Vulpecula


Constellations
Constellations listed by category

Click the first letter of the constellation you want to look up.

Arthropods | Birds | Fish | Geographical Features | Mammals | Monsters | People | Reptiles | Ship | Tools

This list contains all 88 constellations arranged by the months during which they are best seen at 2100 local time.

Arthropods

Cancer

Musca

Scorpius

Birds

Apus

Aquila

Columba

Corvus

Cygnus

Grus

Pavo

Phoenix

Tucana

Fish

Dorado

Pisces

Piscis Austrinus

Volans

Geographical Features

Eridanus

Mensa

Mammals

Aries

Camelopardalis

Canes Venatici

Canis Major

Canis Minor

Cetus

Delphinus

Equuleus

Leo

Leo Minor

Lepus

Lupus

Lynx

Taurus

Ursa Major

Ursa Minor

Vulpecula

Monsters

Capricornus

Centaurus

Draco

Hydra

Monoceros

Pegasus

Sagittarius

People

Andromeda

Aquarius

Auriga

Bootes

Cassiopeia

Cepheus

Coma Berenices

Gemini

Hercules

Indus

Ophiuchus

Orion

Perseus

Pictor

Sculptor

Virgo

Reptiles

Chamaeleon

Hydrus

Lacerta

Serpens

Ship

Carina

Puppis

Pyxis

Vela

Tools

Antlia

Ara

Caelum

Circinus

Corona Australis

Corona Borealis

Crater

Crux

Fornax

Horologium

Libra

Lyra

Microscopium

Norma

Octans

Reticulum

Sagitta

Scutum

Sextans

Telescopium

Triangulum

Triangulum Australe


Constellations
Constellations listed by month

Click the first letter of the constellation you want to look up.

January | February | March | April | May | June | July | August | September | October | November | December

This list contains all 88 constellations arranged by the months during which they are best seen at 2100 local time.

January

Caelum

Dorado

Mensa

Orion

Reticulum

Taurus

February

Auriga

Camelopardalis

Canis Major

Columba

Gemini

Lepus

Monoceros

Pictor

March

Cancer

Canis Minor

Carina

Lynx

Puppis

Pyxis

Vela

Volans

April

Antlia

Chamaeleon

Crater

Hydra

Leo

Leo Minor

Sextans

Ursa Major

May

Canes Venatici

Centaurus

Coma Berenices

Corvus

Crux

Musca

Virgo

June

Bootes

Circinus

Libra

Lupus

Ursa Minor

July

Apus

Ara

Corona Borealis

Draco

Hercules

Norma

Ophiuchus

Scorpius

Serpens

Triangulum Australe

August

Corona Australis

Lyra

Sagittarius

Scutum

Telescopium

September

Aquila

Capricornus

Cygnus

Delphinus

Equuleus

Indus

Microscopium

Pavo

Sagitta

Vulpecula

October

Aquarius

Cepheus

Grus

Lacerta

Octans

Pegasus

Piscis Austrinus

November

Andromeda

Cassiopeia

Phoenix

Pisces

Sculptor

Tucana

December

Aries

Cetus

Eridanus

Fornax

Horologium

Hydrus

Perseus

Triangulum


Andromeda

Names

Abbreviation	And

Latin name	Andromeda

Genitive form	Andromedae

English name	Andromeda, the Princess of Ethiopia

German name	Andromeda

Position

Right ascension	1 hour

Declination	+40 degrees

Area	722 square degrees

Zodiacal	NO

Best seen	November

Named stars

Alpheratz	21 And	α And

Mirach	43 And	β And

Almach	57 And	γ1 And

History and mythology

Princess Andromeda was the daughter of King Cepheus and Queen Cassiopeia.

Cassiopeia boasted that she and her daughter were more beautiful than any of the sea nymphs known as the Nereids. This offended Poseidon, the god of the sea, and as punishment Andromeda was chained to a rock by the seashore to await being devoured by the sea monster represented by the constellation Cetus.

The hero Perseus beheaded Medusa, and used her head as a weapon against the sea monster. Medusa was a gorgon, a creature so hideous that anyone who looked at her was turned to stone. When Cetus saw the head of Medusa he turned to stone and Andromeda was spared. After rescuing Andromeda from the sea monster, Perseus married her and presented the gorgon's head to the goddess Athena.


Antlia

Unavailable in the shareware version of CyberSky.


Apus

Unavailable in the shareware version of CyberSky.


Aquarius

Unavailable in the shareware version of CyberSky.


Aquila

Unavailable in the shareware version of CyberSky.


Ara

Unavailable in the shareware version of CyberSky.


Aries

Unavailable in the shareware version of CyberSky.


Auriga

Unavailable in the shareware version of CyberSky.


Bootes

Unavailable in the shareware version of CyberSky.


Caelum

Unavailable in the shareware version of CyberSky.


Camelopardalis

Unavailable in the shareware version of CyberSky.


Cancer

Unavailable in the shareware version of CyberSky.


Canes Venatici

Unavailable in the shareware version of CyberSky.


Canis Major

Unavailable in the shareware version of CyberSky.


Canis Minor

Unavailable in the shareware version of CyberSky.


Capricornus

Unavailable in the shareware version of CyberSky.


Carina

Unavailable in the shareware version of CyberSky.


Cassiopeia

Unavailable in the shareware version of CyberSky.


Centaurus

Unavailable in the shareware version of CyberSky.


Cepheus

Unavailable in the shareware version of CyberSky.


Cetus

Unavailable in the shareware version of CyberSky.


Chamaeleon

Unavailable in the shareware version of CyberSky.


Circinus

Unavailable in the shareware version of CyberSky.


Columba

Unavailable in the shareware version of CyberSky.


Coma Berenices

Unavailable in the shareware version of CyberSky.


Corona Australis

Unavailable in the shareware version of CyberSky.


Corona Borealis

Unavailable in the shareware version of CyberSky.


Corvus

Unavailable in the shareware version of CyberSky.


Crater

Unavailable in the shareware version of CyberSky.


Crux

Unavailable in the shareware version of CyberSky.


Cygnus

Unavailable in the shareware version of CyberSky.


Delphinus

Unavailable in the shareware version of CyberSky.


Dorado

Unavailable in the shareware version of CyberSky.


Draco

Unavailable in the shareware version of CyberSky.


Equuleus

Unavailable in the shareware version of CyberSky.


Eridanus

Unavailable in the shareware version of CyberSky.


Fornax

Unavailable in the shareware version of CyberSky.


Gemini

Unavailable in the shareware version of CyberSky.


Grus

Unavailable in the shareware version of CyberSky.


Hercules

Unavailable in the shareware version of CyberSky.


Horologium

Unavailable in the shareware version of CyberSky.


Hydra

Unavailable in the shareware version of CyberSky.


Hydrus

Unavailable in the shareware version of CyberSky.


Indus

Unavailable in the shareware version of CyberSky.


Lacerta

Unavailable in the shareware version of CyberSky.


Leo

Unavailable in the shareware version of CyberSky.


Leo Minor

Unavailable in the shareware version of CyberSky.


Lepus

Unavailable in the shareware version of CyberSky.


Libra

Unavailable in the shareware version of CyberSky.


Lupus

Unavailable in the shareware version of CyberSky.


Lynx

Unavailable in the shareware version of CyberSky.


Lyra

Unavailable in the shareware version of CyberSky.


Mensa

Unavailable in the shareware version of CyberSky.


Microscopium

Unavailable in the shareware version of CyberSky.


Monoceros

Unavailable in the shareware version of CyberSky.


Musca

Unavailable in the shareware version of CyberSky.


Norma

Unavailable in the shareware version of CyberSky.


Octans

Unavailable in the shareware version of CyberSky.


Ophiuchus

Unavailable in the shareware version of CyberSky.


Orion

Unavailable in the shareware version of CyberSky.


Pavo

Unavailable in the shareware version of CyberSky.


Pegasus

Unavailable in the shareware version of CyberSky.


Perseus

Unavailable in the shareware version of CyberSky.


Phoenix

Unavailable in the shareware version of CyberSky.


Pictor

Unavailable in the shareware version of CyberSky.


Pisces

Unavailable in the shareware version of CyberSky.


Picsis Austrinus

Unavailable in the shareware version of CyberSky.


Puppis

Unavailable in the shareware version of CyberSky.


Pyxis

Unavailable in the shareware version of CyberSky.


Reticulum

Unavailable in the shareware version of CyberSky.


Sagitta

Unavailable in the shareware version of CyberSky.


Sagittarius

Unavailable in the shareware version of CyberSky.


Scorpius

Unavailable in the shareware version of CyberSky.


Sculptor

Unavailable in the shareware version of CyberSky.


Scutum

Unavailable in the shareware version of CyberSky.


Serpens

Unavailable in the shareware version of CyberSky.


Sextans

Unavailable in the shareware version of CyberSky.


Taurus

Unavailable in the shareware version of CyberSky.


Telescopium

Unavailable in the shareware version of CyberSky.


Triangulum

Unavailable in the shareware version of CyberSky.


Triangulum Australe

Unavailable in the shareware version of CyberSky.


Tucana

Unavailable in the shareware version of CyberSky.


Ursa Major

Unavailable in the shareware version of CyberSky.


Ursa Minor

Unavailable in the shareware version of CyberSky.


Vela

Unavailable in the shareware version of CyberSky.


Virgo

Unavailable in the shareware version of CyberSky.


Volans

Unavailable in the shareware version of CyberSky.


Vulpecula

Unavailable in the shareware version of CyberSky.


Sun

Overview | Interior | Surface | Atmosphere | Properties

Overview

The sun, a typical star of spectral type G2 and luminosity class V, is one of several hundreds of billions of stars that make up our galaxy, a spiral galaxy called the Milky Way. The sun and the rest of our solar system are located on the inner edge of a spiral arm, about 8,500 parsecs (28,000 light years) from the center of the Milky Way. The sun revolves around the galactic center once every 240 million years.

The sun is also the center of our solar system. The planets, asteroids, comets, and meteoroids all revolve around it. The solar system formed from an immense cloud of interstellar gas and dust about five billion years ago. The sun contains about 99.86 percent of all the mass in the solar system. The remaining 0.14 percent formed the other bodies of the solar system.

Like all stars, the sun is a globe of incandescent gas. Analysis of the spectrum of light we receive from the sun allows us to determine that the it is made of about 92 percent hydrogen, 7 percent helium, and 1 percent other gases such as carbon, oxygen, nitrogen, and neon. In fact, helium was discovered in the atmosphere of the sun before it was discovered here on the earth.

Computer models indicate that the sun has been active for about five billion years, and will remain so for another five billion years. This value for the sun's age agrees with the ages measured for the planets, and suggests that the sun and its planetary system formed at the same time.

