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Protestants spread the myth that people in the middle ages thought the earth was flat to try and discredit Catholicism.
Ancient Greeks proved the earth was a sphere as early as the 4th century BC.
And Eratosthenes calculated its circumference to within a few hundred kilometers because he treated the earth as a perfect sphere instead of the oblate spheroid it is
My favorite is that in the same vein, Aristarchus estimated the size of the sun to be much larger than the earth (although he still severely underestimated it because it's so hard to measure), and therefore proposed that the earth should orbit around the sun. And the main problem with his theory was not any religious objection, but rather that his model would imply that there should be parallax visible among the stars. Unless they are, you know, ridiculously far away.
And how big it is, and how big and far away the moon is, and wrote a decent guess at the distance and size of the sun (although they made a measurement error with that one).
Before that, the competing theory was that it's a cylinder with the ends to the east and west. Anyone with eyes for stargazing can see they've obviously rotated when they move a significant distance north-south, so nobody with a long-distance trade network would thought it was flat.