On March 31, 1851, Léon Foucault demonstrated for the first time in public that the Earth turns on its axis. His famous pendulum, installed for the occasion inside the Pantheon, in Paris, offered elegant proof of this imperceptible movement. Today, such pendulums exist in different locations around the world, each swinging with the same enigmatic tranquility. Discover the science and the serenity of Foucault’s pendulum.
Foucault's pendulum, at the Centre national des arts et métiers, in Paris
(Image: Flickr / alexdecarvalho)
The place is as simple and elegant as the experiment taking place within its walls. In a former chapel, brightened by the diffuse light passing through its stained glass windows, a pendulum swings. Steadily. Silently. This space of the Musée des arts et métiers in Paris is empty this afternoon, apart from the security guard seated, unmoving, on his chair. Perhaps he, too, is hypnotized by Foucault’s pendulum, which shows before our very eyes that the Earth is turning.
The beautiful golden sphere swinging at the end of a wire, whose origin is hidden at the top of the vault, 17 meters above the ground, was donated to the museum in 1869 by its inventor, Léon Foucault. Constructed in 1851 of steel, lead and brass, it is the same sphere that, four years later, astonished the public at the Exposition Universelle held in Paris—a surprising reception for a simple pendulum, nothing more than a metal ball 18 centimeters in diameter, weighing 25 kilograms, attached to a bit of piano wire.
Today, in the deconsecrated church of Saint-Martin-des Champs, built on the site of a 6th century Merovingian funerary basilica, the pendulum swings, left to right. Its oscillations continue with reassuring regularity, its movement maintained by an electromagnetic device in the base of the installation. But stay a moment and you’ll see it move, changing the orientation of its steady back-and-forth. In the chapel, this movement is marked by a dozen little metal pins, arranged in a circle on the glass table below the pendulum. With time, the direction of the swinging turns, and the ball knocks the pins down in its slow advance. The occasional sound of metal clinking against glass bears witness to the phenomenon taking place.
And yet, once a pendulum has been set in motion, explains François Mathias, a science communicator with the CNAM, it does not change the direction in which it swings. “The pendulum is only attached to the Earth at a single point, which allows it to maintain its direction of oscillation.” Another phenomenon, then, must be responsible for the progressive fall of the pins. Indeed, it is not the pendulum that turns, but the Earth, turning beneath the pendulum. The rotation of the instrument is only apparent; it is us, the chapel, the glass table and 12 little pins, all firmly attached to the Earth, that are turning around the pendulum.
This is what Mr. Foucault—a surgeon by training, physicist, inventor and journalist by trade—was looking for: direct proof that the planet rotates, without us noticing the slightest movement. “People already knew, at the time, that the Earth turned,” François Mathias says. “The cycle of night and day was already very convincing proof, but Foucault wanted to prove it without resorting to any celestial elements.” That had already been done 218 years earlier by Galileo, who was forced to recant before the Inquisition his theory of the rotation of the Earth. An appropriate choice, in the end, to install the instrument that confirmed his theory in the serenity of a restored church. Between religion and science, the pendulum has often swung, but this pendulum has found the perfect balance.
Foucault's pendulum at the Pantheon, Paris (Image: Flickr / y.caradec)
This text was translated from French by the author. The original can be found here:
Le Pendule de Foucault : Entre deux extrëmes
Find out more:
"About Foucault Pendulums and How They Prove the Earth Rotates!", California Acadmy of Sciences
The Foucault pendulum, from Physclips, a multi level, multi-media resource
Recantation of Galileo (June 22, 1633)