An imperfection in the imaging properties of a lens or mirror.
Aberration, Stellar: (Aberration of starlight)
The aberration of starlight is the apparent displacement of the path of light from a star caused by the orbital motion of the Earth.
A measure of the actual rate of energy output of a star or other celestial object as opposed to the apparent luminosity, which depends on the distance to the object.
Absolute magnitude: (of a star)
The magnitude a star would appear to have if it were at a standard distance of 10 Parsecs. Absolute magnitudes are a method of comparing actual luminosities of stars on an arbitrary scale.
A sharp dip in intensity over a narrow wavelength in a Continuous Spectrum. In a spectrum produced by a typical Spectrograph, in which the light passes through a narrow slit before being dispersed, absorption lines have the appearance of dark lines cutting across at right angles to the direction of dispersion.
Absorption Nebula: (Dark Nebula)
A dark interstellar cloud that absorbs the light from bright objects behind it. Absorption Nebulae range in size from Globules to large clouds visible to the unaided eye. A good example would be the Horsehead Nebula in the constellation Orion.
A Continuous Spectrum in which absorption features of some kind are seen. May be absorption lines or possible Diffuse Interstellar Bands.
Achernar is the brightest star in the Constellation Eridanus, Arabic for "the end of the river" and marks the southern edge of the constellation at a Declination of -57. Acherner is a B-star of magnitude 0.5.
The process by which small particles of material coalesce, or accumulate on a larger mass, under the influence of their mutual gravitational attraction or as a result of a collision, creating larger body.
Adhara: (Epsilon Canis Majoris)
The second brightest star, after Sirius, in the constellation Canis Major. Adhara is a giant B star of magnitude 1.5 and has an eighth magnitude companion.
Asteroid Adonis 2101 has a diameter of 2 km and is a member of the Apollo asteroid group that came to within 2 million kilometres of the Earth in 1937. Adonis was discovered by E. Delporte in 1936.
Adrastea: (Jupiter's satellite).
Adrastea [a-DRAS-tee-uh] is the second inner-most known satellite of Jupiter. Adrastea was the daughter of Jupiter and Ananke and the distributor of rewards and punishments.
An Aeon (U.S. Eon) is a period of time equal to one-thousand million years.
An airy disc is the smallest image a telescope can make of a point source of light, such as a star. Defraction as the light passes through the telescope aperture causes the image of even a point source to have a finite size. It is rarely possible to achieve images as small as the Airy disc because of turbulence in the Earth's atmosphere which distorts the image.
Alamak or Almach, meaning the "badger" or "boot", is considered to be one of the finest double stars for small telescopes. The brighter star is golden, while its companion is aqua. The companion is itself a double with its own 6.3-magnitude star. This star is also a Spectroscopic Binary. So what we have here is really a quadruple system.
Albedo is a measure of the reflecting power of an object or surface. It is mathematically defined as the ratio of the amount of light reflected from a surface to the amount that is incident upon it, often expressed as a percentage.
Albireo is regarded as one of the most beautiful Double Stars in the entire sky. The primary shines at a magnitude 3.09 and its companion is 5.11. The colours have been described as "topaz and sapphire", or "golden and azure". This pair is best observed with a medium size telescope set to only about 30 power.
A blue-white double star in the constellation Taurus. The brightest star in the Pleiades cluster at visual magnitude 2.86.
A Red Giant star in the constellation of Taurus with a magnitude of -0.7 and spectral type K5 lying at a distance of 76 light years.
Algol, with Bayer designation Beta Persei, is a well-known Variable as well as a Binary Star. Its light variations, explained by John Goodricke in 1783, are caused by mutual eclipses of two stars moving in orbits around their common centre of gravity, within a period of about 69 hours.
Algol variables (Beta Persei stars)
A subclass of Eclipsing Binary Stars named after Algol. The more massive star is still on the main sequence while the less massive companion has become a subgiant.
A camera with a very wide-angle lens capable of photographing most of the visible sky in one exposure.
The two ton meteorite that fell in Mexico in 1969, covering an area of 48 by 7 kilometres. The meteorite was of the carbonaceous type and one of the largest known.
Almach: the "badger" or "boot",
An alternative name for Alamak, Gamma Andromedae.
