Carl Sagan explains how the ancient Greeks knew the Earth was round and calculated its circumference over 2,000 years ago. Eratosthenes' method involved measuring the shortest shadow cast by the sun in Alexandria on the same day that there was no shadow at noon in Syene. He also hired a man to pace the distance between the two cities, which he determined to be 5,000 stadia, or about 800 kilometers. Eratosthenes then used simple proportions to calculate the Earth's circumference: 7.2 degrees is 1