Hipparchus

Hipparchus (c. 190-120 bc), the most important Greek astronomer of his time. Hipparchus was born in Nicaea, Bithynia (now İznik, Turkey). He was extremely accurate in his research, a record of which was preserved in the Almagest, the scientific treatise by the Alexandrian astronomer Ptolemy, who was greatly influenced by Hipparchus. By comparing his own celestial studies with those of earlier astronomers, Hipparchus discovered the precession of the equinoxes (see Ecliptic). His calculation of the tropical year, the length of the year measured by the sun, was within 6.5 minutes of modern measurements. Hipparchus devised a method of locating geographic positions by means of latitudes and longitudes. He cataloged, charted, and calculated the brightness of perhaps as many as 1000 stars. Hipparchus also compiled a table of trigonometric chords that became the basis for modern trigonometry. See Astronomy; History of Astronomy.