Greek astronomer, geographer, mathematician, and librarian Eratosthenes calculates Earth’s circumference.
Eratosthenes was the librarian of the great Library of Alexandria. His fans called him Pentathalos, a champion of multiple skills. His detractors called him Beta, because he came in second in every category.
He invented the Sieve of Eratosthenes, an algorithm for finding prime numbers that’s still used in modified form today. He sketched the course of the Nile from the sea to Khartoum, and he correctly predicted the river’s source would be found in great upland lakes.
Eratosthenes knew that at noon on the summer solstice, the sun was observed to be directly overhead at Syene (modern-day Aswan): you could see it from the bottom of a deep well, and sundials cast no shadows. But there were shadows at Alexandria, to the north, where the sun wasn’t directly overhead. Therefore, the earth must be round—already conventionally believed by the astronomers of his day.
Eratosthenes computed that the angle of the shadow in Alexandria was one-fiftieth of a full circle. He then estimated the distance between the two locations and multiplied by 50 to derive the circumference. We don’t know the exact length of the measurement unit Eratosthenes was using when he calculated the figure of 252,000 stades. Depending on which classical source you trust, that’s somewhere between 24,663 and 27,967 miles. The accepted figure for equatorial circumference today is 24,902 miles. Pretty darn good for a guy without modern measurement tools.
Eratosthenes also computed the tilt of Earth’s axis to within a degree and suggested that calendars should have a leap day every fourth year, an idea taken up by Julius Caesar two centuries later (see here).—RA