He established a system of mathematical astronomy which was used until the 16th century.
Image : The Ptolemaic system

Ptolemy placed the earth at the center of the universe, around it went the moon, sun, and planets at various speeds. His theories are preserved in a 13-volume work known as the 'Almagest'.

In addition Ptolemy wrote on geometry, a five-book treatise on optical phenomena, a three-book treatise on music, and eight books of geographical text and maps. In it he underestimated the size of the ocean, a mistake that later encouraged Columbus to undertake his famous voyage.