Lecture 7 E. Foucault's Pendulum

At the north pole, a heavy enough pendulum has enough inertia (mass) it won't be significantly affected by small tugs and pulls as its support rotates with the Earth. Consequently, such a pendulum will oscillate in a plane that stays put while the Earth rotates around it. So it will appear to us -- moving with the Earth -- that the pendulum's plane is slowly rotating!

This effect is dimished at lower latitudes, but does not vanish -- e.g., the Smithsonian's Foucault pendulum* rotates more slowly, but still rotates. (At the equator, no rotation exists; below it, the rotation is in the opposite sense.)