Claudius Ptolemy. Portrait of the Egyptian-Greek astronomer Claudius Ptolemy (about AD 90-170), or Ptolemy of Alexandria. He was famous for his book Almagest, the Greatest. This summarised five centuries of Greek astronomy and held the incorrect belief that the Sun and planets orbited the Earth. Ptolemy described the motions of these bodies on a geocentric basis. Their orbits were mainly circular but he had to use devices such as epicycles to match the theory to 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. This portrait is by Raphael (1483-1520).