The Device That Lets You See The Earth Turn: The Foucault Pendulum

In a famous 1851 experiment, Leon Foucault would, once and for all, prove the Earth spins.

The Device That Lets You See The Earth Turn: The Foucault Pendulum
Foucault Pendulum Arnaud 25/Wikimedia Commons

In 1851, a French medical school dropout and amateur photographer shocked the scientific establishment of Paris. This man was Leon Foucault and he had, for the first time, created an experiment to definitively show that the Earth did indeed spin.

Just prior to his demonstration he sent out a note to a group of scientists stating:-

"You are invited to see the Earth turn"

They were astounded and a month later he repeated it in the magnificent Pantheon in Paris for all to see. From this moment onwards he would forever be immortalized in history for his beautiful and elegant apparatus.

Foucault just so happened to also provide an interesting technique for measuring the speed of light. Quite an astounding man.

Did Foucault prove the Earth spins?

In short yes, but he wasn't the first. In the mid-1800's it was a widely accepted belief that the Earth did indeed rotate on its axis. 

In fact, as early as classical antiquity, philosophers had postulated that this might be the case. Many scholars from the Pythagorean school in ancient Greece made this very hypothesis millennia ago.

Some of the earliest propositions were by Philolaus who drew support from Hicetas, Heraclides, and Ecphantus in the 4th Century BC. Aristotle would famously later disagree, a notion that became the established 'truth' for some time until the great Copernicus adopted his heliocentric system in the 14th Century.

Kepler, Galileo, and Newton would further add they're significant weight to the theory and the rest, as they say, is history.

Foucault's contribution was to provide a way to demonstrate a fairly complex scientific concept in a way that the public could easily grasp. His pendulum, in a pinch, dispelled any lingering doubts in people's minds at the time.

The Foucault Pendulum would forever settle the issue and established the phenomenon as fact. 

His experiment was an instant success and drew massive crowds of members of the public. Foucault would become, in his own way, a rock star of the times. 

Pendula based on Foucault’s calculations began appearing worldwide—and are still iconic features of many science museums in the U.S. and other countries.


When was the Foucault pendulum invented? Who invented the pendulum?

The famous Foucault Pendulum is a relatively simple device that was named after the French physicist Leon Foucault. Leon conceived of a means of easily demonstrating that the long-held belief that the Earth spun around its axis.

The problem was, at the time, that there was no definitive, or simple, means of showing this was true. Foucault would soon solve this once and for all.

The pendulum was introduced in 1851 and was the first experiment to provide a simple, and direct means of proving the this was indeed the case. There are many such pendula around the world today, like the one at the Griffith Observatory in Los Angeles.

Foucault pendulum LA
Foucault Pendulum in the W.M. Keck Foundation Central Rotunda, Griffth Observatory. Source: Griffith Observatory

The very first public demonstration of his pendulum was in the Meridian of the Paris Observatory in 1851. This was made before a group of dignitaries including Napoleon III. 

Several weeks later the larger, and now famous pendulum was built using a 28 kg brass-coated lead bob suspended on a 67-meter long wire hanging from the dome of the Pantheon in Paris.


This pendulum swings clockwise at around 11.3 degrees an hour and makes a full rotation every 31.8 hours because of Paris' latitude on the Earth. 

This is because the direction of the plane of oscillation (swing) of a pendulum with respect to the Earth rotates with an angular speed proportional to the sine of its latitude. So, for example, one at 45 degrees latitude rotates once every 1.4 days and one at 30 degrees every 2 days.

As the sine factor was difficult for people to understand it was one of the influencing factors that prompted Foucault to build his device in the first place. It also prompted him to create his gyroscope in 1852.

The original 1851 bob at the Pantheon was later moved to the Musée des Arts et Métiers in Paris in 1855 with another temporary installation made in 1902 for its 50th anniversary.

in the 1990's reconstruction work at the Musée des Arts et Métiers required the pendulum to be relocated to the Pantheon and it was later returned in 2000.


