Hipparchus (Greek Ἵππαρχος; ca. 190 BC – ca. 120 BC) was a Greek astronomer, geographer, and mathematician. He lived in the Hellenistic period. He was born in Nicaea (now Iznik, Turkey), and probably died on the island of Rhodes. He worked as an astronomer at least from 147 BC to 127 BC. Hipparchus is considered the greatest ancient astronomical observer. Some people see him as the greatest overall astronomer of antiquity.
Hipparchus was the first Greek whose quantitative and accurate models for the motion of the Sun and Moon survive. To do this, he used the observations and perhaps the mathematical techniques accumulated over centuries by the Chaldeans from Babylonia. He had a trigonometric table. It looks like he solved some problems of spherical trigonometry with the table. With his solar and lunar theories and his trigonometry, he was perhaps the first to develop a reliable method to predict solar eclipses. His other well-known achievements include the discovery of precession, and the compilation of the Hipparchus Catalog, the first comprehensive star catalog of the western world.
He might also have invented the astrolabe, and the armillary sphere which first appeared during his century and which he used in making the star catalog. It would be three centuries before Claudius Ptolemaeus' synthesis of astronomy would make the work of Hipparchus obsolete; it is heavily dependent on it in many areas.
Craters on the Moon and on Mars are named for Hipparchus, as are an asteroid and the Hipparcos satellite.