Foucault's pendulum (foo-kohz ) A simple pendulum that demonstrates the rotation of the Earth, first shown by J.B.L. Foucault in 1851. It consists of a massive metal ball that is suspended by a long wire and can swing freely with a minimum amount of friction. The pendulum swings steadily, tracing a straight line on the floor beneath it. The plane in which it swings, however, is observed to rotate during the day as a result of the Earth's rotation. The period of rotation of the plane depends on the latitude of the place: at the poles the plane would appear to move through a complete circle in one sidereal day (23h 56m 4s) while on the equator it would not rotate at all. The pendulum is in fact swinging in a plane that is fixed relative to the stars.