Tragedy struck in April of 2010 when the suspension cable snapped resulting in irreparable damage to the pendulum and marble floor underneath. Today an exact copy of the device currently operates under the dome of the Pantheon in Paris since 1995.

Foucault Pendulum Paris
Current replica of the Foucault Pendulum at the Pantheon, Paris. Source: Arnaud 25/Wikimedia Commons

Amazingly this was not the first time the pendulum flirted with disaster. In May of 2009, a party reveler at the Musée des Arts et Métiers grabbed the pendulum and swung it into a security barrier - luckily it was not damaged. 

The destruction of the original was a real loss to the scientific and historical community with many mourning its passing. Some would even compare this to the news that one of the statues at the Vatican had been broken.

How does a Foucault pendulum work?

In short, a Foucault Pendulum swings in a fixed plane whilst the Earth, and building the pendulum resides, rotates around this plane. This might sound a bit odd, but the principle, when you see it animated, is actually pretty simple and self-explanatory.


"But", we hear you say, "How doesn't the pendulum rotate with the Earth?". After all isn't it attached to the ceiling of something that is also spinning with the Earth? 

The special thing about Foucault's pendulum, apart from the fact they tend to be beautifully crafted objects, is its pivot point. Foucault's insight was to design it in such a way as to allow the pendulum to swing in any direction in the vertical. 

Foucault Pendulum sketch 1895
An 1895 sketch of the original pendulum. Source: T. O'Conor Sloane/Wikimedia Commons

In this sense, it is not restricted to remain in a single defined linear direction - like a Grandfather clock for instance. It can keep swinging in one plane regardless of the relative position of the structure the pivot is attached. 

Still confused? Here's a great animation to explain the phenomenon. 

Foucault Pendula rotations vary depending on latitude

In order to visualize what's going, let's take a Foucault Pendulum and place it in different parts of the globe. 

First off we'll plonk it on the North or South Pole. Once you get the pendulum moving it will continue to swing in a fixed plane relative to the Earth. Forget the fact that the Earth is itself orbiting the Sun - after all, it was designed to show that the Earth was spinning, nothing more. 


If you set up some way to show it had indeed moved from its original position, e..g. marking out a path in some sand or setting up objects to knock over, you would see that it actually completed a full rotation, 360 degrees, in 24 hours.

Now let's take the same setup and place it in Paris - as Foucault did. Here you would see that the pendulum's plane rotates a full 264 degrees, give or take, in 24 hours

What about at the equator? At the equator, all things being equal, and the pendulum's swinging plane will not rotate at all. 

Simple right? 

foucault pendulum animation
Source: Nbrouard/Wikimedia Commons

This phenomenon (i.e the apparent force due to Earth's rotation) is commonly called the Coriolis force or effect. It is this that is responsible for much of the Earth's weather patterns and ocean currents.

How does a Foucault pendulum keep swinging?

Pendulum experiments of this nature require careful precautions to ensure it is not acted upon by external forces (other than the gravity of course). When starting one, for example, the pendulum is held at an angle by a string. 


This string is then burnt to release it. Any other action, like cutting by hand or released from the hand could impart undesired momentum in a particular direction. 

Long pendula, like Foucault's, with heavy bobs suspended on a rigid wire, have enough momentum to keep swinging for long periods of time. However, the ever-present resistance created by air will eventually rob the pendulum of energy and will come to a halt eventually.

To counteract this, most displays have installed an electromagnetic drive to keep the pendula in motion. This solution is ideal as it provides the extra energy needed without impacting the pendulum's direction of motion.

This system usually comprises of two iron collars that are attached to the pendulum's wire near the top. This is fed through a doughnut-shaped electromagnet built into the ceiling.

Other solutions place the electromagnetic drive on the floor.

As the cable swings back an forth so to do the iron collars inside the hole of the doughnut. As the cable reaches a particular point in its arc it is detected by an electronic device that activates the electromagnet.

When calibrated for just the right time it provides the pendulum with a tiny "kick" in the direction of its natural swing. This restores any energy lost during the swing and keeps the pendulum moving.