Claudius Ptolemy (c. 100-170 CE) was a Greek-speaking astronomer, mathematician and geographer working in Alexandria under Roman rule, and one of the most influential scientific authors who ever lived. Little is known of his life; his importance lies entirely in his books, which became standard authorities for well over a thousand years.
His astronomical masterwork, the Mathematike Syntaxis, is universally known by the name Arabic translators gave it, the Almagest, the Greatest. In it Ptolemy set out a complete mathematical model of the heavens on a geocentric basis: the Earth motionless at the centre, the Sun, Moon and planets carried on combinations of circles, deferents and epicycles, with the refinement of the equant, calibrated to reproduce the observed motions with real precision. It also preserved a catalogue of over a thousand stars arranged in the constellations.
The achievement was that the system worked: it predicted planetary positions well enough to serve astronomers, astrologers and calendar-makers, and so it endured. The Ptolemaic cosmos remained the accepted picture of the universe throughout the medieval Islamic and Christian worlds, until Copernicus displaced the Earth from the centre in 1543 and Kepler replaced the circles with ellipses.