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240 BCE Eratosthenes of Cyrene calculates Earth’s circumference

Eratosthenes of Cyrene was a Greek mathematician, geographer, poet, astronomer, and music theorist. He is best known for being the first person to calculate the circumference of the Earth which he did by comparing altitudes of the mid-day sun at two places a known North-South distance apart. His calculation was remarkably accurate. He was also the first to calculate the tilt of the Earth's axis.

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