Who was the ancient Greek astronomer who first proposed a heliocentric model?

The ancient Greek astronomer who first proposed a known heliocentric model was Aristarchus of Samos.

Aristarchus lived in the 3rd century BC and argued that Earth rotates on its axis and travels around the Sun. This placed the Sun at the center of the planetary system, in contrast to the geocentric models later associated especially with Aristotle and Ptolemy.

His original work describing the heliocentric system has not survived. The main ancient account comes from Archimedes' Sand-Reckoner, which summarizes Aristarchus's proposal. Aristarchus also studied the relative sizes and distances of the Sun and Moon. Although his numerical estimates were inaccurate, they helped him reason that the much larger Sun was a more plausible central body.

The model did not become the dominant view in antiquity. Most later astronomers favored a stationary Earth, and Ptolemy's geocentric system shaped astronomy for many centuries. Nicolaus Copernicus revived heliocentrism in the 16th century, but Aristarchus is generally recognized as its earliest known Greek advocate.