Eratosthenes alleged measurement has been shown to not be so genius after all www.youtube.com/watch?v=fIhgFT-OipU The story makes for a great math concept but doesn't prove the earth to be a sphere rotating around the sun. check out the experiment and results in the youtube video link . Eratosthenes didnt account for refraction of light in the upper atmosphere , and he assumed the earth was a sphere.