In The Almagest, Ptolemy outlines antiquity’s geocentric model of the universe. The model is based on the assumptions that the universe is spherical, the Earth is a sphere, the Earth is at the center of the universe, and the Earth does not move. Although modern scientific advances have determined that many of these assumptions are false, Ptolemy correctly conjectures that the Earth is spherical (and yes, I know that the Earth is not a perfect sphere) which is an accomplishment considering that Ptolemy wrote The Almagest in 150 AD, a time when modern astronomical observation instruments were unavailable.
Great Books of the Western World 