Claudius Ptolemy (2nd century AD) - an ancient Greek scholar, the author of the treatise “Great Mathematical Composition of Astronomy in Thirteen Books” (the Almagest, in its Arabic translation), which for more than a thousand years remained the compendium of astronomical knowledge. In the Almagest Ptolemy determined the length of the year, gave methods of calculating lunar and solar eclipses, described the astrolabe, included a catalogue of 1028 stars, explained the phenomenon of precession and was the first to introduce a correction for refraction, among much else. He gave a finished form to the geocentric theory of the universe (the Ptolemaic system of the world), which had been developed before him by Eudoxus of Cnidus, Aristotle and Hipparchus. Although Ptolemy's system of the world explained the visible motions of the planets and made it possible to calculate their future positions, it was very complicated and mistaken in its foundation. In time it was replaced by the system of the world of N. Copernicus. Ptolemy's works also concerned geography and optics (he studied the refraction of light).