Foucault's pendulum is a weight, usually a sphere, suspended by a wire that is attached at the ceiling with a joint that allows the weight to swing in any direction.
The motion of this pendulum proves that the earth is rotating underneath the pendulum. The angle of the rotation of the pendulum per hour remains fixed at any location and equals:
where q is the latitude of the observer. At the equator, the plane in which the pendulum moves does not change.
Foucault's Pendulum is also the English name for an Italian novel by Umberto Eco.