Pythagoras is credited as the first Greek to think the earth spherical, but this idea was probably founded on mystic reasons rather than scientific. The Pythagoreans found conclusive evidence in favour of a spherical earth after it was discovered that the moon shines by reflecting light, and the right explanation for eclipses was found. The earth’s shadow on the moon’s surface suggested that the shape of our planet was spherical.
Aristotle’s book “On the Heavens” summarizes some astronomical notions before his time. He says, for example, that Xenophanes of Colophon claimed the earth below us is infinite, that it has “pushed its roots to infinity”; others believed the earth rested upon water, a claim whose original author seems to be Thales (according to Aristotle); Anaximenes, Anaxagoras, and Democritus believed the earth was flat which “covers like a lid, the earth beneath it”.