Foucault's pendulum was a pendulum designed by Jean Foucault, a French physicist, to demonstrate the rotation of the earth. Although the rotation of the earth had been demonstrated before, it was only through use of the stars and their relative positions in the sky. The original pendulum, first used by Foucault in 1851, was the first experiment from the ground that clearly showed the earth's rotation.
A Foucault pendulum (a pendulum based on Foucault's original pendulum) shows the rotation of the earth by highlighting an apparent change in direction made by the pendulum through the course of the day. A line set under the original path of the pendulum and supporting lines to measure how many degrees the path has changed by show that the pendulum does not follow the same path. It is not, however, the pendulum that changes direction, as no force is applied to it to make it change direction. Instead, it is the position of the earth relative to the motion of the pendulum that changes.