More than 2,000 years ago, pretty much every educated human knew the Earth was round. If you travel south, you see stars and constellations you’ve never seen before (because they’re blocked by Earth’s curvature). Finally, when Earth's shadow falls on the moon in a lunar eclipse, the shadow is a circle. I'm going to show you how to measure the Earth’s size using Lego pieces. You can see that if the sun is in line with the Syene pole, it won’t be in line with the Alexandria pole.