Pendulums generally swing back and forth in a single plane fixed in space. On Earth however, if the path of a pendulum's swing is traced out over time, the orientation of the plane appears to change. This provides evidence for Earth's rotation because it shows that Earth is rotating underneath the unchanging orientation of the pendulum's swing.

This animation shows two views of a pendulum swinging over the North Pole. The drawing shows the path traced out by the swinging pendulum.

  !   Click the image to see the animation.