Aristarchus of Samos
Aristarchus was considerably ahead of his time. While he preceded Ptolemy, he is among the earliest to propose a heliocentric view, in which the Earth orbits the Sun. His ideas were rejected in favor of the incorrect geocentric models of Aristotle and Ptolemy, for centuries.
Unfortunately much of his writings were lost. However Archimmedes references his works and describes his heliocentric ideas. Nicolaus Copernicus, who is credited with formalizing a heliocentric view of the solar system, also credits Aristarchus as the originator of the heliocentric theory.
In addition to this, Aristarchus also correctly suspected that the stars were other suns, and that they were far enough away to have no observable
parallax. This was unprovable at the time, since
Stellar Parallax is only detectable with telescopes.
Ironically, the only surviving work attributed to Aristarchus uses geometry to calculate the size and distance to the sun and the moon, in a
geocentric view. His calculations were incorrect due to the lack of precision in the measurements he had access to, but his geometry was sound. The descriptions from Archimedes of his other works show that he had better numbers and calculations at other times, and his surviving work isn't the best example.