Heraclides of Pontus was a Greek philosopher who proposed the theory that the apparent daily revolution of the fixed stars is in fact due to the actual daily rotation of the earth about its own axis.
Background
Heraclides of Pontus was born circa 388 BC in Heraclea Pontica (present Karadeniz Ereğli, Turkey), the son of Euthyphron, a wealthy nobleman. He was a descendant of one of the founders of Heracleia. He was probably forced to leave Heracleia in 364/363 B. C. because of the tyranny of Clearchus.
Education
Heraclides studied at the Platonic Academy in Athens under its founder Plato and under his successor Speusippus. He also attended Aristotle’s lectures.
Career
Upon the death of Speusippus (339 BC), Heraclides was one of the candidates to succeed him, but his candidacy was defeated by Xenocrates and he returned to Pontus.
Heraclides was a prolific writer, composing dialogues on ethics, natural science, literary criticism, music, rhetoric, and the history of philosophy. However, his works are now known only through some quotations by later writers. His most interesting theories are those in astronomy. Contradicting the Platonic and Aristotelian doctrine that the earth must stand still, he hypothesized that it rotates once daily on its own axis. On this assumption he was free to assert as a corollary that the universe is infinite, then to speculate on the existence of other earths in the stellar systems that appear to be fixed stars. This daring theory could not explain as much about motion as did Aristotle's and therefore was regarded merely as an oddity.
Heraclides also possibly indicated, in one fragment which can be otherwise interpreted, that he believed that Venus and Mercury revolve around the sun, a theory also contrary to Aristotelian physics. This deduction, if his, was probably due only to the identity of the sidereal periods of the sun and the two inferior planets, as these sidereal periods were the common criteria for determining the order of the planetary spheres.
Another theory doubtfully attributed to Heraclides is that the apparent anomalous motion of the sun can be explained as well by the earth's revolving about the sun as by the sun's about the earth. The mathematical identity of the geocentric and heliocentric systems, however, is irrelevant to the problem of anomalous motion, and the meaning of the fragment is thereby rendered very dubious indeed.