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Ariel
Ariel
The fourth largest satellite of Uranus, 1,158km across. It takes just over two days for Ariel to orbit Uranus at a distance of 191,000km from the surface. It is an icy world, but there are large trenches and craters on its surface, suggesting that it may have been geologically active in the past.
Aristotle
Aristotle lived in Greece between 384 and 322 b.c. He was one of the most outstanding thinkers and scientists of all time.
He was fascinated by many things, including the nature of the Earth and the way that planets and stars moved in the sky.
He was the first to figure out that the Earth was a sphere and not a flat disk, as had previously been believed. He also made the first calculation of the diameter of the Earth.
From his studies, and the works of earlier writers, he concluded that the Earth was the center of the Universe and that the other planets, the Sun, and all of the other stars revolved around it.
This view was changed slightly by Ptolemy, but was unchallenged until the work of Copernicus
and Galileo in the 17th century a.d. By this time the Earth as
the center of the Universe was a traditional part of the thinking of the Christian church. Changing these theories to the ones we now believe in was a difficult process, as is reflected in the life of Galileo.
Asteroid number
Asteroids are each numbered as they are discovered. There is no other meaning to the number.
Ida shown in the photo top right is asteroid number 243.
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Asteroids
There are many rocky fragments in our Solar System that have never been swept together to
form planets. Most of
the larger ones—called asteroids—swarm in a
belt between Mars and Jupiter, although a few have paths that take them near the Earth; they are known as the Apollo group.
To understand how asteroids were formed, it is important to look back to the formation of the Solar System. Very early on in the history of the Solar System, when all of the planets were forming from accretion (the collision of small planets or planetesimals), one of these planets grew at a far faster rate than the others. This
 Asteroids—The asteroid belt lies between Mars and Jupiter. Both planets are shown here for reference purposes.
 Asteroids—Ida is a potato-shaped asteroid with a “moon” or satellite, Dactyl, seen far right. Ida is in the main asteroid belt shown below.
planet became Jupiter. As Jupiter grew, its gravity affected large planetesimals—some the size
of the Earth. A number of the planetesimals were pulled toward Jupiter, and others were flung from the Solar System.