Flat earthers reject these assumptions and posit that the sun is much smaller and closer to the earth so that its rays are not parallel. Eratosthenes’ experiment alone does not prove that the earth is a globe because his assumptions must be true in order for his conclusions to be valid. He reasoned that the distance between the two cities must therefore constitute 7.2 degrees of a circle, which indicates a circumference of about 25,000 miles.Įratosthenes made two assumptions here: that the earth is a globe and that the sun is distant enough that its rays are essentially parallel. This angle corresponds to the solar zenith angle – the angle between the sun and the point in the sky directly overhead.
On that day, when the sun was at its highest in the south, he found that a stick in Alexandria cast a shadow at a 7.2 degree angle. He knew that each year on the summer solstice the sun would pass directly overhead and illuminate the bottom of a well in the city of Syene, about 500 miles south of Alexandria on the Tropic of Cancer. By B.C, the Greek astronomer Eratosthenes devised a way to measure the circumference of the earth.
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