Foucault’s Pendulum — Earth’s Rotation Cycle

Sunny Kim (11)

Sunny Kim is a staff writer and unofficial physics expert at BUDDY!. Sunny is also an avid fan of Earth’s rotation (as we all should be).

In her newest article, Sunny delves deeper into her passion and investigates the majestic and mysterious Foucault’s pendulum.

Imagine being a 28kg ball hanging from the ceiling on a 67m-long steel wire. Pretty great, right? It’s an experience I would love to have — being a Foucault pendulum. 

The Foucault Pendulum (From Wikipedia)

The Foucault’s pendulum, despite its imposing name, is just like any run-of-the-mill pendulum — but only much bigger. Like any pendulum, it is composed of a ball suspended by a string and displays the same repetitive back-and-forth motion

The Foucault pendulum (pronounced foo-koh), is named after the French physicist Jean Foucault, who created the first Foucault pendulum. His most famous demonstration showcased a 28kg brass sphere attached to a 67m cable in 1851 at the Pantheon in France. 

At first glance, it seems nothing more than a gigantic pendulum that serves no specific purpose. However, the real value of this pendulum lies in its ability to prove Earth’s rotation without the need to observe celestial objects. Imagine a pendulum at the North pole. It is swinging back and forth, but Earth is rotating underneath it, so the pendulum would seem to be rotating to us Earthly inhabitants, when it is actually the Earth rotating. Here is a video to demonstrate: 

The period of the pendulum’s rotation also varies with its location. If the pendulum were to be located in the North or South Poles, it would take 1 sidereal day, or 23.93 hours, for it to complete one rotation. This is because the plane of the pendulum’s swing is perpendicular to the direction of Earth’s rotation. In other words, when you look at the pendulum at the North Pole, it looks like it is spinning, but what actually is happening is that the Earth is rotating, not the pendulum. Likewise, if the pendulum was at the equator, it would not seem to be rotating at all, since the pendulum would be rotating with the Earth. Here is another gif to demonstrate: 

(From Wikipedia)

The time it takes for the pendulum to complete one rotation --given its location-- can also be calculated with a (very hard to derive) formula by Foucault: to find the time, divide 23.93 (one sidereal day) by the sine of the degree of latitude. For example, the time it took for the original Foucault pendulum to rotate once was about 32 hours, or 11.25 degrees per hour, and Foucault predicted this with his formula. If there was a Foucault pendulum in Beijing, it would take about 37.3 hours for it to rotate once (Beijing’s latitude is 39.9042 degrees North). Since the period of the pendulum is different depending on the location, you can also prove to Flat-Earthers that the Earth is indeed round. 

Composed of such simple ingredients, the Foucault pendulum seemed to be easy to make. In fact, I have considered making one on my own. What I didn’t consider was where I would find a ceiling high enough to hang a 60m cable with a steel ball, and even if I somehow managed to find one, I wouldn’t be able to keep it going. In order for the pendulum to work properly, gravity should be the only force acting on it. That means that air resistance and the spin of the ball has to be zero, but since air resistance can not be zero (unless it is in a vacuum), there has to be some other motion that compensates for the lost energy. Actually, museums use an electromagnet to push the pendulum in the direction of its motion to keep it going. 

There, my friends, is the Foucault pendulum. When I first came across it, I thought it was just for show. After researching, I realized how remarkable Foucault’s pendulum is: who could have thought that such a simple structure could display Earth’s rotation? It is fascinating that one simple pendulum could take complex scientific theories and simplify them, making science accessible and interesting for a wider audience. Although the Foucault pendulum is not needed to know that Earth is rotating, it still serves as a source of inspiration for children and adults alike, kindling or rekindling their interest in science.