Foucault Pendulum

The plane of swing of a pendulum on the surface of the Earth rotates at a rate which depends on the latitude of the location of the pendulum. This problem was first worked out by Foucault in the 1800s and demonstrates that the Earth rotates on its axis and is not stationary under a rotating Celestial Sphere.

To understand how Foucault Pendula work, consider a pendulum placed at the North Pole (see left). The plane of swing of the pendulum remains fixed with respect to a distant observer, one sitting far away from the Earth, while an observer sitting on the Earth at the North Pole would see that the plane of swing of the pendulum rotates once every day, actually once every 23 hours and 56 minutes or so (see right). The movement of the plane of swing happens because the Earth-bound observer is carried around by the spin of the Earth.

The period of swing depends on the latitude of the pendulum. At the equator of the Earth where the latitude is 0 degrees, the plane of swing of the pendulum does not rotate. At arbitrary latitude on the Earth, the period of swing of the plane of the pendulum is given by

P ~ 1 day / sine (latitude)

The latitude of Eugene, OR is around 45^o N. The plane of swing of a Foucault pendulum then rotates with a period of

P = 1 day / sine 45^o ~ 34 hours