Heraklides of Pontus (388–315 BC)

Greek philosopher and astronomer who may have been the first to realize that the Earth turns on its axis, from west to east, once every 24 hours. He also thought that the observed motions of Mercury and Venus suggested that they orbited the Sun rather than the Earth.

Born in Heraklea, near the Black Sea, Heraklides migrated to Athens and studied at the Academy of Plato. He is said also to have attended the schools of the Pythagorean philosophers, and would thus have come into contact with Aristotle. All his writings are lost, so his astronomical theories are known only at second hand.

In proposing the doctrine of a rotating Earth (not to be accepted for another 1,800 years), Heraklides contradicted the accepted model of the universe put forward by Aristotle. Heraklides thought that the immense spheres in which the stars and planets were assumed to be fixed could not rotate so fast. He did not completely adopt the heliocentric view of the universe stated later by Aristarchus, but proposed instead that the Sun moved in a circular orbit (in its sphere) and that Mercury and Venus moved on epicycles around the Sun as centre.