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 calculate the radius of the spherical Earth. You can do it too, and it doesn’t require any fancy equipment. I’m going to show you how to measure the Earth’s size using Lego pieces.
Of course, Eratosthenes didn’t have Legos. But he knew that at noon on the summer solstice, the sun shone straight down a vertical well in Syene, Egypt. This meant the sun was directly overhead. So what did he do? He stuck a pole in the ground in Alexandria, and at noon on the same day he found that it cast a shadow, meaning the sun wasn’t overhead there.
In the picture below, I’ve used a pole in Syene (not to scale, obviously) instead of a well, but it’s the same idea. You can see that if the sun is in line with the Syene pole, it won’t be in line with the Alexandria pole. This could only mean the Earth is curved. But, yeah, he knew that.
Illustration: Rhett Allain