In 1851,
Leon Foucault built a large
pendulum and observed its behaviour : besides the expected back-and-forth motion, the pendulum's swinging plane underwent a very slow rotation, achieving a 360° turn in about 31 hours. He was, as he later put it, "seeing the earth spinning round".
You may understand this more easily if you place the pendulum on the
North Pole instead of some
Parisian cave. Imagine the
Earth, with the pendulum on the top of it. The Earth spins around its axis, but why would the pendulum follow it ? It keeps swinging in the same plane, regardless of the outside world's orientation. Thus, for an observer located at some spot on the planet's surface (and therefore following its spinning motion), the pendulum actually seems to rotate with a 24 hours
period.
Rotation also occurs on other locations, but with a longer period, and it becomes perfectly still on the
Equator. The period is 28 hours in
Saint-Petersburg (
Russia), 37 hours in
New York (
United States), 119 hours in
Phnom Penh (
Cambodia) and 6329 hours in
Quito (
Equator).