The aluminum pendulum swinging back and forth in the center of the counter leaves a record of each oscillation in the tray of sand below.
Known as the Foucault Pendulum, after the French scientist who devised it, the demonstration is a graphic verification of the fact that the earth revolves about its axis.
Having once been set in motion, the pendulum obeys Newton’s Law of Motion by moving in a fixed plane. The earth, meanwhile, moves steadily eastward and describes a circle beneath the plane of the pendulum’s arc.
The length of time required for the pendulum to return to its starting point varies with latitude. At the equator there would be no change at all. At both poles the pendulum would make one apparent revolution about its base every 24 hours. Here in Seattle, the pendulum’s return to its starting point requires a little over 36 hours.
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