The Almagest ("The Greatest Compilation"), the earliest and most important work of the Greek astronomer Ptolemy (AD 100-170), was the standard work on astronomy until the time of Copernicus. A compilation of the astronomical knowledge of the past, as well as of Ptolemy's own research, the work is composed of 13 books, or chapters.
An almanac is a book or table comprising a calendar of the year. Today, almanacs also catalogue miscellaneous events that occurred during the previous year and publish selected statistical data. Besides showing the days, weeks, and months, early almanacs registered feast days and saints' days, recorded astronomical phenomena, and sometimes contained meteorological and agricultural forecasts and miscellaneous advice.
The first letter of the Greek alphabet, used in Stellar Nomenclature to mark the brightest star in the constellation.
Alpha Centauri: (Rigel Kentaurus),
Alpha Centauri is the closest star to the Earth, next to the sun. Being a mere 4.34 Light Years away, the star is one of the finest visual binaries in the sky with an orbital period of 80.089 years. The brighter star is very close to our sun in size. As seen from Alpha Centauri, our sun would be a first Magnitude star, with the earth too dim to be visible.
The alpha particle is a stable combination of two Protons and two Neutrons. It is emitted from the nuclei of many heavy radioactive atomic nuclei during decay. The alpha particle is also the nucleus of the atom of helium-4 and can acquire two electrons to become a neutral helium Atom.
Altair is the 12th brightest star in the sky. It is also one of the closest, 9 times more luminous than the sun, and 1.5 times the size. Altair has one of the fastest rotations known, making a complete rotation in 6.5 days, compared to over 25 days for the sun. Because of this, it must be rather flattened with an equatorial diameter about twice the polar diameter.
A telescope mounting incorporating two independent rotation axes allowing movement of the instrument in Altitude and Azimuth. A simple form of telescope mounting making it easy to move the telescope about both axes at the same time in order to track the motion of celestial objects across the sky. Not suitable for small motor-driven telescopes.
Alpheratz: (Alpha Andromedae),
Alpheratz is a Spectroscopic Binary star with a period of 96.7 days. The name is said to mean "the horse's navel".
In astronomy, navigation, and surveying, the altitude of a celestial object is its Angular Distance above or below the celestial horizon. The angular distance is measured along the vertical circle, the circle passing through both the celestial object and the Zenith, an imaginary point directly above the observer.
Amalthea: (Jupiter's satellite).
Amalthea [am-al-THEE-uh] is one of Jupiter's smaller moons. It was named after the nymph who nursed the infant Jupiter with goats milk. It is extremely irregular, having dimensions of about 270 by 165 by 150 km in diameter. It is heavily scarred by craters, some being extremely large relative to the size of the moon.
A group of asteroids that come within perihelion distance of the planet Mars. They have perihelion distances of between 1.018 and 1.3 Astronomical Units. The group was named when the asteroid Amor was discovered in 1932.
Andromeda, named after the princess of Greek mythology, is a constellation most prominent during autumn in the Northern Hemisphere. Situated between the constellations Cassiopeia and Pegasus, Andromeda's brightest star, Alpheratz, forms one corner of the square of Pegasus. Andromeda contains the famous Andromeda galaxy, M 31.
The Andromeda galaxy is the nearest spiral galaxy beyond the Milky Way, but, at a distance of 2.2 million Light-Years, it is also the most distant celestial object visible to the naked eye.
A meteor shower associated with Comet P/Biela in 1852, but not observed since 1940. The first recorded appearance was in 1741 and then again in November 1872 and 1885. The shower is also known as the Bielids.
A plane angle is the measure of the amount of rotation when a line segment rotates in a plane about a fixed point. By convention, a counter-clockwise rotation is positive, and a clockwise rotation is negative.
The angstrom is a unit of length used principally for expressing the wavelengths of radiation in the optical range. It is also used for smaller distances, such as those involving atoms and molecules. The unit was named after Anders Jonas Angstrom of Sweden who, in 1868, first attempted to measure the wavelengths of light in metric units.
Angular acceleration is the rate of increase of Angular Velocity with time, measured in radians per second.
The rate in which a body moves about a fixed axis, expressed as the rate of change of angular displacement, measured in radians per second.
Angular diameter is determined by the combination of true diameter and distance. The apparent diameter of an object in angular measure, that is, radians, degrees, arc minutes or arc seconds.
The length of an Arc expressed in angular measure (i.e. radians, degrees, arc seconds or arc minutes) as the angle subtended by the arc at the observer. The angle between imaginary lines from the observer in the directions of the two points.
