Measuring the Circumference of the Earth — Eratosthenes' Shadow Method

Around 240 BC the Greek scholar Eratosthenes, chief librarian at Alexandria, measured the size of the whole Earth with nothing but a vertical stick, its shadow, and a known distance. He knew that at noon on the summer solstice the Sun stood directly over Syene (modern Aswan) — sunlight reached the bottom of a deep well and cast no shadow. At Alexandria, far to the north, a vertical rod at the same moment cast a shadow of about 7.2°. That angle is one-fiftieth of a full circle, so the Earth's circumference had to be fifty times the distance between the two cities. This blueprint reproduces his measurement with a gnomon and a plumb line — and lands within a few percent of the true value, as he did.