He measured the size of the entire Earth using nothing but a stick, a shadow, and some men walking through the desert. So why did the intellectual elite of the ancient world sneeringly nickname him "Beta" — number two?

This deep dive explores the life of Eratosthenes of Cyrene (c. 276–194 BC), the polymath who ran the Library of Alexandria and laid the foundations of modern science. We trace how a generalist in a world obsessed with specialists became the intellectual center of gravity for the ancient world, and why his peers couldn't decide whether he was a master of everything or a master of nothing.

• How Eratosthenes calculated Earth's circumference using the shadow of a vertical rod (gnomon) in Alexandria and a shadowless well in Syene — landing within roughly 1% of the true figure.
• Why two false geographical assumptio ...