Foucault Pendulum

A freely swinging pendulum tends to keep its plane of oscillation oriented relative to an approximately inertial frame, provided outside forces and disturbances are small. The ground and the observer, however, rotate with Earth. Consequently, the swing appears to turn relative to the floor: the pendulum is not simply being steered around by a hidden motor. Long wires, heavy bobs and careful control of air currents and friction make the gradual change easier to observe. Many installations use a timed electromagnetic impulse to replace energy lost to friction; when properly arranged, this sustains the swing without setting its direction of rotation.

The apparent rotation rate depends on latitude. In the idealized case, its magnitude is Earth's rotation rate multiplied by the sine of the latitude. At either pole, the swing's plane appears to complete one full turn relative to the ground in about a sidereal day; at the equator, it does not rotate relative to the ground. At intermediate latitudes, it turns more slowly, with opposite apparent directions in the two hemispheres. This pattern follows from observing the pendulum in a rotating frame and is also described through the Coriolis effect. In real demonstrations, friction, air motion, suspension imperfections and how the pendulum is started can affect the observed motion.

French physicist Léon Foucault first used a pendulum to give a direct, laboratory-based demonstration of Earth's rotation in 1851. He initially demonstrated the experiment at the Paris Observatory and later presented a celebrated public installation beneath the dome of the Panthéon in Paris. The simple visual evidence helped audiences connect Earth's rotation—already established by astronomical evidence—with motion they could watch in a room. Foucault pendulums subsequently became familiar exhibits in science museums and universities.

The pendulum is an accessible demonstration of rotational motion and of the difference between an inertial reference frame and one attached to Earth's surface. Its rate of precession can, in principle, be related to the latitude of the installation. Today its main role is educational: modern geodesy and astronomy measure Earth's rotation with more precise instruments, but the pendulum remains a striking way to make the rotation visible without relying on looking at the sky.