Eratosthenes

June 19, 240 BC - Today was the day that a Greek mathematician and all-around genius named Eratosthenes estimated the circumference of the Earth - and despite the limited resources he had at the time, his estimate was remarkably accurate - within 2% of the actual measurement.

Eratosthenes was the head librarian at the Great Library of Alexandria. He’s remembered as the father of geography, and the first to lay down a grid of meridians and parallels.

If you have a couple minutes to burn, check out the video below for a quick explanation about how Eratosthenes measured the Earth:

In the mid-20th century we began launching satellites into space that would help us determine the exact circumference of the Earth: 40,030 km. But over 2000 years earlier, a man in Ancient Greece came up with nearly the exact same figure using just a stick and his brain.