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The Greek astronomer Eratosthenes used simple geometry to calculate the size of the earth. He noticed the position of the sun on the first day of summer at two different places. Assuming the sun’s rays to be parallel to each other and knowing the distance between both the places that he had made his observations at, he calculated the earth’s circumference to be 25,000 miles. His calculations have been proved to be remarkably accurate by later scientists.
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