PTOLEMICSYSTEM

PTOLEMAIC SYSTEM

, in astronomy, theory of the order and action of the heavenly bodies, advanced in the 2d century ad by the Greek astronomer Ptolemy. The Ptolemaic theory held that Earth is stationary and at the center of the universe; closest to Earth is the Moon, and beyond it, extending outward, are Mercury, Venus, and the Sun in a straight line, followed successively by Mars, Jupiter, Saturn, and the so-called fixed stars. Later, astronomers supplemented this system with a ninth sphere, the motion of which supposedly produced the precession of equinoxes. A tenth sphere or primum mobile, which was thought to motivate the other heavenly bodies, was also added. To explain the various observed motions of the planets, the Ptolemaic system described them as having small circular orbits called epicycles; the centers of the epicycles, on circular orbits around Earth, were called deferents. The motion of all spheres is from west to east. After the decline of classical Greek culture, Arabian astronomers attempted to perfect the system by adding new epicycles to explain unpredicted variations in the motions and positions of the planets. These efforts failed, however, to resolve the many inconsistencies in the Ptolemaic system, which was finally superseded in the 16th century by the Copernican system.