A scientist named Foucault (pronounced Foo-koh, or Foo-calt) used a pendulum to demonstrate that the earth rotates. The logic of this device is easiest to understand if we consider a pendulum at the north pole. There the vertical direction is the axis of rotation of the earth, so that "upwards" remains the same direction throughout the day as the earth makes one rotation. Under these circumstances, a swinging pendulum will swing along the same direction in space, so that it will be left behind as the earth turns beneath it. From the point of view of the observers, who rotate with the earth, the pendulum seems to change direction of swing. You could use the direction of swing for a 24-hour clock. The situation is more complicated elsewhere on earth: now the vertical direction does change throughout the day (which is why the sun rises and sets). The result is that it takes longer than a day for the Foucault pendulum to make a complete rotation, in a way that depends on the latitude. At the equator, the Foucault pendulum doesn't do anything at all.