Almost nothing is known about the life of Ptolemy. It is generally believed that he was born at Ptolemaius Hermii, a Grecian city in Egypt. The period of his life is estimated from the dates of his astronomical observations. It is believed that he made his observations and wrote in Alexandria.

The most important of Ptolemy's works was the Almagest, a thirteen-volume abstract of the astronomical science of the Alexandrian Greeks. In it Ptolemy explained his system of astronomy, giving Hipparchus credit as his chief authority. The system Ptolemy advanced was one in which the earth was a motionless globe. Around it revolved a spherical transparent shell in which were located forty-eight constellations and seven planets: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. While making his observations, he discovered the irregular motion of the moon in orbit called

*evection*.

Although Ptolemy is thought to have studied mathematics to make his astronomical observations more meaningful, his mathematical discoveries proved to be of greater value than those in astronomy. By explaining the mathematical theories of Hipparchus, he is considered by some to be the founder of plane and spherical trigonometry.

As a geographer, Ptolemy was almost entirely in error. He did, however, summarize all that the ancient world knew about the surface features of the earth. His main contribution to geography was an eight-volume

*Guide to Geography*.