a famous author who lived at Alexandria in the second century after Christ. Little is known of his life, but he left a work on astronomy and another on geography. The latter was the standard for geography until the discoveries of the fifteenth and sixteenth centuries. The former contained all that was known of astronomy at that time and formed the basis of all astronomy until the beginning of the sixteenth century. During the Middle Ages the book was lost, but was found in an Arabic translation. According to the Ptolemaic system, the earth is a globe and the center of everything. Around it revolves the hollow sphere of the heavens, so large that in comparison with it the earth is but as the point of a pin. The moon and the sun revolve around the earth, but in circles of which the earth is not the center. There were seven planets, arranged according to distance from the earth in this order: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn. See COPERNICUS; PLANET.