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Earth's Rotation Day

The fact that the earth rotates on its axis is common knowledge today, but until the mid-nineteenth century, it was merely conjecture. French physicist Jean-Bernard-Léon Foucault, commonly known as Léon Foucault or Jean Foucault, is known for proving through his experiments that the earth rotates on its axis.

On January 8, 1851, Foucault performed an experiment in the cellar of his home, in which he swung a five-kilogram weight attached to a two-meter-long pendulum. He put sand underneath it to mark the pendulum's path, allowing him to see any changes in it. He observed a slight clockwise movement in the plane—the floor, and thus the earth, were slowly rotating; the pendulum kept its position. His experiment showed that the earth rotated on its axis. No longer was it just a hypothesis.

Shortly thereafter he demonstrated his findings. He sent out invitations to members of the French Academy of Sciences, and they attended a demonstration on February 3, 1851, at the National Observatory in Paris. Here Foucault showed his findings with an eleven-meter long pendulum.

France's President Louis Napoleon requested Foucault to make a public demonstration of his experiment. In March of 1851, Foucault suspended a twenty-eight-kilogram brass coated sphere filled with lead from a sixty-seven-meter wire. It was attached to the dome of the Panthéon in Paris. A pin was placed at the bottom of the sphere to better measure where it swung in relation to the plane. This pendulum fittingly became known as the "Foucault Pendulum." The results showed that the swing of the pendulum rotated 11 degrees clockwise in an hour, making a full circle in 32.7 hours. This first public demonstration brought large crowds, and interest in pendulums in Europe and the United States grew greatly after it.

Foucault determined that the amount of time that it took for rotation depended on the latitude of where the experiment took place. It took twenty-four hours for rotation at the poles, but it didn't rotate at all at the equator. It varied in time in the areas in between, depending on the distance from the poles and the equator.

How to Observe

Here are some ways to celebrate:

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