P A N

i don't think it has been posted. i haven't watched it, but i know who eratosthenes was. he put posts in the ground. one in alexandria, and one in syrene. i can't remember the distances exactly, but it's somewhere between 600 and 800 kilometers between them. he used the divergent shadows of the posts under sunlight to "prove" the earth is round, and i think he is even credited with making a pretty decent guess at it's circumference.

the thing about those shadows is that they are caused by the sun coming in through the atmosphere. in my mind, he actually proved the earth was flat and that the sun is close. if the sun were 93,000,000 miles away (as it's supposed to be) the light rays from the sun would come at us parallel. and if they were coming at us parallel, they would hit the atmosphere, it would act like a lens, and the light would actually focus a bit, rather than diverge. the resultant sunrays (the kind you see peaking through clouds and spreading out) are called crepuscular sun rays, and you can use them (and trig) to determine that the sun is only about 3000kms away, and 32 miles wide.

all that aside, eratosthenes put forward an oft-cited original proof of a ball earth. he did it on a horse using distances exceeding 600kms. his quality checks could not possibly have been up to snuff with this method.