Today in History – January 6, 1851 – Foucault physically demonstrates the rotation of the Earth. Now this isn’t so easy to do because the earth rotates pretty slowly. How slowly? Well, it takes a full day to go around once, eh? So in a minute it’s going to rotate about, oh, 0.0007 revolutions. Not much. Certainly nothing you’re going to feel. So how to show it? Well, you could always point to the fact that the stars and sun go around us once a day, and figure that should be enough of a demonstration for a reasonable person. But people are often unreasonable!

So to make it all very real and exciting, Jean Bernard Léon Foucault decided it wasn’t enough just to have a cool name (you have to admit – that is a pretty cool one) and dreamed up what’s become known as the Foucault pendulum.  In 1851, Jean he obtained a nice chunk of metal (to the tune of 28 kilograms), and suspended it from the ceiling of the dome of the Pantheon in Paris. And when I say suspended, I mean SUSPENDED. 67 meters of suspension (or about 220 feet for those of you who still aren’t 100% metricized). That’s a BIG pendulum.

Since then there have been a whole bunch of these Foucault pendulums set up and they’re pretty nifty. Basically all you do is pull the weight to the side and let go. It then ticks back and forth (no surprise). And the earth rotates underneath. Which means that the ground slowly misaligns itself as the pendulum happily continues in its original tick-tock direction.

Usually they’ll do something like place little pegs in a big circle on the floor and there’ll be a tip extending from the bottom of the pendulum that slowly, ever so slowly, gets closer and closer to one of the pegs as it does its tick tock thing. And then it’ll juuuust brush the peg. Then nudge it. Then BOOM! Over it goes. Some minutes later the next one goes. And the next. All the way up until closing, when the guy in charge has to collect all the pegs and then, before the doors open the next day, set ‘em all up again. Hey, it’s a living.

Anyway, it’s a very satisfying way to waste a few minutes. I recommend it highly.

For more information, see the Engineering Pathway’s resources on Foucault’s Pendulum and pendulums. For related educational resources, visit the Engineering Mechanics or the Mechanical Engineering Education disciplinary communities