Ptolemy, Latin Claudius Ptolemaeus c.90-168AD
Egyptian astronomer and geographer

He flourished in Alexandria. His 'great compendium of astronomy' seems to have been denominated by the Greeks megiste ('the greatest'), from which the Arab name Almagest by which it is generally known was derived. His Tetrabiblos Syntaxis is combined with another work called Karpos or Centiloquium, because it contains a hundred aphorisms - both treat astrological subjects, so have been held by some to be of doubtful authenticity. There is also a treatise on the fixed stars or a species of almanac, the Geographia and other works dealing with map-making, the musical scale and chronology. As astronomer and geographer, Ptolemy was the main influence on scientific men down to the 16th-17th centuries, but he seems to have been not so much an independent investigator as a corrector and improver of the work of his predecessors. For example, in astronomy he depended largely on Hipparchos. However, as his works form the only remaining authority on ancient astronomy, the system they expound is called the Ptolemaic System. The system of Plato and Aristotle, this was an attempt to reduce the common understanding of the motions of the heavenly bodies to scientific form. The Ptolemaic astronomy, handed on by Byzantines and Arabs, assumed that the Earth is the centre of the universe, and that the heavenly bodies revolve round it. Beyond and in the ether surrounding the Earth's atmosphere were eight concentric spherical shells, to seven of which one heavenly body was attached, the fixed stars occupying the eighth. The apparent irregularity of their motions was explained by a complicated theory of epicycles. As a geographer, Ptolemy is the corrector of a predecessor, Marinus of Tyre. His Geography contains a catalogue of places; with latitude and longitude; general descriptions; and details regarding his mode of noting the position of places. He also calculated the size of the Earth, and constructed a map of the world and other maps. His Earth-centred view of the universe dominated cosmological thought until swept aside by Copernicus in the 16th century.