In 1851, the French physicist Léon Foucault provided an experimental proof of the Earth’s rotation using a pendulum. Although Foucault is best known for this ingenious experiment, he also made several important contributions to physics including the discovery of eddy currents as well as measuring the speed of light. Foucault is one of the 72 scientists whose names are engraved on the sides of the Eiffel tower.
At the Arts et Métiers museum in Paris, visitors can witness Foucault's pendulum experiment in action twice daily. The setup features a 17-meter metal cable suspended from the ceiling, with a brass-encased lead sphere attached to its lower end. As the pendulum swings, it gradually knocks over wooden pegs arranged on a circular table below, demonstrating the Earth's rotation.
According to Maela Levigne of the Arts et Metiers museum, “the pendulum starts to oscillate in the direction in which it is launched with no lag observed. The oscillation period (of this pendulum) is between seven and eight seconds. Between each oscillation, the Earth shifts by one millimetre.”
She explained that despite the pendulum never shifting from its plane of oscillation, it manages to knock out the skittles because of the shifting of the earth below it.
What moves is the earth which will shift, relative to the plane of oscillation of the pendulum. “By shifting, the Earth moves the table, which moves the skittles that are laid on the table. They then come into contact with the pendulum and end up falling.”
Foucault's Pendulum
Foucault's experiment was the clearest proof at the time that the the Earth rotated. The simple demonstration not only amazed people, but also gave clear, visible evidence that Earth rotates.
Its attraction comes from how easy it is to understand and the deep meaning it carries, connecting the movement of a swinging pendulum to the spinning of Earth on its axis.
How the Pendulum Works
The Foucault pendulum is made up of a large, heavy weight, or bob, hanging from a long wire, able to swing in any direction.
Once it starts swinging, the pendulum moves back and forth in a fixed direction. However, unlike a normal pendulum that always seems to swing the same way, Foucault's pendulum slowly changes the direction of its swing.
This happens because the Earth is rotating underneath the pendulum, something Foucault clearly showed during his famous experiment at the Pantheon in Paris.
When Foucault first showed his pendulum in the Pantheon in Paris, it was considered a groundbreaking experiment. Although scientists already knew that Earth rotated - thanks to earlier observations by astronomers like Galileo and Copernicus - Foucault’s pendulum was the first simple, mechanical proof that didn’t depend on looking at the stars.
By hanging a 28-kilogram iron ball from a 67-meter wire inside the Pantheon, Foucault allowed people to see Earth's rotation with their own eyes. Over time, they could watch the pendulum’s direction slowly change, proving that the Earth was turning beneath it.
The original pendulum was moved from the Pantheon to the Musée des Arts et Métiers in Paris in 1855 where a replica still swings today.
More Than Just an Experiment
Foucault’s pendulum wasn’t just an experiment; it was a powerful example of how simple physical ideas can show deep truths about the universe.
It was important not only for showing Earth's rotation but also for connecting to other ideas in physics, like how rotating systems work, which also explains things like the way hurricanes spin and how ocean currents move.
In a time when science demonstrations were not as common as they are today, Foucault’s pendulum brought the reality of Earth's movement to the public, helping everyone understand that we are part of the grand, rotating journey of our planet through space.
