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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