Over 2 thousand years ago, almost every educated person knew that the Earth was round. After all, there are some pretty obvious clues. If you go south, you will see stars and constellations you have never seen before (because they are blocked by the curvature of the Earth). When a ship enters a port, you can see the top of the ship before the bottom (because the ocean surface is curved). Finally, when the Earth’s shadow falls on the Moon during a lunar eclipse, the shadow is a circle. I mean, come on!
But it’s impressive: around 240 BC, the Greek mathematician Eratosthenes, head of the renowned Library of Alexandria in Egypt, came up with an ingenious way to calculate the radius of the spherical Earth. You can do it too and it doesn’t require any fancy equipment. I’ll show you how to measure the size of the Earth using Lego bricks.
Of course, Eratosthenes didn’t have Legos. But he knew that at noon on the summer solstice, the sun shone directly into a vertical well in Syene, Egypt. This meant that the sun was directly overhead. So what did he do? He drove a pole into the ground in Alexandria and at noon the same day found that it cast a shadow, or sun there wasn’t there above your head.
In the image below, I used a pole in Syene instead of a well (not to scale, of course), but the idea is the same. You can see that if the sun is aligned with the pole of Syene, it will not be aligned with the pole of Alexandria. This can only mean that the Earth is curved. But yes, he knew it.
Illustration: Rhett Allain