Interior

The sun's energy, which is necessary for life on this planet, originates in the sun's core. The temperature at the core is about 27 million Celsius (15 million Fahrenheit). At such high temperatures the nuclei of hydrogen atoms move at over 1000 kilometers (620 miles) per second and undergo many collisions with other nuclei. A very small percentage of these nuclei take part in reactions that result in four hydrogen nuclei fusing together to form one helium nucleus. The helium nucleus is 0.71 percent lighter than the four hydrogen nuclei, and this excess mass is converted into energy in the form of gamma rays.

The sun is in a state of equilibrium, meaning that for any given piece of the sun the pressure of the gases below just balance the weight of the gases above. To maintain its equilibrium, the sun must convert 600 million tons of hydrogen into helium every second, meaning that 4 million tons of mass are converted into energy every second.

Surrounding the core is a layer in which the energy generated in the core is transported outward by way of radiation. Surrounding this zone of radiative transport is another layer in which convection replaces radiation as the means by which energy moves to the surface. In this layer, warmer columns of solar material, called convective cells, move upward while cooler columns move downward. Such convective cells are typically 1000 kilometers (620 miles) in diameter, and are usually visible for five or 10 minutes before sinking below the photosphere. The energy generated in the sun's core spends over a million years traveling through the sun, first by radiation, then by convection, until it escapes into space.

The sun will not continue to shine indefinitely. When all of the hydrogen in the sun's core has been used up it will shrink and increase its temperature, while the sun's atmosphere will expand and the photosphere will become cooler. The conversion of hydrogen into helium will begin in a thin shell surrounding the core, causing the core to shrink and the outer layers of the sun to expand. After a period of adjustment the sun will continue to live as a red giant star for another billion years. Its atmosphere will expand until it swallows the inner planets. Finally, it will blow its atmosphere into space and the exposed core will remain as a type of star known as a white dwarf. Such is the fate of a star such as the sun. Stars that are much more massive than the sun end their lives as neutron stars or black holes.

Surface

The sun's "surface", called it's photosphere, is the innermost layer of the sun that is opaque to visible light. The photosphere is only about 400 kilometers (250 miles) thick. It has a granulated appearance, due to the turbulent eruptions of convective cells when they reach the surface. The temperature in the photosphere is about 5,500 Celsius (9,950 Fahrenheit).

Sunspots are associated with strong magnetic fields in the photosphere. These fields disrupt the movement of convective cells and therefore cool a small region of the photosphere. Such regions are still very hot, having a temperature of about 4,000 Celsius (7,200 Fahrenheit), but appear darker in comparison to their surroundings. Most sunspots, which are irregular in shape and are typically about 10,000 kilometers (6,200 miles) in size, form and then disappear in only a week. But, larger ones may last for a month or longer, and can be observed to disappear from one limb only to reappear again a couple of weeks later on the opposite limb.

The number of sunspots on the photosphere varies with time, in cycles of roughly 11 years. The equatorial regions of the sun rotate in about 25 days, slightly faster than the polar regions which rotate in 30 days. Deeper regions of the sun appear to rotate in 27 days. It is this differential rotation that causes the subsurface magnetic field to build up and eventually burst through the photosphere, disrupting the normal flow of solar energy. Only a few sunspots may be observed at the minimum of the solar cycle, while more than 100 may be observed at maximum. The formation of so many sunspots releases the built-up magnetic energy, allowing another cycle to begin.

Atmosphere

The region above the sun's photosphere is called the chromosphere. The chromosphere extends about 10,000 kilometers (6,200 miles) above the photosphere. It is here where absorption lines are produced in the sun's spectrum.

Flares are short-lived phenomena that arise in the chromosphere. They are filaments of hot gas that emerge from regions that contain sunspots. Flares typically shoot away from the sun at about 200 kilometers (120 miles) per second. Energetic flares can produce aurorae in our atmosphere and can interfere with terrestrial communication.

The outer region of the sun, extended far above the chromosphere, is the corona. The corona is the halo of light that appears around the sun during a total solar eclipse. The corona's density is very low, but its effective temperature is very high, well over 1,000,000 Celsius (1,800,000 Fahrenheit).

Prominences arise in the corona. Prominences are immense arcs of ionized gas that streak out into space above the surface of the sun. Such prominences occur when the ionized gas escapes the confines of the sun's low-level magnetic field.

The particles that make up the corona escape the sun, continuously flowing outward through the solar system. This outflow, known as the solar wind, interacts with the magnetic fields of planets and is also responsible for the light displays we call aurorae.

Properties

Distance from the earth	147.0 to 152.0 million kilometers

91.34 to 94.45 million miles

0.983 to 1.016 astronomical units

Absolute magnitude	4.8

Diameter	1,392,000 kilometers

864,900 miles

109.1 times that of the earth

Rotation period	25.04 days

Average density	1.41 grams per cubic centimeter

Surface gravity	27.90 times that of the earth

Surface temperature	5,780 K

5,500 C

9,950 F

Escape velocity	618 kilometers per second

384 miles per second


Moon

Unavailable in the shareware version of CyberSky.


Mercury

Unavailable in the shareware version of CyberSky.


Venus

Unavailable in the shareware version of CyberSky.


Earth

Unavailable in the shareware version of CyberSky.


Mars

Unavailable in the shareware version of CyberSky.


Jupiter

Unavailable in the shareware version of CyberSky.


Saturn

Unavailable in the shareware version of CyberSky.


Uranus

Unavailable in the shareware version of CyberSky.


Neptune

Unavailable in the shareware version of CyberSky.


Pluto

Unavailable in the shareware version of CyberSky.


Solar and sidereal time

This diagram illustrates the difference between solar and sidereal time.

At point A the sun and a distant star lie in the same direction. As the earth moves along its orbit from point A to point B, it rotates exactly once with respect to the star. At point B the star has returned to the same position in the sky it had at point A. But, because the earth has moved along its orbit by approximately one degree, the sun is now located at a different position in the sky. As the earth moves along its orbit from point B to point C it rotates approximately another degree bringing the sun back to the same position it had at point A.

It takes the earth 23 hours, 56 minutes, and 4 seconds to move from point A to point B, and 24 hours to move from point A to point C. These intervals are known as a sidereal day and a solar day, respectively.


Parallax

This diagram illustrates the geometry responsible for parallax, the angular displacement in the apparent position of a celestial body when observed from two widely separated points.

Hold your thumb at arm's length in front of your face, and then look at your thumb first with one eye, and then with the other. As you switch from one eye to the other you'll see your thumb's position shift in relation to objects that are farther away. Repeat this experiment, but hold you thumb much closer to your face. You'll see that the shift in your thumb's position will be larger than it was before.

This technique is very useful, because it allows us to measure the distances of nearby stars. The position of a star is measured very precisely at one time, and then this measurement is repeated half a year later when the earth has moved to the other side of its orbit. The star's position will shift in relation to stars that are much farther away, and the amount of the shift allows the distance of the star to be calculated.

A star's parallax, p, is equal to half its shift in position, and is usually measured in arc seconds. Once this value is known the star's distance in parsecs, D, can be calculated using the formula D = 1 / p. For example, if a star's parallax is measured to be 0.5 arc seconds, then we calculate its distance to be 2 parsecs, or about 6.5 light years.


Planetary configurations

This diagram illustrates the terminology used to describe planetary configurations.

The smallest circle represents the orbit of an inferior planet such as Mercury or Venus. The middle circle represents the orbit of the earth. The largest circle represents the orbit of a superior planet such as Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto.


Retrograde motion

This diagram illustrates the reason for the retrograde motion of a superior planet such as Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto.

The earth moves faster along its orbit than the superior planet moves along its orbit. The superior planet appears to slow down in the sky and reverses its direction near opposition. This temporary east to west motion is referred to as retrograde motion. After opposition the superior planet again slows down and reverses its direction, moving west to east as it did initially. This west to east motion is referred to as prograde or direct motion.

The inferior planets Mercury and Venus also exhibit retrograde motion, but in their case the reason is that they move faster along their orbits than the earth moves along its orbit.


Greek alphabet

The lowercase letters of the Greek alphabet are used in the system of stellar nomenclature devised by Johann Bayer in 1603. The brightest star in a given constellation is designed α, the second brightest β, the third brightest γ, and so on.

Number	Uppercase	Lowercase	Name

 1	Α	α	alpha

 2	Β	β	beta

 3	Γ	γ	gamma

 4	Δ	δ	delta

 5	Ε	ε	epsilon

 6	Ζ	ζ	zeta

 7	Η	η	eta

 8	Θ	θ	theta

 9	Ι	ι	iota

10	Κ	κ	kappa

11	Λ	λ	lambda

12	Μ	μ	mu

13	Ν	ν	nu

14	Ξ	ξ	xi

15	Ο	ο	omicron

16	Π	π	pi

17	Ρ	ρ	rho

18	Σ	σ	sigma

19	Τ	τ	tau

20	Υ	υ	upsilon

21	Φ	φ	phi

22	Χ	χ	chi

23	Ψ	ψ	psi

24	Ω	ω	omega


Magnitude scale

Magnitudes are used in astronomy for historical reasons. In ancient times, 129 BC to be exact, the Greek astronomer Hipparchus ranked the stars in six classes of magnitude when he produced the first well-known star catalog. The brightest stars were assigned to the first magnitude and the faintest stars just visible to the naked eye were assigned to the sixth magnitude.

More recently, this system had to be expanded to cover stars brighter than first magnitude and fainter than sixth magnitude. The following table lists some typical magnitude values.

Object	Magnitude

Sun	-26.7

Full moon	-12.5

Venus at brightest	- 4.4

Mars and Jupiter at brightest	- 2.8

Sirius, brightest star other than the sun	- 1.5

Arcturus, Capella, Vega	  0.0

Saturn at brightest	  0.2

Aldebaran, Antares, Betelgeuse	  1.0

Polaris	  2.0

Faintest star seen in city	  5.0

Uranus	  5.6

Faintest star seen in perfect conditions	  6.0

Neptune	  8.2

Pluto	 13.7


Brightness ratios

In 1856 the English astronomer Norman R Pogson proposed that a difference of five magnitudes correspond exactly to a brightness ratio of 100 to 1. The brightness ratio of two stars that differ by one magnitude is the fifth-root of 100, or 2.512, a quantity known as the Pogson ratio. The following table lists some brightness ratios for given magnitude differences.

Magnitude difference	Brightness ratio

 0	        1.00

 0.1	        1.10

 0.2	        1.20

 0.3	        1.32

 0.4	        1.45

 0.5	        1.58

 0.6	        1.74

 0.7	        1.91

 0.8	        2.09

 0.9	        2.29

 1	        2.51

 1.5	        3.98

 2	        6.31

 2.5	       10.0

 3	       15.9

 3.5	       25.1

 4	       39.8

 4.5	       63.1

 5	      100

 6	      251

 7	      631

 7.5	    1,000

10	   10,000

15	1,000,000


Brightest stars

Bayer designations, proper names, and magnitudes of the twenty-five brightest stars in the sky.

