Which Greek astronomer first discovered the precession of the equinoxes?

The Greek astronomer generally credited with first discovering the precession of the equinoxes was Hipparchus. Working in the 2nd century BC, he compared his own stellar observations with earlier measurements attributed to Timocharis and Aristillus.

Hipparchus noticed that the position of the bright star Spica relative to the autumnal equinox had shifted by about 2 degrees. He concluded that the equinoctial points themselves were slowly moving westward against the background of the stars. This gradual change is caused by the slow wobble of Earth’s rotational axis.

He also distinguished the tropical year—the time between successive equinoxes—from the sidereal year, measured against the distant stars. His estimate of the rate was too slow by modern standards, but his underlying discovery was correct. Claudius Ptolemy later preserved Hipparchus’ account in the Almagest. Hipparchus is also associated with an early star catalogue and major advances in trigonometry, but precession remains his most famous astronomical discovery.