At noon on the summer solstice, while priests marveled at a shadowless well in southern Egypt, a librarian 500 miles north was about to measure the entire planet.

The Day Archimedes' Shadow Measured the Earth

How a Greek polymath's forgotten student used a well, a stick, and geometry to calculate our planet's circumference

A Greek librarian measured Earth's circumference using just a stick, a well, and geometry—and got it 98% right.

The sun climbed toward its zenith over the Egyptian city of Syene, and Eratosthenes of Cyrene—head librarian of the Great Library of Alexandria—was thinking about shadows. Or rather, their absence.

It was the summer solstice, and he knew that at this precise moment, 500 miles to the south, something remarkable was happening: the sun hung directly overhead, its rays plunging straight down a deep well without casting a single shadow on its walls. Local priests had long noted this phenomenon, considering it a quirk of geography, perhaps a blessing from Ra himself.

But Eratosthenes saw something else entirely—an experiment waiting to happen.

Back in Alexandria, he planted a gnomon, a simple vertical stick, into the ground and watched. At the exact moment when Syene's well swallowed the sun whole, his stick cast a measurable shadow. The angle? Approximately 7.2 degrees—one-fiftieth of a complete circle.

💡 Eratosthenes' colleagues nicknamed him 'Beta' (second letter of the Greek alphabet) because he was considered second-best in multiple fields—yet his 'second-rate' work on Earth's measurement remained unsurpassed for over 1,500 years.