Foucault's Pendulum

An iron ball hangs from a cable and swings back and forth in a steady rhythm. That's Foucault's Pendulum. A swinging object like this is called a pendulum, and "Foucault" is a surname: the French physicist Léon Foucault used this pendulum in an experiment to prove that the Earth rotates, and the name stuck.

So how can a pendulum prove the Earth's rotation? Rotation means the Earth spinning once on its own axis. We all learned in school that day and night change because the Earth rotates once a day. But in Foucault's time, the 1850s, some people still believed that the Sun moved, and that this was why day turned to night. Galileo and Copernicus had already argued that it was the Earth that moved, not the Sun, but no scientific experiment had ever demonstrated it.

Foucault decided to settle the question by watching a pendulum swing for an entire day. He marked a scale and numbers on the floor to track its movement. The path of the pendulum's back-and-forth swing is called the plane of oscillation, and as time passed, this plane slowly turned clockwise. Nobody had touched the pendulum, yet its plane had shifted. That could only mean the Earth itself was rotating beneath it. The pendulum matters because it let people watch the Earth's rotation with their own eyes.

You might expect a scientific instrument like this to sit in a science museum. So why is it in the Panthéon, the national mausoleum of France's great figures? Because Foucault staged the experiment here, in front of several hundred Parisians. The Panthéon wasn't his first venue, though. He began in a cellar, hanging a light 5 kg pendulum from a short 2 m cable. To show the Earth's slight movement, a pendulum has to keep swinging for at least two hours, but a light weight on a short cable kept stopping too soon. The Panthéon's soaring dome was the perfect fix. The pendulum swinging in front of you weighs 28 kg and hangs from a 67 m cable.

Later, when Napoleon III seized power in a coup and became emperor, the Panthéon was turned from a national mausoleum back into a church, and the pendulum was moved to the Musée des Arts et Métiers in Paris. After the Panthéon became a mausoleum again, a replica was made and has been on display here since 1995. That's why you can see Foucault's Pendulum in the very place where he carried out his experiment.