In 1851, Léon Foucault showed a pendulum like this in Paris. “Incredible!”, “Wow!”, “Fantastic!” were just some of the audience’s reactions. Why was everyone so excited? Because that was the very first proof that the earth rotates!
Go ahead and look at the pendulum. In which direction does it swing back and forth? Go look again a few hours later. Now it looks like the pendulum has changed direction. It hasn’t. A pendulum cannot change direction by itself. For that, a force has to act on it. And that is not happening here. So, if the pendulum is still moving in the same direction, the earth must have rotated underneath it. That is why some rods have been knocked over.
About every 24 hours, the earth rotates once around its axis. We call that … a day.
