Hipparchus (astronomer)

Hipparchus fl. 2d cent. BC, 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.

Hipparchus of Nicaea (c.190–c.120bc)Greek astronomer, geographer, and mathematician, born in modern Turkey. He put Greek astronomy on a more scientific footing, introducing arithmetic and early trigonometric methods. His many accurate astronomical observations resulted in a catalogue of 850 stars, in which he gave their coordinates and divided them into six magnitude classes. Ptolemy incorporated the catalogue and other findings by Hipparchus in the Almagest. Hipparchus made surprisingly accurate measurements of the precession of the equinoxes, the length of the year, and (from observations of eclipses) the Moon's distance. He may have been the inventor of the astrolabe.

Hipparchus (active 2nd century bc) Greek astronomer. Hipparchus estimated the distance of the Moon from the Earth and drew the first accurate star map. He developed an organization of the universe that, although it had the Earth at the centre, provided for accurate prediction of the positions of the planets.