Not only did Eratosthenes of Cyrene calculate the circumference of the Earth with impressive accuracy, but he did it without leaving Egypt. First, the Greek astronomer figured out the sun's angle of elevation on the summer solstice in Alexandria. Finding it to be around 7 degrees -- or approximately one fiftieth of a circle's circumference -- he measured the approximate distance between Alexandria and Syene, where he knew that the sun was directly overhead. Eratosthenes then multiplied that number by 50 to reach a circumference of 250,000 stadia (ancient Greek units of length). Modern scholars disagree over how his stadia would have translated to modern units, but it seems that Eratosthenes' result varied from the actual circumference of the planet by as little as 0.5% or as much as 17%. But even 17% is pretty stunning accuracy given the tools he had to work with.
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