He did, but the earlier Greeks—Thales of Miletus and Anaximander, both dating from the early sixth century BCE—did not think that the world was round.

The real clinchers in evidence for a round Earth are the ships' masts becoming visible before the ships themselves; and the Earth casting a circular shadow on the Moon during lunar eclipses, no matter what the orientation of the Earth with respect to the Moon. I wrote a story once (of which ewx may possibly have read an early version) narrated from the perspective of a flat-Earther in the time of Columbus (by which time everyone and his dog knew the Earth was round—even in the time of the Church Fathers, Cosmas Indicopleustes and Lactantius were laughed out the building for trying to revive flat-Earth cosmology), the narrator of which went to great lengths to explain these phenomena without recourse to a round-Earth cosmology.