Eratosthenes was the first person to calculate the circumference of the Earth around 240 BCE. (This was just one of his many remarkable feats.) The story of how he figured out that circumference is brilliant, as it shows how a little bit of math, legwork, and logic can result in a better understanding of our world.

In 1961, Charles and Ray Eames made this animated film about how Eratosthenes did it using a well in Egypt, a knowledge of angles, and a chance observation of the sun's reflection in a water well. Enjoy:

In the series Cosmos, Carl Sagan told the same story, using a nice physical map (Eratosthenes would have enjoyed that, being "the father of geography"):

If video isn't your thing, check this Khan Academy page about Eratosthenes.