Earth’s Circumference

Greek polymath Eratosthenes was born in Cyrene, an ancient Greek city in what is now northeastern Libya, in 276 BC. By the time he was 40, he had become well-known for his knowledge in a variety of subjects, including mathematics, poetry, writing, and philosophy, earning him a place at the renowned Library of Alexandria.

The calculation of the Earth's circumference was Eratosthenes' most significant accomplishment, however he made far too many contributions to early mathematics to include them all here. He achieved it by comparing the lengths of the shadows that were cast on the summer solstice in Alexandria and Syene, which is now Aswan, two different Greek cities.

He determined the Earth's diameter to be around 40,000 km, or 24,900 miles, using that information and an approximation of the distance between the two cities from traveler accounts. He had estimated the distance between the cities just a little bit incorrectly, so he was off by around eight kilometers.