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As The World Turns

An ornate Foucault Pendulum at the Forest Lawn Mausoleum

How do we really know the Earth spins on its axis? One way to see for yourself is to go to a science museum and watch a Foucault Pendulum.

A pendulum is a string with a weight on it that swings back and forth. You can build one to observe the Earth spinning by using a long enough cable and a heavy enough weight. The length and weight are important, so that it can keep swinging for hours before friction slows it to a stop.

Still confused on how this corresponds with the Earth spinning on its axis? Remember a pendulum always swings in the same direction, unless an outside force acts on it. If the pendulum swings back and forth in the same direction on a rotating Earth, the direction of the pendulum's swing will seem to gradually change through the day, because the Earth is turning.


Once you have your Foucault Pendulum in place, set up a circle of pegs around the pendulum. The swinging weight will slowly knock them down as the day progresses and you can see that the Earth turns. The size of the effect depends on where you are on Earth. It's greatest at the poles and vanishes at the equator.

The physicist Jean Bernard Léon Foucault built the first Foucault Pendulum in the nineteenth century. It was displayed at the Pantheon in Paris. Today you can find these pendulums in science museums around the world.

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