(276-194 BCE) Greek scientist, mathematician, poet, and chief librarian at the Library of Alexandria. He is most famous for calculating the Earth’s circumference. This work began when Eratosthenes read from a papyrus scroll that in Syene (now called Aswan), a frontier town along the Upper Nile River, an object sticking up from the ground cast no shadow at noon on the longest day of the year. At that same time, the Sun’s rays shone directly down into a deep well, rather than hitting any side of the well. This meant that the sun was directly overhead, something that never happened in Alexandria. Eratosthenes hired someone to measure the precise distance to Syene. They found that it was roughly 800 km away. Using measurements of shadows of poles of standardized lengths at the two locations, Eratosthenes was able not only to prove that the Earth was a sphere, but also that the two cities differed by approximately 7.2 degrees of an arc, or 1/50th of a circle. Based on that, he calculated the Earth’s circumference to be 50 x 800 = 40,000 km, which is just about what we know it to be today (40,075 km)!

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