Foucault Pendulum And The Pantheon

In the old Latin quarter in Paris, stands a magnificent 18th century building—the temple to all the gods, the Pantheon. Originally constructed as a church dedicated to St. Genevieve, it was later converted into a mausoleum containing the remains of distinguished French citizens such as Voltaire, Jean Jacques Rousseau, Victor Hugo, Emile Zola, Marie and Pierre Curie, and Jean Moulin. Today, the Pantheon resembles less of a church and more of a museum. The church had long been stripped off its altar. In the apse, instead, hangs from under the central dome, a large metal pendulum that challenges the very beliefs the Church once held.

Foucault pendulum at the Pantheon, in Paris.

This is the famous Foucault pendulum, one of the best-known experiments in the history of science that has been recreated thousands of times across the world in schools, museums, observatories, public parks and even churches. It created a sensation when it was first displayed to the public, and it continues to awe scientists and laymen till today.

For thousands of years people debated whether the sun revolves around the earth (the geocentric model) or the earth revolves around the sun (the heliocentric model). The church maintained that the earth was stationary but astronomers such as Copernicus and Galileo provided enough proof to change the views of the scientific world to heliocentrism. Ever since then, it had been taken for granted that the earth rotates on its axis. But nobody had actually demonstrated the fact. For a normal observer, the earth seemed stationary and aside from the apparent spin of the sky, there was no observable effect that could be attributed to the rotation.

Jean Bernard Léon Foucault was the leading experimental physicist of his day. While working on a clock with a conical pendulum, Foucault placed a steel rod in a lathe and then accidentally bumped the rod and set it into motion. Foucault noticed that the rod tended to maintain its plane of vibration even when rotated. He perceived that if an oscillating pendulum maintained its plane of vibration, as the rod did, it might provide a mechanical demonstration of the earth's rotation.

Foucault determined that the period of rotation of the plane of the pendulum's oscillation varied by latitude—at the poles it would take the pendulum exactly 24 hours to complete one rotation, while at the equator no rotation would be observed. In between the poles and the equator the period would be inversely proportional to the sine of the latitude where the pendulum oscillated.

Foucault first demonstrated his discovery to the Academy of Sciences at the Paris observatory in January 1851. By graduating the circumference of the circle described by the pendulum's arc of oscillation, Foucault demonstrated that the earth had moved from the pendulum's plane of oscillation, providing compelling and tangible proof of the earth's rotation. In March 1851, at the request of President Louis Napoléon Bonaparte, Foucault performed the experiment at the Pantheon with a gigantic pendulum 28 kg in weight hanging from a wire 67 meters in length.

The experiment drew a large crowd. People were fascinated to see the earth’s rotation rendered actually visible before their very eyes. It was such a practical, evident and majestic demonstration of the rotation of the earth that it could seen and understood even by an unscientific observer.

News of the Foucault pendulum spread like wildfire, and before long people all over Europe and in America began demanding demonstration and explanation of the phenomenon. Experiments were repeated, both in private and in public, by scientists and enthusiasts, in laboratories, private homes, public places, government buildings, schools and universities and churches. Never before a scientific experiment had so captured the imagination of the public.

“The Foucault pendulum's popularity was a result of its highly visual effect, its illustration of a basic physical principle, its use of readily available apparatus, and its ability to fascinate observers,” writes Michael F. Conlin.

“No one made religious objections to the demonstration of the earth's rotation with a Foucault pendulum [in a church]” observes Conlin. “Instead, churchmen participated in the scientific controversy. The Reverend John L. Dagg, a leading Baptist theologian and president of Mercer University, presented a geometric proof of the Foucault pendulum.”

The remarkable experiment eventually helped erase the last traces of lingering doubt the Church had against the heliocentric model.

“Foucault’s definitive proof of the rotation of the earth helped vindicate Galileo, Copernicus and Giordano Bruno,” writes Amir D. Aczel, author of Pendulum: Léon Foucault and the Triumph of Science. “After Foucault successfully demonstration of the earth’s rotation Church scholars themselves embraced the heliocentric Copernican view of the world and openly wrote about Foucault’s proof. In 1911, the Jesuit priest J.G. Hagen wrote a major study called ‘The Rotation of the Earth: The Earth’s Mechanical Proofs Ancient and New.’”

Today, there are a number of churches where one can see a Foucault pendulum in motion, such as Saint Isaac's Cathedral in Saint Petersburg, St. Peter and St. Paul’s Church in Kraków, San Petronio Basilica in Bologna, Grote Kerk, in Veere, to name a few.