Amiga ISO Collection/AmigaUtilCD2.iso/WordProcessors/DIGITAWORDWORTH3-2.DMS/in.adf/Archive/Tutorials.lzh/Book Text

WordWorth | 1994-03-10 | 10.7 KB | 128 lines

Chapter One - The Earth
The Earth is a sphere, slightly flattened at the poles - a shape called an
oblate spheroid. The diameter at the pole is 127,414 km and it is 43 km greater
at the equator. For a sphere of this size it is comparatively smooth and is not
much rougher than a billiard ball. The highest mountain, Mt. Everest, is less
than 10 km above sea level and the deepest ocean is only slightly more than 10
km deep.
The Earth spins on its axis once every 24 hours, which makes our day and night.
At the same time, it orbits the sun once every 365¼ days to make our year.
Someone living at the equator, even when he is asleep, is travelling through
space at 1,700 km an hour as the Earth turns on its axis, and at 100,000
km/hour as it travels round the sun.
The Earth's natural satellite is the moon, which orbits the Earth once every
27¼ days. The moon is 400,000 km away and has a diameter of 3,500 km. Like the
planets, it does not produce light of its own but it becomes visible to us on
Earth by the light it reflects from the sun. The gravitational pull of the moon
on our seas tends to heap up the water which causes the tides. When the
gravitational pull of the sun and the moon act in the same direction, the high
spring tides occur; when the two pulls are at right angles, we have the low
neap tides.
Chapter Two - The Sun
The sun, our nearest star, is at the centre of our solar system. Compared with
other stars if is not very large, but to us it is the biggest and brightest.
Films taken when it is eclipsed by the moon show that the sun is surrounded by
a fiery atmosphere (the solar corona) which reaches many thousands of miles
from the surface. In some places enormous jets (solar flares) gush out even
beyond that.
The sun itself is a vast ball of glowing gas. Its outside layer of gas (the
photosphere) has a temperature of about 6000 ºC - about twice as hot as
white-hot metal. In places, there are blackish spots (sunspots) which sometimes
appear in great numbers and then die away. These sunspots are cool areas but
they are still hotter (about 4000 ºC) than the filament of an electric lamp.
Sunspots are not yet fully understood but we know that they occur in greater
numbers, about every eleven years, and that they interfere with radio
communications.
Chapter Three - The Solar System
Unlike the stars, which shine with their own light, the planets of our solar
system can only be seen because they reflect the light of the sun. As they are
much closer that the stars, we follow their movements across the sky.
The solar system stretches from the sun, at its centre, to Pluto, the most
remote planet, a distance of nearly 6.000 million km. Astronomers call Mercury,
Venus, the Earth and Mars the inner planets and Jupiter, Saturn, Uranus,
Neptune and Pluto the outer planets. Nearest to the sun is Mercury, the
smallest of the planets. It revolves very slowly on its axis, taking about 59
Earth days to make one revolution. This creates a very large difference in
temperature between the side facing the sun and the dark side. The side facing
the sun has a temperature of about 395 ºC; there is no atmosphere and the
surface looks very much like that of the moon.
The brightest object in the sky, apart from the sun and the moon, is Venus. It
is about the size of Earth, with an atmosphere of brilliant white clouds, which
prevents its surface from being seen clearly. There is no evidence of oxygen,
nitrogen or water vapour in its atmosphere, although there is plenty of carbon
dioxide. Plants and animals such as those which live on Earth could not survive
on Venus because the temperature under the clouds is probably about 475 ºC.
Mars is about half the size of Earth and, like Venus, has an atmosphere of
carbon dioxide. There are dark unaccountable markings on the surface of the
planet which seem to change with the period of the Martian year. Early
observers thought that they saw long straight lines which they called 'canals'.
