Who first proposed the principle of the heliocentric theory?

Aristarchus of Samos, Aristarchos (approx. 320 approx. 250 BC), was the Greek astronomer and mathematician from Alexandria. He believed that the Earth revolves around the Sun, which is the centre of the world, and the occurrence of the day and night is related to the Earth's rotation around its axis. He also tried to mathematically determine the size of the Sun and the Moon, as well as their distance from Earth.