Eratosthenes of Cyrene

In the very first episode of Cosmos, Carl Sagan explains how Eratosthenes, a Greek scholar and chief librarian at the Library of Alexandria, discovered over 2000 years ago that the earth is a sphere. Given the geographer, mathematician, poet, historian, and astronomer’s incredible list of accomplishments — a system of latitude and longitude, a map of the world, a system for finding prime numbers — this may not even rank as his highest achievement.

In the Cosmos clip above, Sagan explains Eratosthenes’ scientific method: he made observations of how shadows change length given the position of the sun in the sky. Estimating the distance between the cities of Syene (now Aswan, Egypt) and Alexandria, he was then able to mathematically calculate the circumference of the earth.

Although “several sources of error crept into Eratosthenes’ calculations and our interpretation of them,” he nonetheless succeeded almost perfectly. His estimation: 250,000 stadia, or 25,000 miles. The actual circumference: 24,860 miles (40.008 kilometers).

Video: CarlSagan.com (on YouTube)
Words: adapted from OpenCulture