Dr. Hans Mark - "Exploring the Earth and Cosmos: A Study of Western Civilization" 

Aristotle did _not_ believe that the Earth was flat. The reason for his preference of circular celestial motions was precisely because of his belief in the spherical nature of the heavens. Sphericity was a generally accepted view among learned Greeks. Aristotle's teacher Plato for that matter even gave several proofs for the Earth's sphericity. Eratosthenes' achievement was not of any conceptual nature, it was merely in the devising of a method for measuring the circumference. "Flat earthers" generally don't subscribe to a spherical heaven because that would require some serious explaining for the vertical antipode of the flat Earth. "Flat earthers" rather subscribe to a domed heaven which makes any circular motion of heavenly bodies absurdly impractical.