A Foucault pendulum, or Foucault's pendulum, is a tall pendulum free to oscillate in any vertical plane and capable of running for hours. It is used to demonstrate Coriolis force and the rotation of the earth. It is named after the inventor, Léon Foucault.
A Foucault pendulum at the equator does not rotate. A pendulum at the pole rotates once a day. A pendulum anywhere on the earth rotates with a speed proportional to the sine of its latitude; thus one at 45° rotates once every 1.4 days and one at 30° every 2 days.
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