Ok. Can you explain me the hypothesis made by erathostenes? He was making two hypothesis. One was that the earth is a sphere. The othe is that tha rays of sun are parallel. Isnt it?
Well, you have to remember that for at least 2 centuries before Erathostenes, the Greeks had concluded the Earth was curved for a number of reasons, including that there were star constellations visible in Egypt that were never visible in Greece. Initially, there were some who thought it was a cylinder instead of a sphere, but the math didn't work.
I think it is reasonable to assume suns rays are parallel, or to be more precise, light travels in a straight line (putting aside gravity and general relativity for the moment). No reason for light to bend, only because it enters a well.