Claudius Ptolemy was born in 85 AD in Egypt. One of the most famous Greek astronomers he introduced the geocentric theory, stating that the earth was the center of the Solar System. This theory dominated science for more than fourteen hundred years. Spending most of his life in Alexandria, Egypt, Ptolemy made astronomical observations for about twenty-four years. Two of Ptolemy's works are the Geography and the Almagest. In the Almagest, Ptolemy gives the mathematical theory of the motions of the planets, Moon, and Sun. He supplied details for the movements of the planets. It wasn't until 1543 that Ptolemy's work was outdated, by Copernicus's heliocentric theory. He approximated pi, 3.14166. Ptolemy died in Alexandria, Egypt, in or around 165 AD.