In this clip from Genius by Stephen Hawking, learn how to calculate the circumference of the Earth. Three volunteers learn by measuring the flatness of the lake that they will be able to calculate the size and shape of the Earth. Using a powerful laser that projects a straight beam of light and a boat, the volunteers shoot the beam across the lake. This experiment shows the curvature of the lake. This was first discovered by the ancient Greek philosopher, mathematician and geometer Eratosthenes. He proved the Earth wasn't flat through observing the sun and the direction it cast shadows. If the Earth was flat, the sun would always shine at the same angle no matter what time of day it was. Using all the data collected from the curvature of the lake the volunteers are able to calculate the circumference of the Earth.