Before you ask, no I didn’t have any encounters with people who claimed Earth was flat. I’m just curious.
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There was once a man named Eratosthenes in Ancient Greece.
He found in some books a record of a notable occurrence in a town called Syene, in which on the summer solstice at noon, objects would not cast shadows, and you could look all the way down wells.
He realized that meant the sun would have to be EXACTLY overhead in that moment, and since he knew for a fact that on that day and time in Alexandria (where he was) shadows DID get cast, he realized that the apparent angle of the sun must be changed with the distance. The only way this would work would be if the Earth were a sphere.
Since Syene was almost directly south of Alexandria, he took the known distance between to two towns, and the length of shadows in Alexandria on the solstice, and using some Trigonometry found the size of the Earth.
He was less than 1% off modern figures with this crude method
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