Hipparchus of Nicaea was a Greek astronomer, geographer, and mathematician of the Hellenistic period. His recorded astronomical observations place his working life between approximately 162 and 127 BCE. Born in Nicaea in Bithynia, he conducted his research from there and the island of Rhodes, where he made his discovery of the precession of the equinoxes in 128 BCE.

His major scientific treatises are lost but are known through later summaries, chiefly by Claudius Ptolemy. His only surviving work is a prose Commentary on the Phaenomena of Aratus and Eudoxus. His other writings included works on the length of the year, the motions of the sun and moon, and a foundational star catalog.

Hipparchus is considered the greatest astronomical observer of antiquity. He founded trigonometry, discovered the precession of the equinoxes, and compiled the first comprehensive star catalog. His critical, empirical methodology and mathematical models directly influenced Ptolemy and dominated Western and Islamic astronomy for centuries.