Ptolemaic System
Ptolemaic System, in astronomy, theory of the order and action of the heavenly bodies. It was advanced in the 2nd century ad by the Alexandrian 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 (see Ecliptic ). 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 cl...