Ptolemy, astronomer, geographer and mathematician who lived in the Greek culture of Roman Egypt. Ptolemy is one of the most famous astronomers of antiquity. The Ptolemaic system described in his Almagest documented previous Greek thought, and dominated Western science for over a thousand years. The Almagest summarised five centuries of Greek astronomy and represented the conventional view of the Universe until the Copernican revolution. He described the motion of celestial bodies on a geocentric basis with mainly circular orbits but he had to introduce various devices such as epicycles to match the theory with the observations. He also worked out distances and sizes for the Sun and the Moon, compiled a catalogue of 1028 stars and described various astronomical instruments. Taken from a 14th century fresco.