Aristarchus of Samos (c.310-c.230 BCE): Greek astronomer, discoverer of heliocentric system.
Aristarchus of Samos was the first to catch a glimpse of the immense size of the universe. Until then, everyone had believed that the moon, sun, planets, and stars all revolved around the earth. Perhaps the stars were as far away as twenty of forty radiuses of the earth, perhaps hunderd or thousand (the maximum size according to Archimedes of Syracuse), but nobody thought that it would be much bigger.
But Aristarchus argued that the sun was about nineteen times the size and distance of the moon. The consequence of his calculation was that the sun was larger than the earth, and this in turn led to the conclusion that it was possible that it was not the sun, but the earth that was moving. Although several ancient astronomers, a.o. Seleucus of Babylon, subscribed to this correct interpretation of the phenomena, it was ignored until Copernicus. The correctness was not proven until Bessel in 1839 measured the parallax.