Angular momentum is a measure of the energy of a rotating object or system of objects, such as a Planetary system. The amount of angular momentum depends on the speed of rotation, mass, and mass distribution of the object or system. Mass distribution is known as the moment of inertia.
The Angular Distance between the closest details which a telescope can pick out or resolve.
The rate at which a rotating body sweeps out. Angular velocity can be measured in radians, degrees, or revolutions per unit time.
The displacement of the position of a star due to the Earth's annual motion around the sun.
A solar eclipse in which the Moon does not completely obscure the sun and a thin ring of sunlight remains visible. Annular eclipses occur when the Moon is at its furthest point from the Earth.
The angular deviation from the sun of a planet from its perihelion (point of orbit nearest to the sun).
Antares is the 15th brightest star in the sky. The name comes from Greek, and means "Rival of Mars" due to its pronounced reddish colour. Antares is one of the largest stars known, estimated to be 700 times the size of our sun.
The anthropic principle propounds the idea that 'the universe is the way it is because if it were different we would not be here to observe it'. The principle arises from the observation that if the laws of science were even slightly different, it would have been impossible for intelligent life to evove. For example, if the electric charge on the electron were only slightly different, stars would have been unable to burn hydrogen and produce the chemical elements that make up our bodies. Scientists are undecided whether the principle is an insight into the nature of the universe or a piece of circular reasoning.
Antimatter is a form of matter in which each of the particles that compose ordinary matter, the proton, neutron, and electron, is replaced by its corresponding antiparticle, that is, the antiproton, anti neutron, and positron, respectively. Anti particles have the same mass and spin as their respective particles, but they have opposite values of such electromagnetic properties as charge and magnetic moment.
A small, faint constellation in the southern hemisphere of the sky, introduced by Lacaille in 1752. Contains no stars brighter than fourth magnitude.
Used by amateur astronomers to describe the quality of Seeing. A scale from 1 to 5 devised by the French astronomer Eugenios Antoniadi (1870-1944).
The scale is as follows -
1) Perfect seeing.
2) Slight undulations with periods of perfect seeing.
3) Moderately good seeing.
4) Poor seeing making observations difficult.
5) very bad seeing permitting no useful observation.
The aperture of a telescope, or other optical system or instrument, is the opening through which light is admitted. The size of the aperture is limited by the size of the lens mount.
1) The point on the Celestial Sphere in the constellation Hercules (RA 18h, Decl 30) towards which the solar system appears to be moving with respect to other stars in the sun's vicinity.
2) The point on the Celestial Sphere towards which the Earth appears to be moving due to its orbital motion around the sun.
In positional astronomy, or Astrometry, the solar apex is the point on the celestial sphere toward which the sun and its solar system appear to be moving, relative to nearby stars, at a velocity of about 20 km per sec (12 mi per sec). This point lies in the constellation Hercules near the star Vega, approximately at a Right Ascension of 18 hours and a Declination of 30 degrees north.
The point furthest from the sun in the orbit of a body, such as a planet or comet that is travelling around the sun.
A vast highland region on the surface of Venus situated mainly in the southern hemisphere. Several volcanoes lie at its eastern edge, including Maat Mons.
A group of Asteroids with perihelia inside the orbit of the Earth. Included in the group are Icarus and Oljato. The group is named after the asteroid Apollo which was discovered in 1932.
The Apollo program was the successful conclusion of the U.S. effort to achieve, within the decade, the goal set by President John F. Kennedy on May 25 1961, of landing a man on the Moon and returning him safely to Earth. It followed the Gemini manned-flight program conducted in 1966-67 to develop the necessary techniques of orbiting, docking, and extra vehicular activity (EVA). The main elements of the Apollo project were the three-man Apollo spacecraft; the two-man Lunar excursion module (LEM), or Lunar Module (LM); and the Saturn family of rockets, consisting of the Saturn 1, the Saturn 1B, and the Saturn 5.
Adjective used in conjunction with astrophysical quantities (Meaning - the altitude or brightness as perceived by an observer on Earth from a particular location).
The Luminosity of a star or other object as it appears to an observer on Earth.
A measure of the relative brightness of a star or other celestial object as perceived by an observer on Earth.
The time when the sun's centre is seen to cross the Meridian.
