Q:     At the Hansen Planetarium in Salt Lake City, UT, USA there is a giant pendulum that keeps track of time by knocking over pegs placed around a circle.  What kind of pendulum is this and how does it work?

A:     This museum exhibit is called "The Foucault Pendulum" and can be seen in many museums around the world.  Characteristic for the Foucault Pendulum is the very, very long cord (or wire, or beam) that supports it to a fixed point above the mass of the pendulum.   When the oscillating pendulum knocks over successive pairs of diametrically opposite pegs, what moves is in fact not the vertical plane in which the pendulum oscillates but the surface of the earth with which the pegs are connected.  After each twenty-four hour time period, one full rotation of the earth is completed and the Foucault Pendulum will knock over the same pair of pegs.

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