What does a Foucault pendulum demonstrate?
The Earth’s rotation
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First demonstrated in public by the device’s inventor, the pendulum swings by degrees as the Earth rotates on its axis. This Dartmouth professor does a better job of explaining it.
The Foucault pendulum, or Foucault’s pendulum, named after the French physicist Léon Foucault, is a simple device conceived as an experiment to demonstrate the rotation of the Earth. While it had long been known that the Earth rotates, the introduction of the Foucault pendulum in 1851 was the first simple proof of the rotation in an easy-to-see experiment.