"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."
Earth is spherical and its curvature produces observable effects such as changing visible constellations when traveling south, ships appearing top-first in ports, and a circular lunar-eclipse shadow. Around 240 BC, Eratosthenes measured the Earth's radius by comparing sun angles at two locations at noon on the summer solstice. He observed the sun was directly overhead in Syene while a pole in Alexandria cast a shadow, producing an angle θ. That angular difference corresponds to the fraction of Earth's 360-degree circumference, so the planet's size follows from θ and the distance between the two locations.
Read at WIRED
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