This describes a famous experiment conducted by the Greek scholar Eratosthenes around 240 BCE to measure the size of the Earth.Observation: Eratosthenes noticed that at noon on the summer solstice, the Sun was directly overhead in Syene (modern-day Aswan), casting no shadow down a deep well. At the same time in Alexandria, further north, he observed that a vertical stick (gnomon) did cast a shadow.
He measured the angle of this shadow, which was approximately 7.2 degrees, or 1/50th of a circle.
By assuming the distance between the two cities was roughly 5,000 stadia (about 800 kilometers) and multiplying that distance by 50, he calculated the Earth's circumference to be about 40,000 kilometers, a value remarkably close to modern calculations.
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