The photographs sent back in 1969 and 1971 by the Mariner spacecrafts do not
reveal any such marking, but show a surface like that of the moon, dotted with
huge craters. At the poles there are large white ice-caps; these are probably
not frozen water but frozen carbon dioxide. It seems very improbable that life,
even of very elementary kind like lichens, could exist on Mars. Mars has two
tiny moons, Phobos (about 19 km in diameter and Deimos (about 10 km in
diameter).
Jupiter is the largest of the planets, nearly 143,000 km in diameter, and
weighs more that all the other planets put together. It takes nearly 12 years
for Jupiter to circle the sun once. It rotates on its axis faster that any
other planet and its day is only 10 hours long. The speed at its equator is
some 43 000 km per hour. This high speed of rotation causes a bulging at the
equator and a flattening at the poles. There is a deep layer of atmospheric
gases, probably made up of ammonia and methane; when seen through a telescope
from Earth, this deep atmosphere appears to be composed of coloured bands.
Jupiter has 12 moons, two of which, Ganymede and Callisto, are larger that our
own moon.
Saturn, the second largest planet (about 119,000 km in diameter), is the most
extraordinary of the planets because it is surrounded by a system of rings. For
a long time the rings puzzled astronomers, but they are now known to consist of
millions of separate solid particles revolving independently around Saturn like
millions of tiny moons. Evidence for this is that the inner edge of the rings
is moving faster that the outer edge and in some places stars can be seen
shining through them. In addition to the rings, 10 satellite moons of various
sizes revolve around Saturn.
The five planets nearest to the sun have been known from ancient times. They
are all bright and can be easily seen with the naked eye. But planets further
away were no observed until telescopes were invented. Uranus was discovered in
1781 by the great astronomer Sir William Herschel, Neptune was identified in
1846 and Pluto in 1930. One very odd thing about Uranus is that the axis on
which it rotates is in almost the same place as its orbit around the sun which
makes it look as if Uranus is spinning around on its side. Not very much is
known about Pluto but its orbit has been mapped and it looks as if for part of
its path is comes nearer to the sun than Neptune. The path of all the other
planets are quite separate. This suggests that Pluto may at one time have been
a satellite which escaped from the gravitational pull of Neptune and began to
orbit the sun.
Between the orbits of Mars and Jupiter there is a belt of small bodies, called
the asteroids or minor planets. Over 1600 of these bodies have been observed
and there are probably thousands more.
Chapter Four - The Stars
The stars in the northern hemisphere appear to circle around the Pole Star but
this is because the Earth is itself rotating. The farmers in Mesopotamia used
the position of certain stars to decide when to sow there seeds, and travellers
used the stars to guide them. Early observers grouped some stars together into
patterns such as the outlines of men and animals. We still call these groups,
or constellations, by their old names such as the Great Bear and the Southern
Cross. The Stars are so distant that we cannot see if they have planets
revolving around them.
Although the stars are like landmarks in the sky, they do not always stay the
same; they pass through various stages of development called stellar evolution.
A star is born when a mass of hydrogen gas collects in one area of space. The
atoms of the gas are attracted to one another by the gravitational forces
between them. A great ball of hydrogen forms, and the gravitational forces
compress the centre of the ball to such an extent that the temperature rises to
millions of degrees. At these temperatures, thermonuclear reaction begin,
concerting hydrogen into helium with the release of enormous quantities of
energy, some of which is the starlight that makes the star visible. We call the
stars which remain in this state for millions of years main- sequence stars.
When a star has consumed 10% ofits hydrogen, it expands into an enormous
Red Giant. In this state, stars consume hydrogen at a greater rate and some of
the helium is converted into the heavier elements. At this stage, the star may
explode and throw out some of the heavy elements into space. This rave event, a
supernova, has been observed only twive, in 1054 and in 1604.
A White Drawf, the star remaining after a supernova explosion, is much smaller
than a Red Giant. It is thought that bodies such as the Earth and the planets,
which are made of heavy elements, are products of a supernova explosion that
have been attracted into orbits around the sun (itself a main-sequence star).
After all its hydrogen is gone, a star finally dies and becomes a neutron star.