Hipparchus 1. Younger son of Peisistratus, tyrant of Athens, who ruled Athens jointly with his brother Hippias after the death of their father in 527 BC. He was a patron of literature and art. He was murdered by Harmodius and Aristogeiton in 514 BC. See PEISISTRATUS
2. (c.190–after 126 BC), the greatest of ancient astronomers, a Greek born at Nicaea in Bithynia but living mostly in Rhodes. His only surviving work is his commentary on the Phaenomena of Eudoxus and Aratus; Hipparchus criticizes the accuracy of Aratus' source, Eudoxus. Most of our knowledge of his other work comes from the Almagest, the astronomical textbook of Ptolemy (second century AD). His principal work was his catalogue of more than 800 fixed stars arranged in categories of magnitude and brightness, their positions fixed by latitude and longitude in relation to the ecliptic (the geocentric path of the sun). He is famous for his discovery of the precession of the equinoxes (i.e. the slight change in timing in each successive year), achieved by comparing his observation of the position of a particular star with the observation made 160 years earlier; his figure differs by only ten seconds from the modern estimate. He calculated more accurately the length of the natural year (from one equinox to the same equinox again), but his retention of the erroneous geocentric theory (as opposed to Aristarchus' heliocentric theory), made his calculations on the relative size and distance of the sun and moon inaccurate. He is the first person known to have made systematic use of trigonometry, and was probably the inventor of stereographic projection. His researches on the sphere led him to criticize the geographical procedures of Eratosthenes; he himself applied more rigorous mathematical methods to specify location, and again was apparently the first to use latitude and longitude systematically.
