Hipparchus, c.555-514 B.C., Athenian political figure, son of Pisistratus. After the death of his father, he was closely associated with his brother Hippias, tyrant of Athens, in ruling the Athenian city-state. Under Hippias he was a patron of the arts and sponsored the poets Anacreon and Simonides. He was slain by Harmodius and Aristogiton because of his personal vices.

Hipparchus, fl. 2d cent. B.C., Greek astronomer, b. Nicaea, Bithynia. He is the first systematic astronomer of whom there are records. He made his observations chiefly on the island of Rhodes. Ptolemy's geocentric theory of the universe was based largely on the conclusions of Hipparchus, a record of whose researches is preserved in the Almagest of Ptolemy. In it Hipparchus is credited with the discovery of the precession of the equinoxes, the eccentricity of the sun's apparent orbit, and certain inequalities of the motions of the moon. He determined the lengths of the seasons and accurately measured the year. He calculated the sizes of the sun and moon using eclipses. He also made the first known comprehensive chart of the heavens giving the positions of at least 850 stars, and he divided them into brightness classes, a system of magnitudes later expanded by Ptolemy. Hipparchus suggested a method of determining longitude by observing the parallax of the moon in eclipse. He is believed to have been the first to make systematic use of trigonometry, and he computed a table of chords roughly equivalent to trigonometrical sines. Only one of his works, a commentary on the work of Aratus and Eudoxus, survives.

or Hipparchos

(born , Nicaea, Bithynia—died after 127 BC, Rhodes?) Greek astronomer and mathematician. He discovered the precession of the equinoxes (see equinoxes, precession of the), calculated the length of the year to within 6.5 minutes, compiled the first known star catalog, and made an early formulation of trigonometry. His observations were painstaking and extremely accurate. He rejected all astrology but also Sun-centred views of the universe; his views had a profound influence on Ptolemy. His star catalog logged the positions of the stars in terms of celestial coordinates, listed about 850 stars, and specified their brightnesses by a system of six magnitudes similar to today's. He adequately accounted for the irregularities in the Moon's motion that are due to its elliptical orbit. His main contribution to geography was to apply rigorous mathematical principles to the determination of places on Earth's surface, and he was the first to do so by specifying latitude and longitude.

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