Colour portrait of the Egyptian-Greek astronomer Claudius Ptolemy. Ptolemy, who lived in the first and second century A.D., is famous for his book called Almagest, the Greatest. The Almagest summarised five centuries of Greek astronomy and represented the human view of the Universe until the Copernican revolution. He described the motion of celestial bodies on a geocentric basis. Their orbits were mainly circular 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.