A measure of time based on the daily motion of the sun. The motion of the sun is not uniform, it is inclined to the Celestial Equator because the Earth's orbit around the sun is elliptical, not circular.
Apus: (APS) (Bird of Paradise)
A small constellation in the southern hemisphere close to Crux. Apus has no stars above the 4th magnitude.
The Eta Aquarids meteor shower is observed between April 24th and May 20th with maximum occurring around 4th or 5th of May. The Aquarids are associated with Halley's Comet.
Aquarius the Water Bearer, or Water Carrier, is one of the constellations of the Zodiac, the band of sky through which the sun, moon, and planets appear to move. Most prominent during the autumn months in the Northern Hemisphere, Aquarius contains the globular cluster M2, the planetary nebula NGC 7009, also known as the Saturn nebula, and NGC 7293, known as the Helix nebula.
Aquila the Eagle is a constellation of stars prominent in the summer sky in the Northern Hemisphere. It contains the bright star Altair (alpha Aquilae), of visual magnitude 0.77 and spectral type A7.
A constellation of the southern hemisphere of the sky lying south of Scorpius in a rich part of the Milky Way. Objects of interest include 2 star clusters, NGC-6193, containing approximately 30 stars, and globular cluster NGC 6397, a seventh magnitude cluster believed to be the closest to us at a distance of 8000 light years. Ara contains 7 stars above the 4th magnitude.
An arc is a continuous segment of a simple curve. More specifically, any segment of the circumference of a circle is called a circular arc. In a circle with centre at O and two points A and B on the circumference, the Angle AOB is a central angle of the circle; the larger of the two arcs AB is a major arc of the circle; the smaller, a minor arc. If A and B are end points of a diameter, the two arcs AB are the same length, and each arc is a semicircle.
A unit in which very small angles are measured. One arc minute is equal to one-sixtieth of a degree.
A unit in which very small angles are measured. One arc second is equal to one-sixtieth of an Arc Minute.
Archaeoastronomy is the science that concerns the recovery and study of evidence of the astronomical beliefs and practices of ancient or preliterate peoples. It is primarily a tool to gain insight into the intellectual achievements of remote cultures.
Meaning 'The Guardian of the Bear', Arcturus is the 4th brightest star in the sky. It is about 115 times as luminous as the sun, 25 times its diameter but only 4 times its mass. This means that it is around .0003 times as dense as the sun. The heat received on the Earth from Arcturus is equal to that delivered from a candle 5 miles away.
Argo, also known as Argo Navis, is an ancient name for a Southern Hemisphere constellation that contained Canopus (alpha Carinae), the brightest star in the sky after Sirius. It was named for the ship in Greek mythology in which Jason and the Argonauts sought the Golden Fleece. The ship was later broken up into its more manageable components, which we now know as the constellations Carina, the Keel, Vela, the sail, and Puppis, the prow.
Ariel [AIR-ee-al] is a relatively small satellite and is the brightest moon of Uranus. The surface is pock-marked with craters, but the most outstanding features are long Rift Valleys stretching across the entire surface.
Aries, the Ram, is one of the constellations of the Zodiac, the band of sky through which the sun, moon, and planets appear to move. Most prominent during the autumn in the Northern Hemisphere, it contains no stars brighter than 2nd magnitude, but one of them, Gamma Arietis or Mesarthim, reveals itself as a pretty double star in modest telescopes at relatively low magnification. The two components are each of about magnitude 4.8 and separated by some eight seconds of arc.
Aristarchus: of Samos (310-230) BC
The Greek mathematician and astronomer Aristarchus, c.310 BC, is celebrated as the exponent of a sun-centred universe and for his pioneering attempt to determine the sizes and distances of the sun and Moon.
The Ancient Greek philosopher who favoured an Earth-centred universe in which the heavenly bodies rotated on concentric spheres around a spherical Earth. Aristotle was the founder of a famous school in Athens, known as the Lyceum or 'Peripatetic School' in around 335 BC. Amongst his works the following are remembered especially: the Organon, which contains his writings on Logic, the Metaphysics, Physics, On the Soul, Nicomachean Ethics, Economics, Politics, Poetics, Rhetoric. For Aristotle 'physics' signified the qualitative study of all natural phenomena, carried out without the aid of a store of mathematical knowledge.