Number	Bayer designation	Proper name	Magnitude

 1	α Canis Majoris	Sirius	-1.46

 2	α Carinae	Canopus	-0.72

 3	α Bootis	Arcturus	-0.04

 4	α Centauri	Rigelkent	-0.01

 5	α Lyrae	Vega	 0.03

 6	α Aurigae	Capella	 0.08

 7	β Orionis	Rigel	 0.12

 8	α Canis Minoris	Procyon	 0.38

 9	α Eridani	Achernar	 0.46

10	α Orionis	Betelgeuse	 0.50

11	β Centauri	Hadar	 0.61

12	α Aquilae	Altair	 0.77

13	α Tauri	Aldebaran	 0.85

14	α Scorpii	Antares	 0.96

15	α Virginis	Spica	 0.98

16	β Geminorum	Pollux	 1.14

17	α Piscis Austrini	Fomalhaut	 1.16

18	β Crucis	Becrux	 1.25

19	α Cygni	Deneb	 1.25

20	α Crucis	Acrux	 1.33

21	α Leonis	Regulus	 1.35

22	ε Canis Majoris	Adhara	 1.50

23	λ Scorpii	Shaula	 1.63

24	γ Orionis	Bellatrix	 1.64

25	β Tauri	Alnath	 1.65


Spectral types

O | B | A | F | G | K | M

Stars are classified into types based on the presence, absence, and relative strengths of absorption and emission lines in their spectra. These spectral features are dependent on the temperatures and the spectral sequence is therefore a sequence that orders stars by their temperatures.

This classification scheme was developed by Annie Jump Cannon (1863-1941) and her colleagues at Harvard Observatory and was internationally adopted in 1910.

O

Temperature	25,000 - 50,000 K

Color	Light Sky Blue

Percentage of stars	0.1

Examples	Mintaka (δ Ori)

Alnitak (ζ Ori)

B

Temperature	11,000 - 25,000 K

Color	Light Cyan

Percentage of stars	0.1

Examples	Rigel (β Ori)

Regulus (α Leo)

A

Temperature	7,500 - 11,000 K

Color	White

Percentage of stars	1

Examples	Vega (α Lyr)

Sirius (α CMa)

F

Temperature	6,000 - 7,500 K

Color	Pale Yellow

Percentage of stars	2

Examples	Procyon (α CMi)

Polaris (α UMi)

G

Temperature	5,000 - 6,000 K

Color	Gold

Percentage of stars	4

Examples	Sun

Capella (α Aur)

K

Temperature	3,500 - 5,000 K

Color	Orange

Percentage of stars	14

Examples	Arcturus (α Boo)

Aldebaran (α Tau)

M

Temperature	2,000 - 3,500 K

Color	Coral

Percentage of stars	72

Examples	Betelgeuse (α Ori)

Antares (α Sco)


Luminosity classes

Luminosity classes are Roman numerals used to express the size of stars. Small stars have dense atmospheres that exhibit high gas pressures, whereas large stars have immense, extended atmospheres that exhibit low gas pressures.

The characteristics of absorption lines and bands in the spectra of stars are highly dependent on atmospheric pressure, and therefore are an indication of a star's size. These characteristics are studied to determine the luminosity class of a given star.

William W Morgan and Philip C Keenan introduced what is now known as the M-K luminosity classification scheme in 1937.

Ia	Bright supergiants

Ib	Supergiants

II	Bright giants

III	Giants

IV	Subgiants

V	Dwarfs

VI	Subdwarfs

VII	White dwarfs

Stars within each group can be further differentiated by the addition of suffixes such as "a", "ab", and "b". In addition, a star that cannot be classified with certainty may be assigned a hypenated classification such as "IIIa-IIIb".

CyberSky displays luminosity classes that have been simplified. For example, a classification such as "IIIa-IIIb" is displayed simply as "III". To view such information for a given star, click on it with the right mouse button.


Temperature scales

Kelvin	Celsius	Fahrenheit

 373	 100	 212	water boiling point

 273	   0	  32	water freezing point

   0	-273	-460	absolute zero

To convert	Do this

Kelvin to Celsius	Subtract 273

Fahrenheit to Celsius	Subtract 32, then divide by 1.8

Celsius to Fahrenheit	Multiply by 1.8, then add 32


Conversion factors

1 inch	2.54 centimeters

1 foot	12 inches

30.48 centimeters

1 yard	3 feet

91.44 centimeters

1 meter	39.37 inches

3.281 feet

1.0936 yards

1 kilometer	0.6215 miles

1 mile	1.609 kilometers

1 astronomical unit	149,597,870 kilometers

92,955,807 miles

1 light year	9,500,000,000,000 kilometers

6,000,000,000,000 miles

63,240 astronomical units

0.307 parsecs

1 parsec	9,500,000,000,000 kilometers

6,000,000,000,000 miles

3.26 light years

206,265 astronomical units


Glossary

Click the first letter of the word you want to look up.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

aberration

absolute magnitude

absolute zero

albedo

almanac

altitude

analemma

Ångstrom

angular diameter

angular measure

annular eclipse

aphelion

apogee

apparent magnitude

apparent solar time

asterism

asteroid

asteroid belt

astrology

astrometry

astronomical twilight

astronomical unit

astronomy

astrophysics

atmosphere

atmospheric refraction

aurora

autumnal equinox

axial inclination

axis

azimuth

B

Bayer letter

binary star

black hole

bollide

C

caldera

cardinal points

celestial equator

celestial mechanics

celestial meridian

celestial poles

celestial sphere

Celsius scale

chromosphere

circumpolar

civil twilight

color index

coma

comet

conjunction

constellation

core

corona

cosmic rays

cosmogeny

cosmology

cosmos

crater

crescent

cusps

D

dark cloud

dark nebula

daylight saving time

declination

density

diffuse nebula

direct motion

disk

distance

diurnal motion

double star

dwarf

E

east point

eccentricity

eclipse

ecliptic

ecliptic coordinate system

ecliptic latitude

ecliptic longitude

ellipse

elliptical galaxy

elongation

emission nebula

ephemeris

epoch

equation of time

equatorial coordinate system

equinox

escape velocity

evening star

F

Fahrenheit scale

fireball

first quarter

Flamsteed number

full

G

galactic cluster

galactic coordinate system

galactic equator

galactic latitude

galactic longitude

galaxy

Galilean satellites

giant

gibbous

globular cluster

great circle

Gregorian calendar

H

harvest moon

horizon

horizontal coordinate system

hour angle

hour circle

hunter's moon

I

illumination

inclination

inferior conjunction

inferior planet

inner planet

intergalactic

interplanetary

interstellar

irregular galaxy

J

Julian calendar

Julian date

K

Kelvin scale

L

last quarter

latitude

light year

limb

local sidereal time

longitude

luminosity

luminosity classes

lunar eclipse

M

magnitude

main sequence

mantle

mare

mass

mean solar time

meridian

Messier catalog

meteor

meteoroid

meteorite

meteoritics

meteor shower

Milky Way

minor planet

moon

morning star

N

nadir

nautical twilight

nebula

neutron star

new

New General Catalogue

north point

North Star

nova

nuclear fusion

O

occultation

open cluster

opposition

orbit

orbital inclination

outer planet

P

parallax

parsec

partial eclipse

penumbra

perigee

perihelion

photosphere

planet

planetarium

planetary nebula

pole star

precession

prograde motion

prominence

proper motion

proper name

pulsar

Q

quadrature

quasar

R

radiant

radiation

retrograde motion

revolution

right ascension

rotation

S

satellite

semimajor axis

sidereal day

sidereal time

solar day

solar system

solar time

solstice

south point

spectral sequence

spectral types

spectrum

spiral galaxy

star

summer solstice

sun

sunspot

sunspot cycle

supergiant

superior conjunction

superior planet

supernova

supernova remnant

synodic period

T

terminator

total eclipse

transit

tropical year

U

umbra

universal time

universe

V

variable star

vernal equinox

vertical circle

W

wane

wavelength

wax

west point

white dwarf

winter solstice

Y

year

Z

zenith

zodiac

zodiacal constellation

zodiacal light


aberration

Small shift in the apparent position of a celestial object caused by the motion of the earth around the sun. The effect was discovered by the English astronomer Jamers Bradley in 1729 and used by him to prove that the earth moves through space.


absolute magnitude (M)

Magnitude of a celestial object if it was located at a standard distance of 10 parsecs or 32.6 light years from an observer. An object's apparent magnitude tells us how bright the object appears to an observer here on the earth, but we don't know how bright an object really is unless we take into account its distance from us.

The sun's absolute magnitude is about 4.8, meaning that if it were placed 10 parsecs away it would appear as an inconspicuous star in our sky. In contrast, if the star Rigel (β Orionis) were placed 10 parsecs away it would shine at magnitude -8, about as bright as a quarter moon!


absolute zero

A temperature of 0 Kelvin, -273 Celsius, or -460 Fahrenheit, where all molecular motion stops.


albedo

Measure of the reflecting power of the surface of a nonluminous body such as a planet, moon, or asteroid. An albedo of 1.0 represents a white surface that reflects all light that strikes it whereas an albedo of 0.0 represents a black surface that absorbs all light that strikes it.

The surfaces of the planet Mercury and the moon are made mostly of volcanic rock that is a poor reflector of incident light. The albedos of Mercury and the moon are 0.07 and 0.06 respectively.

The surface of the earth consists largely of water and the earth's albedo is therefore higher, about 0.36.

The planet Venus as well as the giant gas planets Jupiter, Saturn, Uranus, and Neptune are shrouded in clouds that are good reflectors of light, and these bodies all have albedos above 0.5.


almanac

A table listing the position of a celestial body at various times. Also called an ephemeris.


altitude (h)

Angular distance of a celestial object above or below the horizon. Altitude and azimuth are coordinates in the horizontal coordinate system.

Altitude is usually measured in units of degrees from 0 degrees to 90 degrees for altitudes above the horizon and from 0 degrees to -90 degrees for altitudes below the horizon.

The apparent altitude of a celestial object is affected by atmospheric refraction. CyberSky calculates this effect for objects in the sky.


analemma

Figure-eight pattern with one loop much larger than the other, depicting the sun's declination and the equation of time over the course of one year.


Ångstrom (Å)

Unit of length used in measuring the wavelength of light and other electromagnetic radiation. One Ångstrom is equal to one ten-billionth of one meter, or about the diameter of one hydrogen atom. A human hair is about 500,000 Å in diameter.


angular diameter

Observed diameter of a celestial body, measured in degrees, minutes, and seconds of arc.


angular measure

Angles are measured in degrees (°), minutes ('), and seconds (") of arc. A full circle contains 360 degrees, a degree contains 60 arc minutes, and one arc minute contains 60 arc seconds.

The diameters of the sun and moon are about one-half degree or about 30 arc minutes across. One arc minute is about the smallest angular measure that can be perceived by the best human eyes without optical aid. One arc second is roughly the angular measure of a typical coin seen from five kilometers or three miles away.


annular eclipse

An eclipse of the sun in which the angular diameter of the moon's disk is not large enough to completely hide the sun's disk.


aphelion

The point in an object's orbit around the sun at which it reaches its greatest distance from the sun. The earth is at aphelion in early January.


apogee

The point in an object's orbit around the earth at which it reaches its greatest distance from the earth.


apparent magnitude

Brightness of a celestial object, on the magnitude scale, as measured by an observer on the earth.

The apparent magnitude of a celestial object depends on its luminosity, its distance from the earth, and on the amount of light absorbed between the object and the earth.


apparent solar time

Time measured with respect to the apparent sun. Sundials display time based on the apparent sun.

The earth's orbit around the sun is not a perfect circle but is an ellipse. The earth does not move along this ellipse at constant speed, but moves fastest when it is closest to the sun and slowest when it is farthest from the sun. The apparent sun therefore moves along the ecliptic at a nonuniform rate.

The nonuniform motion of the apparent sun implies that the length of the solar day is dependent on the position of the earth in its orbit. This situation is not desirable for civil timekeeping. Ideally we want to be able to treat each day as if it was exactly 24 hours long. The mean sun, which moves along the celestial equator at a uniform rate, was invented with this goal in mind.


asterism

Distinctive group of stars that are not one of the 88 constellations recognized by the International Astronomical Union. One example is the Big Digger, which is not itself a constellation but is part of the constellation Ursa Major. The Big Dipper is known as the Plough in Great Britain, and as the Wagon in Germany.


asteroid

One of many small, rocky bodies that orbit the sun. Asteroids are larger than meteoroids, but smaller than planets. Most are found between the orbits of Mars and Jupiter, but some cross the earth's orbit and have the potential to strike our planet at some point in the future.

Asteroids are also known as minor planets.


asteroid belt

A region of asteroids found between the orbits of the planets Mars and Jupiter.


astrology

A pseudoscience that claims to link the apparent and relative positions of the sun, moon, and planets in the sky to human personalities and destinies. There is no scientific basis to astrology.


astrometry

Branch of astronomy concerned with the measurement of the precise positions, sizes, and motions of stars and other celestial bodies.


astronomical twilight

Period between the time the sun is 12 degrees and 18 degrees below the horizon.


astronomical unit (AU)

Unit of length equal to the average distance between the earth and the sun. One AU is equal to 149,597,870 kilometers or 92,955,807 miles. One AU is a long way. A typical commercial airliner would take approximately 17 years to fly from the earth to the sun!

The astronomical unit is mainly used for distances within the solar system. Pluto, the planet most distant from the sun, lies approximately 40 AU from the center of the solar system. The entire solar system is therefore about 80 AU in diameter. The nearest star outside the solar system is about 270,000 AU from the sun, or about 7,000 times the distance of Pluto!


astronomy

Science concerned with the study of the stars and all other bodies in the universe, the space between those bodies, and of the universe as a whole. Major branches of astronomy include astrometry, astrophysics, and celestial mechanics.


astrophysics

Branch of astronomy concerned with the study of the physical and chemical processes associated with celestial bodies and with the space between those bodies. Astrophysics focuses in particular on the production and expenditure of energy in stars and galaxies and seeks to understand how this activity affects the evolution of such systems.


atmosphere

The gaseous outer region of a star, planet, moon, or any other celestial body.

A celestial body can maintain an atmosphere if its escape velocity is sufficiently high. If this condition is not met, the gas molecules can escape into space and over time the body will loose its atmosphere. If the temperature is too hot the lighter gas molecules will travel faster than the planet's escape velocity and will escape into space. If the temperature is too cold the gas molecules will freeze onto the surface of the planet, which may be the case on the planet Pluto.


atmospheric refraction

Bending of the light from a celestial object as it passes through the earth's atmosphere. Atmospheric refraction bends the light toward the earth, causing celestial objects to appear slightly higher in the sky than they really are.

The effect of atmospheric refraction depends on the altitude of the object and is greatest near the horizon. The position of an object at the horizon has been shifted by about 35 arc minutes toward the zenith, meaning that the object is really 35 arc minutes below the horizon but is visible due to atmospheric refraction.

CyberSky calculates the effect of atmospheric refraction for objects in the sky.


aurora

Glowing, often moving, display of colored light seen high in the atmosphere near the north and south magnetic poles of the earth. Auroras are caused by charged particles from the sun that become trapped in the earth's magnetic field. High-altitude air molecules within about 20 degrees of the magnetic poles and approximately 100 kilometers or 60 miles high are excited by these particles and emit light.

An aurora in the northern hemisphere is called an aurora borealis, or the "Northern Lights". A similar effect in the southern hemisphere is called the aurora australis.


autumnal equinox

Time when the sun crosses the celestial equator moving from from north to south. This takes places about September 21 and marks the beginning of fall in the northern hemisphere and the beginning of spring in the southern hemisphere.


axial inclination

The angle between a body's axis of rotation and the plane of its orbit.


axis

The line about which a body rotates, or about which a property such as its magnetic field is symmetrical. These two lines do not have to coincide. For example, the axis of the earth's magnetic field is tilted about 11.4 degrees with respect to it's axis of rotation. A compass points to the magnetic north pole, not to the earth's geographic north pole!


azimuth (A)

Angular distance from the north point measured eastward to the intersection of the horizon and the object's vertical circle. Altitude and azimuth are coordinates in the horizontal coordinate system.

Azimuth is usually measured in units of degrees from 0 degrees to 360 degrees.


Bayer letter

Lower case letter of the Greek alphabet assigned to stars in each constellation.

This system of star names was introduced by Johann Bayer in 1603 in his book Uranometria that catalogs 1277 stars. He assigned the letter α (alpha) to the brightest star in a constellation, β (beta) to the second-brightest star, γ (gamma) to the third brightest, and so on. The letters are usually assigned in this order of brightness, but mistakes were made, so sometimes the star designated β is brighter than the star designated α.


binary star

Pair of stars that are revolving around a common center of gravity. Some components of binary stars are so close to each other that they complete one revolution in a fraction of a day. Others are so far apart that centuries pass before they complete one revolution.

The first such star was discovered by William Herschel (1738-1822), who proved the mutual revolution of Castor in 1803. Binary and multiple stars are common in our galaxy. Perhaps about half of all stars are part of binary and multiple star systems. Binary stars are important to astronomers because observations of their orbital motions provides the only direct way to determine the masses of stars.


black hole

Collapsed star whose gravitational field has become so strong that not even light can escape it. A black hole cannot be observed directly, but its effect on its environment can be studied.


bollide

Term used for a very bright meteor that explodes into fragments near the end of its path through the earth's atmosphere.


caldera

The "crater" at the top of a volcano.


cardinal points

The four principal points on the horizon, called the north point (0 degrees azimuth), south point (180 degrees azimuth), east point (90 degrees azimuth), and west point (270 degrees azimuth).


celestial equator

Great circle formed by projecting the earth's equatorial plane onto the celestial sphere. The celestial equator is 90 degrees from both the north and south celestial poles.

The axis of the earth's daily rotation is tilted with respect to its orbital plane, and therefore the ecliptic and celestial equator are tilted with respect to each other. This angle, known as the obliquity of the ecliptic, is currently roughly 23.5 degrees. The precession of the earth's axis causes this angle to slowly change over time.


celestial mechanics

Branch of astronomy concerned with the study of the motions of celestial bodies under the influence of the force of gravity.


celestial meridian

Projection of the observer's meridian on the celestial sphere. The observer's celestial meridian passes through both celestial poles, and passes through the observer's zenith.

The celestial meridian divides the sky into an east half and a west half. When an object in the sky is in the east half, it is rising, and when it is in the west half it is setting. When the object is on the celestial meridian, it has reached its highest point in the sky.


celestial poles

Projections of the north and south poles of the earth's axis of rotation onto the celestial sphere. The precession of the earth's axis causes the celestial poles to trace out circles on the celestial sphere in a period of about 25,800 years.

The north celestial pole is currently found near the star Polaris, better known as the North Star, or more formally as α Ursae Minoris. The south celestial pole is currently found in the constellation Octans.


celestial sphere

Sphere of immense radius centered on the earth. It provides a convenient surface on which to draw and study the motions of points in the sky as seen from an observation point on the earth. Celestial bodies may be thought of as being projected onto the surface of this immense sphere.


Celsius scale (C)

Temperature scale that sets the freezing point of water at 0 degrees and the boiling point of water at 100 degrees. Because there are 100 divisions between the freezing and boiling points of water, the size of a degree in the Celsius scale is larger than the size of a degree in the Fahrenheit scale which has 180 divisions.

The Celsius scale was invented in 1742 by Swedish astronomer Anders Celsius (1701-1744).


chromosphere

The layer of the sun's atmosphere just above the photosphere, and just below the corona.


circumpolar

Term used to describe stars that are permanently above the horizon at a given location on the earth. All stars with positive declinations are circumpolar at the north pole, and all stars with negative declinations are circumpolar at the south pole.


civil twilight

Period between the time of sunrise or sunset and the time the sun is 6 degees below the horizon.


color index

Difference between the apparent magnitude of a star measured at one standard wavelength of light and another longer wavelength. A star's color index value depends on the distribution of light across the spectrum, and is therefore a measure of the color and temperatures of the star.

Currently, colors indices are measured using the Johnson-Morgan UBV system. The apparent magnitude of a star is measured at three wavelengths, ultraviolet (U) at 3600 Ångstroms, blue (B) at 4200 Ångstroms, and greenish-yellow (V) at 5400 Ångstroms. Subtracting these values yields the U - B and B - V color indices. The more positive the color index, the redder and cooler the star is. The more negative the color index, the bluer and hotter the star is.


coma

The bright diffuse halo of a comet.


comet

Diffuse body of gas and solid particles that orbits the sun in an orbit generally more elliptical than that of planets. Most orbit the sun in very elongated orbits and appear at regular intervals. Others appear only once and then disappear into interstellar space, never to return.

Comets typically consist of three parts. The nucleus is usually about one mile across and is thought to resemble a dirty snowball. The coma surrounds the nucleus. The tail consists of gas and dust and appears only when the comet nears the sun. A comet's tail always points away from the sun.


conjunction

Moment when a planet's elongation is 0 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


constellation

Any one of the areas into which the sky is divided.

Constellations are not physical objects. They were invented to break the sky into smaller, more manageable pieces, thereby making it possible for the human mind to quickly identify the stars visible at a given time.

In ancient times constellations had no fixed limits. A constellation consisted of a group of stars that defined an outline resembling a mythological person, an animals, or a monster. The Greek astronomer Ptolemy listed only 48 constellations. In more recent times more constellations were added for the areas of the southern sky that became visible to European explorers.

It became necessary to standardize the constellations and define precise boundaries separating all constellations. Just as any city on the earth falls within the boundaries of some state, province, or country, all celestial bodies fall within the boundaries of a constellation.

