Callippus (370-310 BC) - Greek astronomer and student of Eudoxus who improved upon his theory by adding 8 more spheres to the universe, leading to a total of 34. He also took accurate measurements of the seasons to obtain a closer to true measurement of the length of the year. Cleomedes - Roman astronomer, wrote a manual on astronomy titled Circle Theory of the Celestial Phenomena, and correctly described the refraction of sunlight above the horizon as a cause of certain astronomical events.