In Aristotelian cosmology the Earth was imperfect and situated at the centre of the Universe. It was composed of the four elements earth, water, air and fire, which were characterised in terms of a rectilinear and sporadic motion. Conversely, the movement of the celestial bodies (the sun, planets and stars, composed of ether or quintessence) was continuous and circular. In order to explain the independent motion of the planets, Aristotle proposed that they rotated on concentric spheres.
After the physics of Aristotle had been 'Christianized' in the thirteenth century, it became the foundation of university instruction in this subject. This system was put into serious doubt with the advent of the heliocentric theory developed by Copernicus.
One of the large Shield Volcanoes in the Tharsis-Ridge region of Mars, 250 kilometres (150 miles) in diameter.
One of the large Shield Volcanoes in the Tharsis-Ridge region of Mars, measuring some 350 kilometres (220 miles) in diameter.
The point at which an orbit crosses the plane going north.
A dim glow that observers claim to see on the dark part of Venus when the planet is at crescent phase. Unfortunately, there is no photographic evidence and the reality is questioned.
1) The position of a planet or the moon relative to the sun, as viewed from Earth. 2) The angle between the rotation axis of a body in the solar system and the radius vector between that body and the Earth.
A loose grouping of stars, containing between ten and a hundred members. About seventy examples are known.
A prominent pattern of stars, usually with a popular name, that does not constitute a complete constellation. An example would be the Plough, or Big Dipper, in Ursa Major.
An asteroid is any one of a large number of small solid objects in the solar system, sometimes called minor planets because they orbit the sun directly. The vast majority are found in a swarm called the asteroid belt, between the orbits of Mars and Jupiter, at average distances of 2.1 to 3.3 Astronomical Units (AU) from the sun. They are given a number (and sometimes then named) when their orbits are well established. More than 5,000 such asteroids are now known.
The vast majority of asteroids in our solar system can be found between the orbits of Jupiter and Mars, approximately 2.0 to 3.3 AU from the sun. Within the belt, there are concentrations of orbits forming groups. There are also regions with no asteroids, known as the Kirkwood Gaps.
Asteroids are assigned a serial number when they are discovered. It has no particular meaning except that asteroid N1 was discovered after asteroid N.
Astrology in its modern form is the use of astronomical phenomena to predict earthly and human events, in terms of an occult system of great antiquity. In its earliest manifestation, however, the practice of astrology was indistinguishable from that of astronomy. In other words, when we hear of the ancient Egyptians or Babylonians or Maya 'practising astrology', they were in effect making astronomical observations according to a scientific system to which archaeologists have merely seen fit not to accord a separate name. Indeed, neither of our post-renaissance terms for the respective disciplines existed in ancient times.
In its mature differentiated form, astrology seeks to analyze the long-observed effects of sun, moon, planets (and, to a lesser extent, the stars) on the Earth and its inhabitants for a specific time and place. For an individual, this starting point is the moment of birth, giving rise to a natal chart, or horoscope, in which the positions of the principal heavenly bodies for that instant are schematically recorded and related to the fixed signs of the Zodiac. The practise of astrology is thus more akin to psychology, seeking origins and causes and making prognoses based on these, using its accumulated stock of symbols and empirically-observed effects as analytical tools for personality and character assessment. In the hands of a skilled exponent, this assessment can be very penetrating.
Astrometry: (positional astronomy)
Astrometry, also called positional astronomy, is the branch of astronomy that deals with determining the positions and motions of celestial bodies. This science is also concerned with measuring other quantities, such as the diameter and polar flattening of the sun and planets, and determining the orbits of double star components.
Astrophotography is the photography of stars and other celestial bodies for the purpose of obtaining observational data. It has long been an important tool in modern astronomical and astrophysical research.
Astronautics is that area of engineering and technology concerned with spaceflight. The discipline encompasses several technical fields, including Astrodynamics, propulsion, structures, power supplies, thermal control, and communications. Astrodynamics, the study of spacecraft motion through force fields in space, is the cornerstone of Astronautics.
An expert in their particular branch of astronomy; an observer of celestial objects.
An astronomical catalogue is a tabular compilation of data about selected celestial objects, whereas an astronomical atlas includes charts or photographs of these objects. Catalogues may be divided into three major categories: finding lists, positional catalogues, and specialized catalogues.
