In 240 B.C., Greek mathematician Eratosthenes heard about an interesting phenomenon happening in the city of Siena. There was a well in the center of the city, and at noon on the first day of summer the sun would be directly overhead, and shine down into the well, casting no shadows. Eratosthenes was living in Alexandria at the time, and noted all the wells there cast shadows. The mathematician began measuring the shadows and using geometry to find out the size of the Earth. He even commissioned a man to walk to Siena and back to measure the distance. According to NASA, he was only off by about 99 miles.
Historian John Schmidt tells us the story of how Eratosthenes first determined the size of the Earth.