Named after the French physicist Léon Foucault, it is a simple device conceived as an experiment to demonstrate the rotation of the Earth.
The experimental apparatus consists of a tall pendulum free to swing in any vertical plane. The actual plane of swing appears to rotate relative to the Earth; in fact the plane is fixed in space while the Earth rotates under the pendulum once a sidereal day. The first public exhibition of a Foucault pendulum took place in February 1851 in the Meridian of the Paris Observatory.
Bottom: Animation of a Foucault pendulum at the Pantheon in Paris (48°52’ North), with the Earth’s rotation rate greatly exaggerated. The green trace shows the path of the pendulum bob over the ground (a rotating reference frame), while the blue trace shows the path in a frame of reference rotating with the plane of the pendulum.