September 18 - Happy Birthday, Jean Foucault

Posted on September 18, 2021

Key Points

This is an update of my post published on September 18, 2010:

Born on this day in 1819, in France, Foucault invented the gyroscope and is known by name because of his pendulum.

A gyroscope is a spinning wheel or disk that has an axle that is free to take on any orientation. This device helps measure or maintain orientation during navigation. For example, airplanes, space ships, and robots use gyroscopes. The Hubble telescope is in space and cannot use a magnetic compass to keep lined up properly, so it uses gyroscopes. Radio-controlled helicopters also use gyroscopes to stay level no matter whether they are rising, sinking, or zooming in some direction.

Foucault’s pendulum is like any other pendulum in that it’s a weight attached to a wire (or string); but it’s different from other pendulums because at the top it is attached to a universal joint that allows the pendulum to rotate freely. However, the pendulum just keeps swinging in the same direction, once it’s started off, unless some force acts on it–it just seems to be rotating because…

Because the Earth rotates beneath it, and the pendulum, with its universal joint, isn’t forced to rotate along with the Earth and almost everything else on it.

A Foucault pendulum is usually very heavy and usually hangs on a very long wire—often from the ceiling of a tall building such as a museum or cathedral. What’s cool about it is that it swings slowly and continues to swing for a long time; as the Earth rotates underneath the pendulum, it seems as if the Foucault pendulum is rotating.

In other words, a Foucault’s pendulum proves that the Earth rotates (as if we needed proof other than the “rising” and “setting” of the sun, moon, planets, and stars!).

Play with gyroscopes and a pendulum today!

Or, at the very least, check out these videos:

Here is a great video that shows gyroscopes spinning and balancing and explains why they work and what they are used for.

And just for fun, here is a video of a “human gyroscope” ride. (Did I say “fun”? I think I meant “And just for tummy-lurching viewing…”)

Here is a video of a Foucault pendulum. It takes a LOT of patience to watch the pendulum knock one of these pins down. If you don’t have the patience, fast forward near the end of the video. But if you have the time, the pendulum swings can be mesmerizing, a whole lot of “moments of Zen,” meditational.