A couple of weeks ago we saw an awesome optical illusion created by a series of pendulums of gradually increasing length. Today we focus on a single pendulum, Foucault's pendulum, and the incredible insights (scientific and philosophical) that it can reveal.
The idea behind this kind of pendulum is quite simple: if you simply release it so that gravity does all the work, a pendulum will swing back and forth in a straight line. Foucault took this simple fact about pendulums and used it to prove empirically that the Earth rotates on its own axis: because the pendulum really does swing back and forth in a straight line and the Earth rotates below it as usual, the pendulum should produce the illusion of slowly rotating around the plane of the Earth, but that's just an illusion created by our point of view, being as we are, standing on a rotating planet!
But the awesomeness doesn't stop there. Because Foucault's pendulum is an instrument to measure rotation, you can always ask what the rotation is relative to... and then things can get spooky, as Darmouth physics professor Jim LaBelle explains: