Aristarchos (310–230 BCE)

Aristarchos of Samos calculated the Earth's circumference very well, and realized that the Earth must revolve around the Sun. Nobody liked this, so we had to wait 1700 years for Nicolaus Copernicus (1473–1543) to once more put forth the idea in the public sphere.

It has been said that Cleanthes (a Stoic) considered Aristarchos a dangerous enemy, (perhaps due to the 'providence or atoms' issue?)