The Greek astronomer Hipparchus (active 162-126 BC) discovered the precession of the equinoxes, founded trigonometry, and compiled the first star catalog.

Born at Nicaea in Bithynia, Hipparchus studied astronomy, perhaps under Theodosius, and made some of his early observations in his native city. From at least 162 B.C. he was on the island of Rhodes, where he especially observed solstices, equinoxes, and lunar eclipses. His last recorded observation was made in 126 B.C. Hipparchus wrote on a variety of subjects connected with astronomy, but of his 14 works known once to have existed, only the commentary on Aratus's Phaenomena is extant. His astronomical work is known chiefly through the Almagest of Ptolemy and the writings of Strabo of Amisela.

Hipparchus seems to have initiated the study of plane trigonometry, devising for that purpose a table of chords of angles ranging from 0 to 180°. He also developed a method of...