In astronomy the traditional method for determining the distance of a celestial body is to measure its Parallax, which is the difference in direction of an object as seen by an observer from two different locations. The greatest precision is obtained by choosing the longest possible Baseline. For the sun and Moon, two widely separated points on the Earth are chosen as observation stations. For objects beyond the solar system, the Semimajor Axis of the Earth's orbit is the adopted baseline.
The interval of time during which the sun is between 102 and 108 degrees below the Zenith Point.
The astronomical unit, used for measuring distances in astronomy, especially in the solar system, equals the mean distance between the Earth and the sun. The abbreviation AU is used for this unit in English. The International Astronomical Union has adopted the value 1 AU = 149,600,000 km (92,956,000 mi).
Astronomy is the scientific study of individual celestial bodies and of the Universe as a whole. It is also the oldest of sciences, having its roots in the astonishingly accurate naked-eye observing of early civilizations in the Middle East. Galileo pioneered telescopic observation in 1609. Karl Jansky founded radio astronomy in 1931. Today, astronomers can peer into the heavens in nearly all wavelengths of the electromagnetic spectrum.
Atlas, the second of Saturn's known satellites, orbits near the outer edge of the A-ring and is about 40 by 20 kilometres (25 by 15 miles) in size. It is probably a shepherd satellite for Saturn's A-ring. Atlas was discovered by R. Terrile in 1980, found from photographs taken by Voyager during its encounter with Saturn.
The atmosphere is the nearly transparent envelope of gases and suspended particles that surrounds the Earth, profoundly influencing environmental conditions on the planet's surface. Without chemical processes involving several of the atmospheric gases, life could not exist. The physical processes that operate in the atmosphere are also of vital importance because they are responsible for the Earth's varied climates.
A light ray's path through the Earth's Atmosphere is a straight line only if the light is moving vertically with respect to the Earth's surface. Light travelling on an oblique path is refracted, or bent, by the atmosphere. This atmospheric refraction is greatest for objects near the Horizon, so that a star that appears to be on the horizon is actually about one-half deg below it. The amount of refraction decreases markedly above the horizon; a light ray travelling at 45 deg above the horizon is bent only one-sixtieth deg, or 1 minute of arc.
An atom is the smallest unit of matter that is recognizable as a chemical Element. Atoms of different elements may also combine into systems called Molecules, which are the smallest units of chemical compounds. In all these ordinary processes, atoms may be considered as the ultimate building blocks of matter. When stronger forces are applied to atoms, however, the atoms may break up into smaller parts. Thus atoms are actually composites and not units, and have a complex inner structure of their own. By studying the processes in which atoms break up, scientists in the 20th century have come to understand many details of the inner structure of atoms. The size of a typical atom is only about 10 (-10th) metres. A cubic centimetre of solid matter contains something like 10 (24th) atoms. Atoms cannot be seen using optical microscopes, because they are much smaller than the wavelengths of visible light. By using more advanced imaging techniques such as electron microscopes, scanning tunnelling microscopes, and atomic force microscopes, however, scientists have been able to produce images in which the sites of individual atoms can be identified.
The atomic clock, the most accurate of time-keeping devices, is based on the measurement of changes in the energy states of atoms. The energy change involved in the most common forms of the atomic clock occurs when the atom absorbs energy, causing an electron to alter its spin characteristics and, subsequently, its magnetic field.
Auriga, the Charioteer, is a Constellation prominent during winter in the Northern Hemisphere. Situated between the head of Orion and the north celestial pole, it contains Capella, the sixth brightest star in the sky, three star clusters (M 36, M 37, and M 38) visible through binoculars, and several important Double Stars.
The Aurora Borealis (northern lights) and Aurora Australis (southern lights) are beautiful, dynamic, luminous displays seen in the night-time sky. The most common form of an aurora is a curtain like luminosity extending east to west.
The "Northern Lights", caused by the interaction between the solar wind, the Earth's magnetic field, and the upper atmosphere. A similar effect happens in the southern hemisphere where it is known as the Aurora Australis.
The equinox that occurs in September.
The period of time it takes a body to make one complete rotation on its axis.
The imaginary straight line about which a body (planet, star etc) rotates, or may be supposed to rotate.
The angular distance of an object eastwards along the horizon, measured from due north, between the astronomical meridian (the vertical circle passing through the centre of the sky and the north and south points on the horizon) and the vertical circle containing the celestial body whose position is to be measured.
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