Leon Foucault, a self-taught scientist of the 1800’s, invented the Foucault pendulum as a demonstration of the rotation of the earth. He took advantage of the fact that pendulums do not like to change the plane in which they are swinging. He saw this effect first with a bit of metal sticking out of the end of a lathe. It was vibrating, up and down, and he could see that the direction of the vibration did not change as the lathe turned. The vibration was still up and down. Not being familiar with lathes, a lot of people don’t quite get what he saw. There was no change in the vibration. No change.

To see what he saw try this. Take a long metal rod, like a car antenna, and stick the end into an electric drill. The antenna needs to be pulled, and allowed to bounce up and down, sort of like twanging the doorstops in a house. Turn on the drill and watch. Nothing looks different. The rod doesn’t move around with the turn of the drill. It will still be moving in the exact same way as it did before the drill started to turn. Quite boring. So boring that, until you wonder why it isn’t moving in a circle, it seems very unimportant.

Foucault realized along with lots of 19th century scientists that vibration of a doorstop, or a chuck in a lathe, or a ruler off the end of a table, is the same as the swing of a pendulum. Said the other way, the swing of a pendulum is vibration, just like the doorstop.
During Good Friday and Easter services I had the opportunity to watch a highly experienced server working with the thurible, which is the item where incense is burned. If the incense is lit too far in advance of the procession getting started, it will goes out while everyone is waiting. The solution is to keep swinging the thurible, and the server did, vigorously. In order not to hit anyone, he was swinging across his body but when the procession finally started, he had to change the direction of the swing, (so the thurible would swing beside him), perpendicular to its previous motion. He had a careful and quick maneuver for accomplishing that directional change. And this is where I get interested.

When people, in general, as opposed to scientists, decided that the earth went around the sun, as opposed to the sun going around the earth, their reasons did not include any kind of direct demonstration, in part because no-one could imagine one. (Galileo thought he had such a demonstration, in the motion of the tides, but he did not.)

In 1680, Newton used Galileo’s true ideas about force to explain gravity, and in the process, ordinary people came to an intuitive understanding that very large objects, like the sun, would not orbit very small ones, like the earth, which is quite small, relative to the sun. There were also religious and national considerations although these pulled ordinary people in all sorts of directions. But there was still no direct demonstration.

Until Foucault.

Years ago I watched a procession in a church that required the servers to turn left twice in order walk down the central aisle from the staging area where they lit the incense. The head server had an elaborate and beautiful way of getting around those corners. But watching the server swing the thurible so carefully, pointed out to me how clearly people have known that pendulums take management. Catholics have been swinging incense for a long time. Servers have developed ways to manage that swing for a long time. Because if they don’t, they will kneecap themselves. And spill the burning incense.

John Heilbron wrote a book back in 2000 called The Sun in the Church. In it he discussed monks using pendulums for experiments in the 1700’s. They wrote about how the pendulums didn’t work or went a little crazy after an hour or so. The monks would keep careful watch and try to fix the craziness, because a pendulum, before it twists, is a really good timekeeper, and time keeping is critical to good science.

Until Foucault thought about it, no-one realized exactly what was going on.

Then Foucault built a 220 foot pendulum using the new steel cable, that was possible because Henry Bessemer had shown how to manufacture it. He put a very heavy weight on the end of the cable, and a gimbal at the top, that allowed the whole apparatus to swing freely in any direction. His setup allowed the pendulum to swing for a very long time, so that people could actually see how the direction of the pendulum seemed to change, although it takes a careful explanation to understand why the pendulum is showing the rotation of the earth. In his own time, other scientists would come from far away to watch Foucault’s pendulum swinging away and demonstrating the turning of the earth.

Extra rabbit holes: The Foucault pendulum is a demonstration not an experiment, since it isn’t being compared to anything. I mention this because some people think science is only experimental but clearly sometimes it isn’t. An experiment would involve two pendulums. One kind of pendulum ‘experiment’ is done just by having these pendulums built at different latitudes on the earth. They shift direction in different amounts of time. At the north pole, where you can’t built one on the Arctic ice, it would take 24 hours for the pendulum to go around. It wouldn’t change at all at the equator.
It has been said that eclipses change the swing but I’ve never heard that anyone has actually managed a real experiment involving eclipses and several Foucault pendulums.