Foucault's Pendulum

Foucault's Pendulum at The Franklin Institute

The Franklin Institute’s four-story Foucault’s Pendulum mimics an experiment undertaken by French physicist Jean-Bernard-Leon Foucault early in 1851. The experiment proved the nature of the Earth’s daily rotation around its axis. The fact that the Earth is spinning and the pendulum is swinging creates this interesting effect. At 9:30 am, the museum starts the pendulum swinging in a North-South direction. During the day, the pendulum knocks down a peg every 20-25 minutes, making it appear to change direction. By 5:00 pm, half of the pegs have been knocked down.

Here in Philadelphia, the pendulum moves with the Earth and the Earth turns underneath the pendulum and appears to change direction throughout the day, an effect caused by the Earth’s rotation.