Aristotle was a famous Greek philosopher and scientist who was the student of Plato. He studied many sciences including astronomy to which he contributed many new and unique ideas. Aristotle wrote down a collection of the ideas of the ancient Greeks on the universe and included his own opinions. Some of these opinions lasted the test of time, standing firm for over 2000 years.
Composition of the Universe
Aristotle believed that the central part of the universe was composed of four elements: earth, air, fire, and water. Each of these elements had its proper place depending on its relative heaviness or "specific gravity". These elements moved in a straight line, earth-down and fire-up into its proper place where it will be at rest. As a result, the terrestrial motion is always linear and comes to a halt. Aristotle explained the constant circular motion of the heavens by the existence of a fifth element which he called aither. Aither does not have the ability to change only the ability to change its position. Aristotle also believed that heavier bodies of a given material, when their shapes are the same, would fall faster than lighter bodies. This theory was accepted as fact until Galileo disproved it with his famous experiment when he dropped weights from the Leaning Tower of Pisa.
Aristotle's Planetary System
Aristotle believed in a finite, spherical universe where the Earth was located at its center. He based this theory upon observations he made of the Moon during an eclipse. He said that the circular shadow that the Earth cast on the Moon during an eclipse proved that the Earth was a sphere. The ancient Greeks already discovered that the stars in the sky change location depending on where you are on Earth. He used this common knowledge to further justify his theory. In Aristotle's planetary system the Earth was stationary in the middle and the Moon and the Sun were nearest and orbited the Earth. He justified the Earth being stationary because he believed that the stars were static and did not move. To explain why the Moon and Sun where closest to the Earth he relied on his observations of Mars. At certain points in its orbit the light from Mars was blocked by the Moon. He also was aware that the Egyptians and Babylonians had observed this also.