In Léon Foucault’s 1851 pendulum experiment, what motion of Earth did the pendulum demonstrate?

In Léon Foucault’s 1851 pendulum experiment, the pendulum demonstrated Earth’s rotation.

Foucault suspended a long pendulum so it could swing in a nearly fixed vertical plane. As hours passed, observers saw the direction of the swing appear to rotate relative to the floor. The pendulum’s plane was largely maintaining its orientation in inertial space while Earth and the building turned beneath it.

The effect is easiest to notice at the poles, where the apparent rotation completes a cycle in about one sidereal day. At the equator, the idealized Foucault effect does not produce a full apparent rotation. At intermediate latitudes, the rate depends on latitude.

The experiment provided a visually persuasive demonstration of Earth’s rotation without relying on astronomical observations. A common mistake is to say that the pendulum itself actively turns because of a motor or magnetic force; its changing floor direction is an effect of Earth’s rotating reference frame.