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 was also the guy who came up with the science we know as Geography. He invented a system of latitude and longitude, and he was the first person to prove that the Earth was round.
If you have a couple minutes to burn, check out the video below for a quick explanation about how Eratosthenes measured the Earth: