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Foucault Pendulum

This pendulum provides visual proof of the Earth's rotation. Designed by Jean Leon Foucault in 1851, it is called the Foucault (foo-koh) Pendulum.

What is a pendulum?

A pendulum is a body suspended from a fixed point so as to swing freely to and fro under the action of gravity.

How does the Foucault Pendulum work?

Once the pendulum is set in motion, it continues to swing back and forth in a vertical plane defined by the track of the ball and cable.

The direction of swing is constant. Although gravity is pulling down on the ball and tension in the cable is holding it up, there is no force acting to change the direction of swing.

Why does it appear to be moving in a circle?

The apparent change in direction of swing is related to the perspective of the observer. Because no force is acting to change the direction of swing, it doesn't change. Instead, the floor, the observer, and the Earth's surface are turning beneath the pendulum.

The Earth's spin is essential to life.

Night and day, darkness and light, cooling and warming- these are the result of the spinning Earth.

The Earth's spin contributes to the movement of ocean and wind currents, which in turn affect climate, weather, food production, and our everyday lives.

The pendulum's direction of swing does not spin with the Earth.

The hardware holding the pendulum is attached to the roof of the Museum and moves with the Earth. But with flexible wire for support, the pendulum swings back and forth in the same plane.

The pendulum is not a perpetual motion machine.

Because of air friction and gravity the pendulum would eventually stop swinging, but a magnet encircling the cable pulls on the cable with each swing. Electric current controlling the magnetism is turned on and off with each swing of the pendulum.

The pendulum is not a clock.

However, the time it takes to go around the circle is predictable. Imagine yourself hovering above the Earth's axis at the North or South Pole. The Earth below appears to be spinning like a merry-go-round. The pendulum would complete the circle in 24 hours.

Now imagine yourself hovering above the equator.

The Earth appears to be rolling like a barrel. As the pendulum swings, it does not move around the circle at all. Latitude determines the length of time required for a complete trip around the circle. Here in San Diego, the pendulum makes its way around the circle in 44½ hours.