Hipparchus of Rhodes (also seen as Hipparchus of Nicaea, as he was born there; c. 190-c. 120 B.C.E.) was one of the greatest Greek astronomers. A partial list of his discoveries includes: being the first to discover the precession of the equinoxes, compiling an extensive star catalogue, assigning “magnitudes” as a measure of stellar brightness, and calculating the length of the year to within 6.5 minutes of the correct value. His planetary models were mathematical, not mechanical. And although Hipparchus did not invent it, he was probably the first person to systematically use trigonometry, which was a necessity for most of his discoveries.
The famous astronomer Nicolaus Copernicus (pictured here) is commonly thought of as the first person to propose a heliocentric (Sun-centered) model of the solar system, but actually Aristarchus surmised the truth centuries before him.