= Foucault pendulum
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{title2=$\dot\psi=-\Omega\sin\varphi$}
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A <Foucault pendulum> is a long <simple pendulum> whose suspension allows the bob to swing in both independent horizontal directions. Its slowly changing swing direction relative to the building reveals <Earth's rotation>. A hinge restricting the bob to one vertical <plane> would prevent this measurement: the support would impose the very direction we want the pendulum to select freely. <Léon Foucault> demonstrated the effect in 1851.<The Foucault pendulum by Joe Wolfe>[The Foucault pendulum, Does the Earth turn?: 1851 demonstration]{ref}
The bob's back-and-forth <oscillation> and the swing direction's <precession> are different motions. The bob traverses its arc in seconds, while successive arcs change direction over hours. In a <conical pendulum> the bob itself circulates around the vertical; a nearly planar <Foucault pendulum> instead reverses along an almost straight horizontal trace, whose orientation changes slowly. The measured quantity is that orientation against a fixed floor or wall reference, rather than the bob's instantaneous <azimuth>.
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