The International Astronomical Union established the boundaries of 88 constellations in 1930. The constellation boundaries follow arcs of right ascension and declination for the epoch January 1, 1875.


core

The central region of a star, planet, moon, or any other celestial body.


corona

The outer atmosphere of the sun, only visible at the time of a total solar eclipse. The sun's corona has a temperature of about 2 million K.


cosmic rays

Highly energetic particles from interstellar space, moving at close to the speed of light, that bombard the earth from all directions.

Cosmic rays were discovered by VF Hess during a balloon flight in 1912.


cosmogeny

Study of the origin and evolution of the universe or one of its components, such as galaxies or the solar system.


cosmology

Study of the structure and evolution of the universe.


cosmos

Everything there is. Another term for universe.


crater

A circular depression observed on the surface of a terrestrial planet or a moon. Craters are formed when a large meteoroid or asteroid strikes the surface of the object.

Craters formed on bodies that don't posses an atmosphere, such as Mercury or the moon, are not subject to erosion and therefore appear as they did when they were formed. Craters formed on bodies that possess an atmosphere, such as the earth or Mars, show signs of erosion due to rain and wind.


crescent

Term used to describe the appearance of a planet or moon when between zero and 50 percent of the its disk is illuminated by sunlight. In other words, when the phase of the planet or moon is between new and first quarter, or between last quarter and new.


cusps

Pointed end of Mercury, Venus, or the moon at crescent phase.


dark cloud

An interstellar cloud of gas and dust that is dense enough to block the passage of light.


dark nebula

An interstellar cloud of gas and dust that is dense enough to block the passage of light.


daylight saving time

Daylight saving time is adopted in some areas for spring and summer use to prolong sunlight into evening hours. One hour is added to standard time to obtain daylight saving time.

In the United States, daylight saving time is currently used from 2:00 AM on the first Sunday in April to 2:00 AM on the last Sunday in October. The states of Arizona and Hawaii, as well as some counties in the Midwest, do not observe daylight saving time.


declination (Decl, Dec, or δ)

Angular distance of a celestial object north or south of the celestial equator. Right ascension and declination are coordinates in the equatorial coordinate system.

Declination is analogous to latitude on the earth's surface. Declination is usually measured in units of degrees, minutes, and seconds from -90 degrees to 90 degrees.


density (ρ)

Amount of mass per unit volume, usually measured in grams per cubic centimeter. The density of water is 1.0. Objects with a density greater than that of water will sink, those with a density less than that of water will float. The density of iron is 7.9. The density of lead is 11.3.


diffuse nebula

A cloud of gas and dust in interstellar space, measuring on average twenty light years across. Diffuse nebulae consist mostly of hydrogen, helium, nitrogen, and oxygen. Such clouds of gas and dust are invisible unless illuminated by the light of nearby stars.

If one or more nearby stars are very hot, their intense ultraviolet radiation will cause the nebula to emit light by the process of fluorescence, causing it to glow brightly.

If all nearby stars are cool the nebula will appear less bright as it will merely reflect their light.


direct motion

Apparent west to east motion of a planet as seen from the earth against the background of stars. Direct motion is also described as prograde motion.

See the Diagrams section of this Help file for a diagram illustrating prograde and retrograde motion.


disk

Visible surface of a celestial body.

CyberSky displays the disks of bodies in the Moon Data, Moon Report, and Planet Data dialog boxes, so you can see which bodies show phases and which do not.


distance

Separation between two celestial objects. CyberSky displays the distances of celestial objects in familiar units such as kilometers or miles, and in units such as astronomical units, light seconds, light minutes, and light years, and in parsecs.


diurnal motion

Apparent daily east to west motion of celestial bodies and the sky as a whole, caused by the west to east rotation of the earth on its axis.


double star

Pair of stars that appear close together in the sky. Such stars may be physically close to each other and revolve around a common center of gravity, in which case they are called binary stars. Or, such stars may simply lie in the same direction in space and be otherwise unrelated to each other.


dwarf

Term used to describe a typical star such as the sun that is generating energy by converting hydrogen into helium. Stars spend most of their lifetime in the "dwarf" or "main sequence" stage of stellar evolution.


east point

The point on the horizon at 90 degrees azimuth.


eccentricity (e)

Measure of the degree of flattening of an ellipse. An ellipse with zero eccentricity is called a circle. Most of the planets and moons in the solar system have orbits with eccentricities under 0.25. Venus has the least eccentric orbit. Its orbit is almost a circle. Pluto has the most eccentric orbit. The orbits of most comets are much more eccentric.


eclipse

Total or partial obscuration of light from a celestial body as it passes through the shadow of another body. Note that a solar eclipse is really an occultation.

Each year at least two and no more than five solar eclipses occur.


ecliptic

Plane of the earth's orbit, and the projection of that plane onto the celestial sphere. It is the line in the sky along which the sun appears to move during the course of a year. The sun moves roughly one degree along this line every day.

Because the orbits of the naked-eye planets have inclinations of seven degrees or less with respect to the ecliptic, you will always find these planets on or very close to this line.


ecliptic coordinate system

Coordinate system in which the fundamental reference circle is the ecliptic and the zero point is the vernal equinox. The coordinates are named ecliptic longitude (λ) and ecliptic latitude (β).


ecliptic latitude (β)

Angular distance of a celestial object above or below the ecliptic. Ecliptic longitude and ecliptic latitude are coordinates in the ecliptic coordinate system.

Ecliptic latitude is usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 90 degrees for latitudes above the ecliptic, and from 0 degrees to -90 degrees for latitudes below the ecliptic.


ecliptic longitude (λ)

Angular distance of a celestial object east of the vernal equinox. Ecliptic longitude and ecliptic latitude are coordinates in the ecliptic coordinate system.

Ecliptic longitude is usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 360 degrees.


ellipse

Geometric curve in the shape of a "squashed" circle. The exact shape of an ellipse is determined by a value known as its eccentricity.


elliptical galaxy (E)

A galaxy that has elliptical contours and that contains no conspicuous interstellar material.

Elliptical galaxies are classified according to their apparent degree of oblateness. Perfectly round elliptical galaxies are classified as type E0, and elongated, lens-shaped elliptical galaxies are classified as type E7.


elongation

Angular distance between the sun and a planet or moon.

An object with an east elongation appears east of the sun in the evening sky, and an object with a west elongation appears west of the sun in the morning sky. An elongation of 0 degrees is called a conjunction; one of 90 degrees is called quadrature; and one of 180 degrees is called opposition.

The greatest possible elongation for Mercury is about 28 degrees and for Venus about 48 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


emission nebula

A type of diffuse nebula that is caused to radiate visible light by the absorption of ultraviolet light of a nearby star.


ephemeris

List of calculated positions of a celestial body or celestial bodies over successive intervals of time.


epoch

Point of time selected as a fixed reference, especially for stellar positions or orbital elements.


equation of time

Difference between time based on the apparent sun and the mean sun. In other words, the difference between time as reported by a sundial and a clock.

The apparent sun runs up to 16 minutes fast with respect to the mean sun, and up to 14 minutes slow.


equatorial coordinate system

Coordinate system in which the fundamental reference circle is the celestial equator and the zero point is the vernal equinox. The coordinates are named right ascension (α) and declination (δ).

Precession of the equinoxes causes right ascension and declination to change slowly over time. Therefore, to correctly specify the position of a celestial object, one must give not only its right ascension and declination, but also the epoch for which those coordinates apply.


equinox

One of the two points at which the ecliptic intersects the celestial equator. This term is also applied to the moment the sun crosses such a point.

See autumnal equinox and vernal equinox.


escape velocity

Velocity a body much reach to break away from the gravity of another body and never return to it. In order for a planet or moon to maintain an atmosphere, its escape velocity must be higher than the velocities of the atoms and molecules that make up its atmosphere. Planets like Venus and the earth are too small to hold on to hydrogen and helium gases, and so their atmospheres are made up of heavier gases such as carbon dioxide, nitrogen, and oxygen. Planets such as Jupiter and Saturn are much more massive and have therefore been able to hold onto the hydrogen and helium gases in their original atmospheres.


evening star

Term used to describe the planet Venus when it shines brightly in the western sky after sunset.


Fahrenheit scale (F)

Temperature scale that sets the freezing point of water at 32 degrees and the boiling point of water at 212 degrees. Because there are 180 divisions between the freezing and boiling points of water, the size of a degree in the Fahrenheit scale is smaller than the size of a degree in the Celsius and Kelvin scales which both have 100 divisions.

The Fahrenheit scale was invented in 1709 by German physicist Gabriel Daniel Fahrenheit.


fireball

Term used for a very bright meteor, one that far exceeds the brightness of the planet Venus. As many as 100,000 fireballs occur in the earth's atmosphere each year.


first quarter

Term used to describe the appearance of a planet or moon when 50 percent of the its disk is illuminated by sunlight and when the planet or moon will subsequently enter a gibbous phase.


Flamsteed number

Number assigned to stars in each constellation.

This system of star names was introduced in 1725 by English astronomer John Flamsteed (1646-1719) in his book Historia Coelestis Britannica, published posthumously. Flamsteed numbered 3,000 stars in order of right ascension within each constellation.


flare

An energetic, temporary outburst of light observed in some stars, including the sun.


full

Term used to describe the appearance of a planet or moon when its disk is fully illuminated by sunlight.


galactic cluster

Small star cluster, containing a few hundred stars at most, located in the spiral arms or disk of our galaxy. Galactic clusters are also know as open clusters.

Class letters for galactic clusters define their richness and degree of concentration.

c	loose and irregular clusters

d	loose clusters

e	moderately concentrated clusters

f	fairly well compressed, compact clusters

g	very rich, compact clusters


galactic coordinate system

Coordinate system in which the fundamental reference circle is the galactic equator and the zero point is the direction of the galactic center. The coordinates are named galactic longitude (l) and galactic latitude (b).


galactic equator

Plane of the Milky Way galaxy projected onto the celestial sphere. The galactic equator and the celestial equator are inclined at an angle of about 62 degrees.


galactic latitude (b)

Angular distance of a celestial object above or below the galactic equator. Galactic longitude and galactic latitude are coordinates in the galactic coordinate system.

Galactic latitude is usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 90 degrees for latitudes above the galactic equator, and from 0 degrees to -90 degrees for latitudes below the galactic equator.


galactic longitude (l)

Angular distance of a celestial object east of the galactic center. Galactic longitude and galactic latitude are coordinates in the galactic coordinate system.

Galactic longitude is usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 360 degrees.


galaxy

Large collection of gravitationally bound stars and interstellar gas and dust. Galaxies typically are composed of hundreds of millions or even hundreds of billions of stars and their diameters range from a few thousand to hundreds of thousands of light years.

Many galaxies are flat disks that have a spiral appearance. Many others are elliptical and show no structural features. The rest are irregular in shape.

The galaxy that contains our solar system is a spiral galaxy called the Milky Way. It is composed of approximately 200 billion stars and is about 100,000 light years in diameter. The solar system is located about 30,000 light years from the center of the Milky Way galaxy.


