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Likely familiar to this crowd, but Carl Sagan's explanation on Cosmos of Eratosthenes' discovery is great:

https://www.youtube.com/watch?v=G8cbIWMv0rI&ab_channel=carls...

That explanation irks me a lot because it implies Eratosthenes knew the Sun was so far away, the rays reaching the earth are practically parallel. And while it might sound a like a detail, the whole calculation depends on it.

If you presume the Earth is flat and the Sun not that far away, Eratosthenes was calculating the distance to the Sun.

Wouldn't observing that shadows at noon do not change as you travel east-west rule that out?

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