One of the most influential Greek astronomers and geographers of his time, Ptolemy propounded the geocentric theory in a form that prevailed for 1400 years. We do not know much about his life but most of his major works have survived. His earliest and most important piece is the Almagest which gives in detail the mathematical theory of the motions of the Sun, Moon and planets. This thirteen-book treatise was not superseded until a century after another mathematician, Copernicus, presented his heliocentric theory in the De revolutionibus of 1543.