How did ancient people know that the earth was round?
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Eratosthenes of Cyrene calculated the radius of the planet pretty accurately by noting that on a given day the sun shown directly down a well in Aswan at noon, and then measured the angle of the sun’s shadow at noon on the same day in Alexandria – about 500 miles away. A bit of math gave a circumference of 25,000 miles. That was ~250 BCE.
Posidonius of Rhodes calculated it by observing the star Canopus, which when it’s on the horizon in Rhodes it was a certain distance above the horizon in Alexandria. His calculations showed a circumference of 18,000 miles.
Columbus knew about Posidonius’ estimate, but not Eratosthenes’ which is why he thought he had travelled farther than he actually had.
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