More than 2,000 years ago, pretty much every educated human knew the Earth was round. There are some pretty obvious clues, after all. If you travel south, you see stars and constellations you’ve never seen before (because they’re blocked by Earth’s curvature). When a ship comes into port, you see the top of it before the bottom (because the ocean surface is curved). Finally, when Earth’s shadow falls on the moon in a lunar eclipse, the shadow is a circle. I mean, c’mon!
But this is impressive: Around 240 BC, the Greek mathematician Eratosthenes, head of the famous Library of Alexandria in Egypt, came up with a brilliant way to