Carl Sagan explains how the Greeks worked out the earth was round, and it's exact circumference using a couple of sticks, over 2,000 years ago.
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@UniverCurious There's also probably a much simpler explanation. The Greeks learnt this from civilisations prior to them. Namely the Arabs of the Orient. Plausible the ancient Chaldeans knew too.
@Tetragr72834810 @UniverCurious Observe the shadow of the earth on the moon at eclipse + the light reflected by the moon after sunset.