Ptolemy (c. AD 100-c. AD 170)

Egyptian astronomer and geographer. His Almagest developed the theory that Earth is the centre of the universe, with the Sun, Moon, and stars revolving around it. In 1543 the Polish astronomer Copernicus proposed an alternative to the Ptolemaic system. Ptolemy's Geography was a standard source of information until the 16th century.

The Almagest (he called it Syntaxis) contains all his works on astronomical themes, the only authoritative works until the time of Copernicus. Probably inspired by Plato, Ptolemy began with the premise that the Earth was a perfect sphere; all planetary orbits were circular, but those of Mercury and Venus, and possibly Mars (Ptolemy was not sure), were epicyclic (the planets orbited a point that itself was orbiting the Earth); and the sphere of the stars formed a dome with points of light attached or pricked through.

He may have been born in the town of Ptolemais Hermii, on the banks of the Nile. He worked in Alexandria and had an observatory on the top floor of a temple.

His chief works are the Mathematike Syntaxis (a summary of Greek astronomy) and the Geographike Huphegesis in eight volumes. Other works are De Apparentiis et significationibus Inerrantium, De Analemmate, and Planispherum. He also wrote on the musical scale and possibly on optics. He produced vivid maps of Asia and large areas of Africa.

In his thesis on astrology, Tetrabiblios, Ptolemy suggests that some force from the stars may influence the lives and events in the human experience.