A freely swinging pendulum slowly turns the direction of its swing on its own — not because anything pushes it, but because the Earth is rotating beneath it, the effect Léon Foucault used in 1851 to let Paris watch the planet turn

In 1851 Léon Foucault hung a heavy bob on a long wire under the dome of the Panthéon and set it swinging. Its path slowly rotated, the first plain, direct sign anyone could watch of the Earth turning on its axis. It does not, though, turn once a day everywhere. The post A freely swinging pendulum slowly turns the direction of its swing on its own — not because anything pushes it, but because the Earth is rotating beneath it, the effect Léon Foucault used in 1851 to let Paris watch the planet