Foucault pendulum

A pendulum with precession, relative to an observer fixed to the earth, produced by the effect of Coriolis acceleration.

If set swinging at the North Pole in a given plane in space, its linear momentum perpendicular to the plane is zero, and it will continue to swing in this invariable plane while the earth rotates beneath it with a period of one day. At a latitude L, the frequency of precession is 1/T sin L where T is one day.