Hipparchus of Nicaea

Hipparchus was born in Nicaea (in what is now northwestern Turkey) around 190 BCE and did most of his observational work on the island of Rhodes. He is widely regarded as the greatest astronomer of antiquity. Almost none of his original writings survive, but his work is known in detail from quotations, tables, and references in later authors, particularly Ptolemy's Almagest.

His most famous discovery, around 130 BCE, was the precession of the equinoxes: comparing the celestial longitudes of stars in his own observations with those catalogued by Timocharis of Alexandria 150 years earlier, he noticed a systematic shift of 2° to the east — roughly 48 arcseconds per year. He correctly inferred that this was a slow motion of the Earth's rotational frame (or, in ancient geocentric terms, of the celestial sphere relative to the ecliptic), not of the stars themselves. The physical cause — the torque of the Sun and Moon on the Earth's equatorial bulge — would not be known for 1,800 years, until Newton derived it from gravitational theory.

Hipparchus also compiled the first systematic star catalogue of the Western tradition (containing around 850 stars, classified by apparent magnitude on a scale whose logic survives today), developed an early version of trigonometry including a table of chords, and worked out accurate values for the lunar and solar distances and the length of the tropical year. He is a foundational figure in ancient mathematical astronomy.