1851

Proof that the Earth Spins

Jean Bernard Léon Foucault (1819–1868)

Our space-age perspective on our planet now makes the fact that the Earth spins a matter of common knowledge. But imagine for a moment going back in time to an era when there were no satellites or space probes or fancy computerized planetarium programs, and try to convince someone that the Earth is actually spinning. It’s not intuitive—the Sun and sky appear to move, not the Earth! If the Earth were spinning as fast as it would need to be spinning to rotate once per day (about 1,000 miles [1,600 kilometers] per hour at the equator), wouldn’t we all get flung off into outer space? Even today, it’s hard to prove to someone that the Earth spins. What is needed is a simple and repeatable experiment that provides a physical demonstration of the Earth’s rotation.

Although a number of such experiments have been proposed and conducted, by far the most famous is the one first performed in 1851 by the French physicist Jean Bernard Léon Foucault. Foucault (pronounced foo-KOH), like any good physicist, understood Newton’s laws and exploited the first law (bodies at rest or in motion remain at rest or in motion unless acted on by an external force) for his experiment. He constructed a long, heavy, stable pendulum using a ball (or bob) made from lead-coated brass that was suspended on a wire 220 feet (67 meters) long from the ceiling to the floor of the Panthéon in Paris. Foucault knew that in the absence of any other forces, once he started the pendulum swinging, it would continue to swing in that same plane—that is, it would remain in the same inertial reference frame relative to the “fixed” stars, rather than to the Earth. By setting up hour markers like those on a sundial (or small obstacles for the bob to knock over), and by compensating for friction in the wire or from the bob’s motion through the air, it became easy to demonstrate that the room (indeed, the whole Earth) was slowly rotating relative to the plane of the pendulum’s swing. The following year, Foucault perfected a gyroscope (a device for measuring velocity and orientation) based on similar principles.

Foucault’s pendulum became a nineteenth-century sensation because of its simplicity; hundreds can still be found around the world in universities, museums, and science centers.

A large Foucault’s pendulum from the Príncipe Felipe Science Museum of the City of Arts and Sciences in Valencia, Spain. In this setup, a little ball-and-stick model is tipped over roughly every 30 minutes because the Earth is spinning underneath the inertially fixed plane of the pendulum’s swing.