## Foucault's Pendulum

Foucault's pendulum illustrates the rotation of the Earth. It consists of a tall pendulum free to swing in any vertical plane. The 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. Relative to the Earth the pendulum appears to rotate relative to the ground.

At either the North Pole or South Pole, the plane of oscillation of a pendulum remains fixed relative to the distant masses of the universe while Earth rotates underneath it, taking one sidereal day to complete a rotation. So, relative to Earth, the plane of oscillation of a pendulum at the North Pole undergoes a full clockwise rotation during one day; a pendulum at the South Pole rotates counterclockwise.

When a Foucault pendulum is suspended at the equator, the plane of oscillation remains fixed relative to Earth. At other latitudes, the plane of oscillation precesses relative to Earth, but slower than at the pole; the angular speedwhereis the latitude, positive north of the equator and negative south.