A demonstration of the rotation of the Earth.
The principle of the Foucault pendulum: After one quarter of a rotation (6 hours) the pendulum appears to swing perpendicular to its inital direction at the pole, giving it a rotation of 15° per hour; at the equator the pendulum is always aligned in the same direction. At intermediate positions the plane of oscillation rotates at a rate of less than 15° per hour and decreases from the pole towards the equator, where it reaches zero (no rotation).
A public demonstration Foucault pendulum in St. Petersburg.