Galilean satellites

Term used to describe the four largest satellites of Jupiter, discovered independently by Galileo Galilei (1564-1642) and Simon Marius (1573-1624) in 1610. Their names are Io, Europa, Ganymede, and Callisto.

Although they are frequently referred to as the Galilean satellites of Jupiter, their names were given them by Marius, not Galileo!


gegenschein

Spot of light in the sky at a point 180 degrees from the sun.

Gegenschein is also called counterglow.


giant

Term used to describe a star that has completed the "dwarf" or "main sequence" stage of stellar evolution, and is generating energy by converting into helium into carbon and oxygen. Stars spend a small fraction of their lifetime in the "giant" stage of stellar evolution.


gibbous

Term used to describe the appearance of a planet or moon when between 50 and 100 percent of the its disk is illuminated by sunlight. In other words, when the phase of the planet or moon is between first quarter and full, or between full and last quarter.


globular cluster

A large star cluster, often containing more than 100,000 stars, located outside the spiral arms or disk of our galaxy.

Class numbers for globular clusters define their richness and degree of concentration.

I	extremely rich and highly compressed

.

.

.

XII	very loose, sparse clusters


great circle

Any circle on the surface of a sphere generated by a plane passing through the center of the sphere. Such a circle has the same radius as the sphere.

Lines of longitude on the surface of the earth are great circles, as are lines of declination on the celestial sphere.


Gregorian calendar

In the Julian calendar established during the days of the Roman Empire, each year was 365 days long and every fourth year was made 366 days long by the addition of a leap day. This made a year 365.25 days long, which is over 11 minutes longer than the tropical year of 365.24219 days. Every 128 years this error accumulated into a full day, and by 1582 the first day of spring was occurring on March 11 instead of March 21.

Pope Gregory XIII was advised to correct this problem. First, he proclaimed that October 4, 1582 was to be followed by October 15, 1582. This eliminated the accumulated error of 10 days. Second, he proclaimed that century years, which are leap years in the Julian calendar, would henceforth only be considered leap years if they were evenly divisible by 400. The Gregorian calendar is correct to 1 day in 3,300 years.

Note that different countries made the switch from the Julian to the Gregorian calendar at different times. The Roman Catholic countries made the switch in 1582. But, England and the American colonies did not switch until 1752. Russia did not make the switch until 1917, and by this time the Julian calendar error had increased such that they had to omit 13 days.


harvest moon

Term used for the full moon closest to the autumnal equinox. The harvest moon gets its name from the fact that it rises soon after sunset, giving farmers more light and extra hours to harvest their crops.


horizon

Great circle that corresponds to the intersection of the earth and the sky.


horizontal coordinate system

Coordinate system in which the fundamental reference circle is the horizon and the zero point is the north point. The coordinates are named altitude (h) and azimuth (A).


hour angle (HA or t)

Angle measured westwards along the celestial equator from an observer's meridian to the hour circle of a celestial body. Hour angle is usually measured in units of hours, minutes, and seconds between 0 hours and 24 hours.


hour circle

Any great circle on the celestial sphere along which declination is measured. A great circle passes through the celestial poles and is perpendicular to the celestial equator.


hunter's moon

Term used for the first full moon following the harvest moon.


illumination

Percentage of the visible disk of a planet or the moon that is illuminated by sunlight.


inclination (i)

Axial inclination is the angle between the rotation axis of a celestial body and a line perpendicular to its orbital plane.

Orbital inclination is the angle between the orbital plane of a planet, satellite, asteroid, or comet and the plane of the ecliptic.


inferior conjunction

Moment at which the elongation of a celestial body on the near side of the sun is 0 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


inferior planet

Mercury or Venus. These planets are referred to as inferior planets because they are closer to the sun than the earth.


inner planet

Mercury, Venus, the earth, or Mars. These planets lie closer to the sun than the main belt of asteroids found between the orbits of Mars and Jupiter.


intergalactic

Between the galaxies. The void between the galaxies is called intergalactic space.


interplanetary

Between the planets. The void between the planets is called interplanetary space.


interstellar

Between the stars. The void between the stars is called interstellar space.


irregular galaxy (I)

A galaxy that does not posses a smooth distribution of stars or rotational symmetry. Neither an elliptical nor a spiral galaxy.


Julian calendar

Calendar established by the Roman Emperor Julius Caesar in 46 BC and modified by Augustus Caesar in 8 AD. The new months of July and August were named after these men. Each year was 365 days long and every fourth year was made 366 days long by the addition of a leap day. This made a year 365.25 days long, which is over 11 minutes longer than the tropical year of 365.24219 days. Every 128 years this error accumulated into a full day, and it was eventually necessary for a new calendar, the Gregorian calendar, to be adopted to solve this problem.


Julian date

Number of days that have passed since noon universal time on January 1, 4713 BC. The Julian day is the number of integral days that have passed, whereas the Julian date includes the decimal fraction of the day that has elapsed since the preceding noon. A Julian date is not a date on the Julian calendar!

The Julian date is very useful because the consecutive numbering of days makes the system independent of the varying lengths of months and years. To find the number of days between two events, simply determine the Julian dates of those events and subtract one date from the other.

The Julian date system was invented in 1582 by the French scholar Joseph Scalinger.


Kelvin scale (K)

Temperature scale that sets the freezing point of water at 273 degrees and the boiling point of water at 373 degrees. Because there are 100 divisions between the freezing and boiling points of water, the size of a degree in the Kelvin scale is larger than the size of a degree in the Fahrenheit scale which has 180 divisions.

The Kelvin scale was invented in 1848 by British physicist William Thomson (1824-1907), also known as Lord Kelvin.


last quarter

Term used to describe the appearance of a planet or moon when 50 percent of the its disk is illuminated by sunlight and when the planet or moon will subsequently enter a crescent phase.


latitude

Angular distance of a terrestrial location north or south of the equator. Longitude and latitude are coordinates in the terrestrial coordinate system.

Latitudes are usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 90 degrees for latitudes north of the equator, and from 0 degrees to -90 degrees for latitudes south of the equator.


light year (ly)

Distance that light or any other form of electromagnetic radiation travels in a vacuum in the course of a year.

Light travels at about 300,000 kilometers per second or 186,200 miles per second. A light year is therefore equivalent to about 9,500,000,000,000 kilometers or 6,000,000,000,000 miles.


limb

Edge of the visible disk of the sun, moon, or any planet, or of any other celestial object that has a detectable disk.


local sidereal time (LST)

Sidereal time at an observer's location. It is equal to the right ascension currently on the observer's celestial meridian.


longitude

Angular distance of a terrestrial location east or west of the meridian that passes through Greenwich, England. Longitude and latitude are coordinates in the terrestrial coordinate system.

Longitude is usually measured in units of degrees, minutes, and seconds of arc from 0 degrees to 180 degrees for longitudes east of Greenwich, and from 0 degrees to -180 degrees for longitudes west of Greenwich.


luminosity (L)

Measure of the intrinsic or absolute brightness of a star. Luminosity is equal to the total energy radiated by a star per unit time.


luminosity classes

Luminosity classes are Roman numerals used to express the size of stars. Small stars have dense atmospheres that exhibit high gas pressures, whereas large stars have immense, extended atmospheres that exhibit low gas pressures.

The characteristics of absorption lines and bands in the spectra of stars are highly dependent on atmospheric pressure, and therefore are an indication of a star's size. These characteristics are studied to determine the luminosity class of a given star.

See the Tables section of this Help file for more information on luminosity classes.


lunar eclipse

Passage of the moon through the earth's shadow.


magnitude

Logarithmic scale used to describe the brightness of an object. The larger the magnitude, the dimmer the object.

Magnitudes are used in astronomy for historical reasons. In ancient times, 129 BC to be exact, the Greek astronomer Hipparchus ranked the stars in six classes of magnitude when he produced the first well-known star catalog. The brightest stars were assigned to the first magnitude and the faintest stars just visible to the naked eye were assigned to the sixth magnitude.

More recently, this system had to be expanded to cover stars brighter than first magnitude and fainter than sixth magnitude. In 1856 the English astronomer Norman R Pogson proposed that a difference of five magnitudes correspond exactly to a brightness ratio of 100 to 1. The brightness ratio of two stars that differ by one magnitude is the fifth-root of 100, or 2.512, a quantity known as the Pogson ratio. A star that is two magnitudes brighter than another star is 2.512 squared, or about 6.3, times brighter than the other star.


main sequence

Term used to describe a typical star such as the sun that is generating energy by converting hydrogen into helium. Stars spend most of their lifetime in the "main sequence" or "dwarf" stage of stellar evolution.


mantle

The inner part of a planet between its central core and its outer crust.


mare (plural: maria)

An impact basin on the surface of the moon that has been filled with dark, basaltic lava. Such lava flows cover about 17 percent of the moon's surface.


mass (M)

Measure of the quantity of matter in a body, usually measured in units of grams or kilograms. Mass is also frequently measured in units of the mass of another body such as the earth or the sun. The mass of a body determines the strength of the gravitational field it produces.


mean solar time

Time measured with respect to the mean sun. Clocks display time based on the mean sun.

The earth's orbit around the sun is not a perfect circle but is an ellipse. The earth does not move along this ellipse at constant speed, but moves fastest when it is closest to the sun and slowest when it is farthest from the sun. The apparent sun therefore moves along the ecliptic at a nonuniform rate.

The nonuniform motion of the apparent sun implies that the length of the solar day is dependent on the position of the earth in its orbit. This situation is not desirable for civil timekeeping. Ideally we want to be able to treat each day as if it was exactly 24 hours long. The mean sun, which moves along the celestial equator at a uniform rate, was invented with this goal in mind.


meridian

Great circle on the surface of the earth that passes through both poles of the earth's axis. In other words, an observer's meridian is the line of longitude he or she is standing on.

In astronomy, when someone says "meridian" he or she usually means the celestial meridian.


Messier catalog

A list of nonstellar objects compiled by French astronomer Charles Messier (1730-1817) and published in 1784. Messier was interested in discovering comets. Between 1760 and 1784 he compiled a list of nebulous objects that might be confused with the comets for which he was searching.


meteor

Streak of light in the sky produced by the motion of a meteoroid through the earth's atmosphere. Meteors are typically observed 100 kilometers or 60 miles high in the atmosphere and burn up before ever getting near the ground.

On a clear, moonless night an average of about 10 meteors can be seen per hour. Much higher counts are possible during a meteor shower. The highest meteor count occurs during the early morning hours when the earth is moving into the cloud of interplanetary particles that cause meteors.

Meteors are commonly called falling stars or shooting stars.


meteoroid

Small particle orbiting the sun in the vicinity of the earth. Most are thought to be fragments of asteroids. Most are also very small, about the size of a grain of sand.

