How to measure the radius of the Earth using Lego

More than 2000 Years ago, almost every educated person knew that the Earth was round. There are some pretty obvious clues, after all. If you travel south, you’ll see stars and constellations you’ve never seen before (because they’re obscured by the Earth’s curvature). When a ship reaches port, you see the top of it before the bottom (because the ocean surface is curved). Finally, when the Earth’s shadow falls on the Moon in a lunar eclipse, the shadow is a circle. I mean come on!

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 a spherical Earth. You can do this too, and it doesn’t require any fancy equipment. I’ll show you how to measure the size of the Earth using Legos.

Of course, Eratosthenes had no Lego pieces. But he knew that at noon on the summer solstice, the sun shines directly down a vertical well in Aswan, Egypt. This means that the sun was directly overhead. So what did he do? He planted a pillar in the ground in Alexandria, and at noon on the same day he found that it was casting a shadow, that is, the sun It wasn’t Up there.

In the photo below, I used a column in Sienne (not to scale, of course) instead of a well, but it’s the same idea. You can see that if the sun was on one line with the Aswan pole, it would not be on one line with the Alexandria pole. This can only mean that the Earth is curved. But, yes, he knew that.

Illustration: Rhett Allen