Roman name: Claudius Ptolemaeus. Greek philosopher who synthesized and extended Hipparchus's system of epicycles and eccentric circles to explain his geocentric theory of the solar system. Ptolemy believed the planets and Sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations, although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit. Ptolemy, who also wrote Tetrabiblos, a work on astrology, described his system in the book Mathematical Syntaxis (widely called the Almagest). The Almagest also included a star catalog containing 48 constellations, using the names we still use today. Ptolemy also investigated the refraction of light in water, but altered his data to fit the form of the mathematical law he postulated to be correct.

Additional biographies: MacTutor (St. Andrews), Firenze, Bonn, Greek and Roman Science and Technology