Erastosthenes - Greek scientist, who was one of the first to attempt to determine the size of the Earth and came remarkably close to the actual number. Also was the first to determine the obliquity of the ecliptic. Eudoxas (408-355 BC) - Greek mathematician who was the first to describe a theoretical explanation of the movements of the planets. Each planet is attached to differing spheres which revolve around the Earth as its center. Each sphere is tilted in respect to the earth, thus accounting for the motions of the planets as seen in the night sky. He calculated there to be 26 spheres.