Twenty three centuries ago a man named Eratosthenes of Cyrene was able to calculate the circumference of the earth with nothing more than a sundial, a compass, some scraps of papyrus, and a basic understanding of mathematics. The calculation Eratosthenes used is one you might find on a high school math exam. On its surface it sounds like an ordinary achievement, something worthy to display on the refrigerator. But it wasnâ€™t the math Eratosthenes used that makes his geodesy so impressive. Instead it was a series of forward thinking realizations that the rest of the world would disregard for almost 2000 years. In time, Eratosthenes would be vindicated by modern technologies. Not only was Eratosthenes vindicated, but it turns out he calculated the circumference of the earth with 99% accuracy.