Hipparchus of Nicaea (Ἵππαρχος ὁ Νικαεύς) was a Greek astronomer, geographer, and mathematician of the 2nd century BCE. He conducted his recorded astronomical observations, primarily from Rhodes, between 162 and 127 BCE [1][2]. His work marks a decisive shift from speculative cosmology to a rigorous, mathematical, and observation-based science [1].

His original writings are almost entirely lost, known chiefly through later sources like Ptolemy’s Almagest [1][2]. One surviving work is his Commentary on the Phaenomena of Aratus and Eudoxus, which critiques the astronomical inaccuracies in those earlier texts [1][2]. Lost contributions, known by reference, include the first comprehensive star catalog, studies on trigonometry, solar and lunar theory, and the discovery of the precession of the equinoxes [1][2].

Hipparchus established astronomy as a precise, predictive mathematical science. His trigonometric models and systematic observations set the standard for antiquity, making him a foundational figure upon whom Ptolemy heavily relied [1][2]. He is often considered the true founder of systematic astronomy [2].

Sources 1. Stanford Encyclopedia of Philosophy: https://plato.stanford.edu/entries/hipparchus/ 2. Encyclopædia Britannica: https://www.britannica.com/biography/Hipparchus-Greek-astronomer