Claudius Ptolemy (Ptolemais Hermiou, c. 100 / Canopus, c. 170) was a Greek astronomer, astrologer, geographer, chemist, and mathematician. He was the author of the astronomical treatise known as Almagest. Heir to the conception of the universe given by Plato and Aristotle, his method of work differed markedly from theirs, for while Plato and Aristotle give a worldview of the universe, Ptolemy was an empiricist. His work consisted of studying the large amount of existing data on the motion of the planets in order to build a geometric model that explained these positions in the past and was able to predict their future positions. The orbits of his system are eccentric, as opposed to the circular and perfect ones of Plato and Aristotle. The Almagest contains a catalog of stars that Ptolemy took from a lost work of Hipparchus of Nicea. Although Ptolemy affirmed that he observed the catalog, it appears from multiple lines of evidence that the catalog was the work of Hipparchus. The Almagest also established criteria for predicting eclipses.
