The Foucault pendulum

The Foucault pendulum (English pronunciation: /fuːˈkoʊ/ foo-KOH; French pronunciation: ​[fuˈko]), 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 rotated, the introduction of the Foucault pendulum in 1851 was the first simple proof of the rotation in an easy-to-see experiment. Today, Foucault pendulums are popular displays in science museums and universities. http://upload.wikimedia.org/wikipedia/commons/8/82/Foucault-rotz.gif