It is estimated that several hundred tons of meteoroids fall on the earth each day. Almost all burn up in the atmosphere and do not strike the ground. Those that do are called meteorites.


meteorite

Portion of a meteoroid that has survived its fall from the sky and has landed on the surface of the earth.

The "thunderstone" that fell near Einsisheim, Alsace in 1492 was the first documented case of a meteorite. Meteorites were not accepted as being extraterrestrial in origin by the scientific community until 1803.


meteoritics

The study of meteorites and of the formation of the solar system.


meteor shower

Profusion of meteors that typically fall within a day or two and appear to radiate from a common point in the sky. Meteor showers are caused by the passage of the earth through a meteor stream. Such streams are composed of the particles left behind by comets as they near the sun.


Milky Way

Galaxy that contains the solar system. The approximately 200 billion stars that compose the Milky Way galaxy are concentrated in a plane, and our view of this plane of stars from within results in the faint band of light we also call the Milky Way. It is necessary to view the sky on a clear, moonless night to see the Milky Way.

The Milky Way galaxy is thought to be a spiral galaxy of type Sb or Sc.


minor planet

Another name for an asteroid.


moon

Any natural body that revolves around a planet. Although we refer to our planet's natural satellite as "the moon" there are in fact many other moons in our solar system. In fact, the only planets that do not possess one or more moons are Mercury and Venus.


morning star

Term used to describe the planet Venus when it shines brightly in the eastern sky before sunrise.


nadir

Point on the celestial sphere that lies directly beneath an observer, 90 degrees below the horizon. This point lies directly opposite the observer's zenith and is therefore never observable.


nautical twilight

Period between the time the sun is 6 degrees and 12 degrees below the horizon.


nebula (plural: nebulae)

Originally, any diffuse, nonstellar object. This term now describes a cloud of gas and dust in interstellar space.

The two major types of nebulae are diffuse nebulae and planetary nebulae.


neutron star

A star more massive than the sun, yet only about 20 kilometers (12.4 miles) in diameter. A neutron star is a dense ball of neutral subatomic particles left at the core of a star after a supernova explosion destroys the rest of the star. A star that is not massive enough to form a neutron star at the end of its life ends up as a type of star called a white dwarf. A star that is too massive to form a neutron star collapses and forms a black hole.


new

Term used to describe the appearance of a planet or moon when its disk is not illuminated by sunlight.


New General Catalogue

Short for New General Catalogue of Nebulae and Clusters of Stars. A list of nonstellar objects compiled by Danish astronomer John Louis Emil Dreyer (1852-1926) and published in 1888.


north point

The point on the horizon at 0 degrees azimuth.


North Star

Polaris, known more formally as α Ursae Minoris, or any other bright star that happens to be within a few degrees of the north celestial pole during a given era.


nova (plural: novae)

A star that experiences a dramatic increase in its brightness for a short period of time, and then slowly returns to its original brightness. During a nova outburst a star can increase its luminosity by hundreds or even thousands of times.

It is thought that novae are binary star systems, and that the increase in brightness happens when one star dumps some of its mass onto the surface of the other star.


nuclear fusion

Process whereby several small particles are combined into a larger particle whose mass is slightly less than the combined masses of the small particles. The lost mass is converted into energy. This is the source of energy in all stars, including the sun.


occultation

Passage of a larger celestial body in front of a smaller one. A solar eclipse is strictly an occultation.


open cluster

Small star cluster, containing a few hundred stars at most, located in the spiral arms or disk of our galaxy. Open clusters are also know as galactic clusters.

Class letters for open clusters define their richness and degree of concentration.

c	loose and irregular clusters

d	loose clusters

e	moderately concentrated clusters

f	fairly well compressed, compact clusters

g	very rich, compact clusters


opposition

Moment when a planet's elongation is 180 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


orbit

Path along which a celestial body moves in the gravitational field of another body. For example, the path of the earth or another planet around the sun, the path of an artificial satellite around the earth, or the path of one star around another star.

Orbits are conic sections. That is, they are either ellipses, parabolas, or hyperbolas. Most orbits are ellipses, and the orbits of the planets of the solar system are almost circular. The orbits of periodic comets are ellipses.


orbital inclination

The angle between a body's orbital plane and the earth's orbital plane.


outer planet

Jupiter, Saturn, Uranus, Neptune, or Pluto. These planets lie farther from the sun than the main belt of asteroids found between the orbits of Mars and Jupiter.


parallax

Angular displacement in the apparent position of a celestial body when observed from two widely separated points.

Measuring the parallax of stars is very useful because it allows us to measure the distances to those stars. The position of a star is measured very precisely at one time, then this measurement is repeated half a year later when the earth has moved to the other side of its orbit. The star will have a slightly different position when viewed from these two perspectives, and the different in these positions allows the distance of the star to be calculated.

The German astronomer Friedrich Wilhelm Bessel (1784-1846) measured the first stellar parallax, that of 61 Cygni, in 1838.


parsec (pc)

Unit of length equal to the distance at which the mean radius of the earth's orbit subtends an angle of one arc second.

Parsecs are used to specify the distances of objects outside our solar system, such as the distances of stars and galaxies. One parsec is equal to 206,265 astronomical units, or 3.26 light years.


partial eclipse

An eclipse in which the eclipsed body is not totally hidden by the eclipsing body.


penumbra

Term used to describe the region of partial shadow in an eclipse, as well as the lighter part of a sunspot.


perigee

The point in an object's orbit around the earth at which it reaches its nearest distance to the earth.


perihelion

The point in an object's orbit around the sun at which it reaches its nearest distance to the sun.


photosphere

The visible surface of the sun or any other star. The photosphere is the region of a star's atmosphere that contributes most of the visible light that we see when we look at the star. Sunspots are observed in the photosphere.


planet

Spherical body that orbits the sun and shines only by reflected sunlight. The nine planets of the solar system are Mercury, Venus, the earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. The name means "wanderer" in Greek, since the ancients observed these bodies to move around with respect to the stars which appeared fixed.


planetarium

An optical device for projecting a representation of the sky on a domed ceiling. Also, a building housing such a device.


planetary nebula

A cloud of gas and dust in interstellar space, measuring on average a light year across. Planetary nebulae consist mostly of hydrogen, helium, nitrogen, and oxygen.

A planetary nebula is an expanding shell of gas ejected from the outer atmosphere of a giant star in the late part of its life.

Planetary nebulae have nothing whatsoever to do with planets. These nebulae are called "planetary" because as seen through a small telescope they resemble the disk of a distant planet.


pole star

Star closest to the north or south celestial pole at any particular time.


precession

Periodic change in the direction of the earth's axis of rotation. Currently the north pole of the axis of rotation points toward the star Polaris, known more formally as α Ursae Minoris.

The earth is not a perfect sphere, but has an equatorial diameter that is slightly larger than its polar diameter. The sun, moon, and other planets of the solar system exert a gravitational attraction on the equatorial bulge of the earth. It is this force that is responsible for precession.

The positions of the celestial poles actually move across the sky, tracing out a circle with a diameter of about 47 degrees on the celestial sphere every 25,800 years. Therefore, the pole star observed thousands of years ago was not the star we see near the north celestial pole today. For example, in Egypt when the pyramids were being built, the north star was Thuban, known more formally as α Draconis, the brightest star in the constellation Draco.

Precession does not alter the constellation outlines or change their boundaries with respect to the stars, but causes a slow drift of the entire celestial coordinate system on the celestial sphere.

CyberSky calculates the effect of precession for objects in the display.


prograde motion

Apparent west to east motion of a planet as seen from the earth against the background of stars. Prograde motion is also described as direct motion.

See the Diagrams section of this Help file for a diagram illustrating prograde and retrograde motion.


prominence

Solar material elevated above the surface by the magnetic forces found in active regions on the solar surface.


proper motion (μ)

Yearly apparent angular motion of a star across the celestial sphere. Stars do actually move through space, and the change in their position with respect to the moving the solar system also causes their apparent position to change.

Because stars are so far away, it takes thousands of years for stars to move far enough for this effect to be noticeable to the naked eye.

Proper motion was discovered by the English astronomer Edmund Halley (1656-1742) around 1715 when he compared the position of the star Arcturus, known more formally as α Bootis, with its position in ancient star charts.

CyberSky calculates the effect of proper motion for stars in the display.


proper name

Name, such as Altair or Betelgeuse, given to one of the brighter stars in the sky.

CyberSky can display proper names for over 200 stars, although only about two or three dozen such names are in common use today.


pulsar

Rapidly rotating neutron stars that emit radiation in narrow beams focused by their strong magnetic fields. These beams act like the beams of light from a lighthouse. When either beam crosses our line of sight we see a "flash" from the pulsar.


quadrature

Moment when a planet's elongation is 90 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


quasar

Star-like objects found at great distances from our own galaxy, now known to be the centers of very young galaxies.

The term "quasar" is a contraction of the phrase "quasi-stellar radio source". Such objects were first detected by radio telescopes due to their strong radio emissions.


radiant

Point from which all the meteors in a meteor shower appear to radiate.


radiation

Energy propagating through space in the form of electromagnetic waves.


reflection nebula

A nebula that reflects the light of cool stars in our direction.


retrograde motion

Apparent east to west motion of a planet as seen from the earth against the background of stars. Such apparent backward motion occurs when the earth is overtaken by an inferior planet near inferior conjunction or overtakes a superior planet near opposition. The result is that the planet appears to slow down, reverses its direction, slows down again, and then continues its "normal" prograde or direct motion.

See the Diagrams section of this Help file for a diagram illustrating prograde and retrograde motion.


revolution

Orbital motion of a body around a gravitational center of attraction.


right ascension (RA or α)

Angular distance of a celestial object east or west of the vernal equinox to the hour circle of the object. Right ascension and declination are coordinates in the equatorial coordinate system.

Right ascension is analogous to longitude on the earth's surface. Right ascension is usually measured in units of hours, minutes, and seconds from 0 hours to 24 hours. The benefit of using these units is that as the earth rotates you see the sky turn by about one hour of right ascension for each hour of time.


rotation

Turning of a body on its axis. Almost all celestial bodies show some degree of rotation.


satellite

Any body, natural or artificial, that revolves around a planet.


semimajor axis

Average distance of an orbiting body from the object it orbits.


sidereal day

Time interval between two successive meridian passages of a star. A sidereal day is divided into 24 hours, and is equal to about 23 hours, 56 minutes, and 4 seconds of mean solar time.

See the Diagrams section of this Help file for a diagram illustrating the difference between solar and sidereal time.


sidereal time

Time measured by reference to the rotation of the earth with respect to the stars. Our common solar time is measured by reference to the rotation of the earth with respect to the sun.

With respect to the stars, the earth takes about 23 hours, 56 minutes, and 4 seconds to spin once on its axis. This quantity is known as a sidereal day. During this period the earth moves along its orbit around the sun. Because of this motion, it takes roughly another four minutes before the earth has rotated once on its axis with respect to the sun. This 24-hour period is known as a solar day.

