Claudius Ptolemy was born 85 AD in Egypt. One of the most influential Greek astronomers and geographers of his time, Ptolemy propounded the geocentric theory that prevailed for 1400 years. Ptolemy made astronomical observations from Alexandria Egypt during the years AD 127-41. He probably spent most of his life in Alexandria.
Ptolemy's two major works are the Almagest and the Geography . The Almagest is the earliest of his works and gives in detail the mathematical theory of the motions of the Sun, Moon, and planets. Ptolemy made his most original contribution by presenting details for the motions of each of the planets. TheAlmagest was not superseded until a century after Copernicus presented his heliocentric theory in the De revolutionibus of 1543.
Ptolemy's geometric models, used only to predict the positions of these bodies, employed combinations of circles known as epicycles, within the framework of the basic Earth-centred system supplied by Aristotle.
Ptolemy devised new geometrical proofs and theorems. He obtained, using chords of a circle and an inscribed 360-gon, the approximation
= 3 
/
= 3.14166
and, using 
3 = chord 60
,
3 = 1.73205.
In a book entitled Analemma he discussed the projection of points on the celestial sphere. In Planisphaerium he is concerned with stereographic projection.
Ptolemy died about 165 in Alexandria, Egypt.
