The theory, developed by Aristotle in the fourth century B.C.E., that the universe consisted of five elements and eight spheres. The elements were earth, air, fire, water, and ether, the sub¬ stance of which the spheres themselves were composed. Earth itself (known to be spherical because its shadow could be seen on the Moon) was stationary and held an immobile position at the center of the eight spheres. The eight etherial spheres revolved around the immovable Earth, carrying with them the Sun, the Moon, each of the known planets (five at the time), and all the stars.
Aristotle’s system of celestial mechanics was based on commonsense naked-eye observations of how the stars appear to move through the sky. Later sky-watchers, however, noted some discrepancies between Aristotle’s system and their own observations. Sometimes the planets seemed to stop in the sky or even to move backward. In the second century C.E., for example, the Alexandrine astronomer Claudius Ptolemy explained these motions through the use of epicycles small spheres within the great etherial spheres in which the planets rotated separately. Ptolemy’s star charts, which used as many as 90 epicycles to account for variations in Aristotle’s celestial spheres, were used by mariners for hundreds of years as navigational aids.