The sidereal time at any location is equal to the hour angle of the vernal equinox.

See the Diagrams section of this Help file for a diagram illustrating the difference between solar and sidereal time.


solar day

Time interval between two successive meridian passages of a sun. A solar day is 24 hours long.

See the Diagrams section of this Help file for a diagram illustrating the difference between solar and sidereal time.


solar system

Sun and a large number of objects gravitationally bound to it. These objects include the nine planets Mercury, Venus, the earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto; the moons of the nine planets; and numerous asteroids, comets, and meteoroids.

The solar system is located about 30,000 light years from the center of the Milky Way galaxy.


solar time

Time measured by reference to the rotation of the earth with respect to the sun.


solstice

One of the two points at which the sun reaches its northernmost or southernmost points on the ecliptic. This term is also applied to the moment the sun reaches such a point. See summer solstice and winter solstice.


south point

The point on the horizon at 180 degrees azimuth.


spectral sequence

Sequence of letters assigned to stars to order them by decreasing temperature in the classification scheme of spectral types. The sequence consists of O, B, A, F, G, K, and M, where O represents the hottest stars and M represents the coolest stars. O stars are rare but very bright. M stars are numerous but relatively faint. The sun is a star of type G.

See the Tables section of this Help file for more information on the spectral sequence.


spectral types

Spectral types are letters and numbers used to express the surface temperatures of stars.

The characteristics of absorption lines and bands in the spectra of stars are highly dependent on temperature, and so these characteristics are studied to determine the spectral type of a given star.

See the Tables section of this Help file for more information on spectral types.


spectrum (plural: spectra)

Display of the dispersal of radiation from an object into its constituent wavelengths. Such a display allows one to observe the wavelengths present in the radiation and their relative intensities.

One familiar example of a spectrum is a rainbow, in which the visible wavelengths of sunlight are dispersed so that the sequence red, orange, yellow, green, blue, indigo, and violet can be observed. In this case sunlight is dispersed when it passes through drops of rain. Starlight can be dispersed by passing it through a prism or a grating.

Because the characteristics of absorption lines and bands in the spectra of stars are highly dependent on both temperature and atmospheric pressure, these physical properties of stars can be termined from their spectra.


spiral galaxy (S or SB)

A galaxy that appears to have stars concentrated in a spherical nuclear bulge or oblong central bar, and a flat disk with young stars concentrated in spiral arms, giving the galaxy the appearance of a pinwheel.

Spiral galaxies are divided into two types, normal spirals and barred spirals.

S	the spiral arms curve outward directly from the spherical central bulge

SB	the spiral arms begin on opposite ends of an oblong central bar

Additionally, spiral galaxies are classified according to the prominence of their nuclear bulge or central bar and the degree of spiral arm winding.

a	nuclear bulge or central bar dominates, spiral arms are tightly wound

b	nuclear bulge or central bar and spiral arms are equally prominent

c	nuclear bulge or central bar is small, spiral arms are loosely wound

The Milky Way galaxy is thought to be a spiral galaxy of type Sb or Sc.


star

Spherical, gaseous body that generates energy by nuclear fusion reactions in its core. These reactions convert hydrogen into helium, and helium into heavier elements, and allow a typical star to shine for billions of years.

Some stars appear bluish, others appear yellowish, and others appear reddish. These different colors are due to a difference in temperature. Bluish stars are the hottest and reddish stars are the coolest.

Many of the stars you see in the sky are much like the sun. Some are many times brighter and hundreds of times larger than the sun. If such a star were placed where our sun is, the planets of the inner solar system, including the earth, would be inside the star.

The sun is a typical star, one of about 200 billion stars that make up the Milky Way galaxy.


summer solstice

Time when the sun reaches the northernmost point of the ecliptic. This occurs about June 21 and marks the beginning of summer in the northern hemisphere and the day having the maximum hours of sunlight. It also marks the beginning of winter in the southern hemisphere and the day having the maximum hours of darkness.


sun

Star that is the center of the solar system. The sun is a typical star in the Milky Way galaxy.

The term sun is sometimes used generally as a synonym for star.


sunspot

A region in the photosphere of the sun that is cooler than its surroundings, and therefore appears dark by contrast.


sunspot cycle

Variation in the number of sunspots, which repeats itself about every 11 years.


supergiant

Term used to describe a star that has completed the "giant" stage of stellar evolution, and is nearing the end of its lifetime. Stars spend a very small fraction of their lifetime in the "supergiant" stage of stellar evolution.


superior conjunction

Moment at which the elongation of a celestial body on the far side of the sun is 0 degrees.

See the Diagrams section of this Help file for a diagram illustrating planetary configurations.


superior planet

Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto. These planets are referred to as superior planets because they are farther from the sun than the earth.


supernova (plural: supernovae)

A star that experiences a catastrophic explosion of its core, resulting in a spectacular increase in its energy output. Such an explosion marks the final stage of evolution of a massive star. During a supernova explosion a star can increase its luminosity by millions of times.


supernova remnant

The rapidly expanding gases produced by a supernova explosion.


synodic period

For the moon, the average length of time between a given phase and the next phase of the same type. For a planet, the average length of time between successive oppositions, conjunctions, or quadratures.


terminator

Dividing line between the illuminated and dark portions of the lunar or any planetary disk. The line of sunrise or sunset on that body.


total eclipse

Complete hiding of one body by another as seen by an observer on the earth. In common usage a total eclipse of the sun is one in which the moon completely hides the sun's disk, and a total eclipse of the moon is one in which the moon's disk passes completely into the umbra of the earth's shadow.


transit

The passage of a celestial body across the meridian.

Also, the passage of a small celestial body in front of a much larger one, such as when Mercury or Venus appear in silhouette against the solar disk, or when a satellite passes in front of Jupiter or Saturn, The shadow of a satellite may also transit the disk of its primary.


tropical year

Time interval between two successive passages of the sun through the vernal equinox. A tropic year is 365.24219 solar days long.


umbra

Term used to describe the region of total shadow in an eclipse, as well as the darker part of a sunspot.


universal time (UT)

Universal time is the mean solar time in the time zone centered on Greenwich, England. It is the same thing as Greenwich Mean Time (GMT).


universe

Everything there is. Another term for cosmos.


variable star

Star whose brightness varies with time. The period of time between successive peaks in the brightness of such a star can vary from hours to years. Some variable stars have very regular periods, and others have irregular periods. Some variable stars change their brightness so little that this change is only noticed after careful observation over time. Other variable stars change their brightness drastically, easily visible to the naked eye in the night sky, but then disappearing from view, only to brighten and reappear months later.

The most common type of variable stars are red giants that appear to pulsate, expanding and contracting like a beating heart.


vernal equinox

Time when the sun crosses the celestial equator moving from south to north. This takes places about March 21 and marks the beginning of spring in the northern hemisphere and the beginning of fall in the southern hemisphere.

The sun crosses the celestial equator where the ecliptic and the celestial equator intersect. The point of the vernal equinox is also called the First Point of Aries and is important because it is the zero point for both the equatorial and ecliptic coordinate systems. That is, both right ascension and ecliptic longitude are measured eastward from this point.

The equinoxes are not fixed but move approximately 50 arc seconds per year due to the effect of precession. The vernal equinox or First Point of Aries is now actually located in the constellation Pisces!


vertical circle

Circle on the celestial sphere, drawn through the observer's zenith and perpendicular to the horizon.


wane

Term used to describe that portion of the lunar revolution between new and full, when the illuminated area of the disk is increasing. Opposite of wax.


wavelength (λ)

Distance between two crests of a wave.


wax

Term used to describe that portion of the lunar revolution between full and new, when the illuminated area of the disk is decreasing. Opposite of wane.


west point

The point on the horizon at 270 degrees azimuth.


white dwarf

A star that is very near the end of its lifetime. Such a star has lost the ability to produce nuclear energy and as a consequence has collapsed to a size approximately that of the earth.


winter solstice

Time when the sun reaches the southernmost point of the ecliptic. This occurs about December 21 and marks the beginning of winter in the northern hemisphere and the day having the maximum hours of darkness. It also marks the beginning of summer in the southern hemisphere and the day having the maximum hours of sunlight.


year

Time interval required for the earth to revolve once around the sun. The choice of reference point determines the exact length of the year.

Tropical year	equinox to equinox	365.24219 days

Sidereal year	fixed star to fixed star	365.25636 days

Anomalistic year	apse to apse	365.25964 days

Eclipse year	moon's node to moon's node	346.62030 days


zenith

Point on the celestial sphere that lies directly above an observer, 90 degrees above the horizon. This point lies directly opposite the observer's nadir.


zodiac

A band approximately 20 degrees wide passing around the celestial sphere centered on the ecliptic.

This band is divided into 12 equal sections 30 degrees in length called signs of the zodiac. The names of these sections correspond to the constellations located in them at the time of Hipparchus, over 2000 years ago.

The signs of the zodiac are called Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. Precession of the equinoxes has shifted the zodiacal constellations by over 30 degrees meaning that the signs of the zodiac and the zodiacal constellations no longer correspond.

For example, spring begins when the sun enters the sign of Aries, but this point is now located in the constellation Pisces. Furthermore, during the course of one year the sun now passes through the constellation Ophiuchus, although this constellation is not considered a sign of the zodiac.


zodiacal constellation

One of the twelve constellations of the zodiac. The zodiacal constellations include Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. Note that the zodiacal constellations are not equivalent to the signs of the zodiac, although they once were. Note, too, that in astrology Scorpius and Capricornus or referred to as Scorpio and Capricorn.


zodiacal light

Faint glow that extends away from the sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Zodiacal light is best seen on a clear, moonless night in the tropical latitudes. Zodiacal light is sunlight scattered by very small dust particles in interplanetary space concentrated in the plane of the solar system.


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To obtain information about a menu command, a toolbar button, or another part of CyberSky's interface, click the Help button. After the pointer turns into an arrow and question mark, click the item you want to learn more about.


Restore command

To restore a window to its previous size and location, click the Restore button in the upper-right corner of the window, or click Restore on the application Control menu.


Move command

To use the arrow keys on the keyboard to change the location of the application window, click Move.


Size command

To use the arrow keys on the keyboard to change the size of the application window, click Size.


Minimize command

To reduce the application window to an icon, click the minimize button in the upper-right corner of the window, or click Minimize on the application Control menu. To restore the window to its previous size and location, double-click the icon.


Maximize command

To enlarge the application window to fill the screen, click the maximize button in the upper-right corner of the window, or click Maximize on the application Control menu. To restore the window to its previous size and location, double-click the title bar.


Close command

To close the application, double-click the upper-left corner of the window, or click Close on the application Control menu.


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Toolbar

Contains buttons that give you quick access to many commands and features. To see the name of a button, point to it with the mouse. To show or hide the toolbar, use the Toolbar command on the View menu.


Status bar

Displays information about a command or toolbar button. To show or hide the status bar, use the Status Bar command on the View menu.