Aristarchus of Samos, a student of Strato of Lampsacus who flourishes in the mid-third century BCE, champions the notion of a Sun-centered universe and makes a pioneering attempt to determine the sizes and distances of the Sun and Moon.

Aristarchus’ only surviving work, On the Sizes and Distances of the Sun and Moon, details his remarkable geometric argument, based on observation, whereby he determined that the Sun is about twenty times as distant from the Earth as the Moon, and twenty times the Moon's size. (Both these estimates are an order of magnitude too small, but the error stems from Aristarchus' lack of accurate instruments rather than in his faultless method of reasoning. Scant evidence exists concerning the origin of his belief in a heliocentric world system; his theory, rejected by his contemporaries, is known only because of a summary statement in Archimedes' The Sand Reckoner.)