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.)