This is a story I find quite fascinating: the ancient Greeks knew that the Earth was round and could measure the size of it!
Eratosthenes (yeah, the guy with the sieve for finding prime numbers) lived in Alexandria. He knew that in Aswan, that lays on the tropic of cancer, the sun would be in zenit on the summer solstice. At the same time in Alexandria the sun casted a shadow with an angle of 7°10′. This together with the distance between Alexandria and Aswan (known at the time, the question is how accurate this measure was) gave the circumference of the Earth. Eratosthenes got the circumference right, give and take ~10%
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