A Foucault pendulum, or Foucault's pendulum, is a tall pendulum free to oscillate in any vertical plane and capable of running for many hours. Its significance is that its motion demonstrates the Coriolis force and the rotation of the earth. It is named after its inventor Léon Foucault, and was first exhibited in 1851 from the ceiling of the Pantheon in Paris.
It can be observed that, at almost any location on the Earth, the plane within which the pendulum swings slowly rotates, with the exception of a Foucault pendulum at the equator, which does not rotate.
The plane of oscillation of a pendulum at either the North Pole or South Pole rotates once a day. The plane of oscillation of a pendulum anywhere on the earth rotates with a speed proportional to the sine of its latitude; thus one at 45° rotates once every 1.4 days and one at 30° every 2 days.