Foucault Pendulum: Getting into the swing

We wrote before about how Galileo's insight into the isochronism of pendulum motion occurred at Pisa in 1581 AD. Then, as though keeping time with a cosmic pendulum, Léon Foucault unveiled his discovery in the year 1851 - an anagram of the number 1581 !

By 1851, it was long known and accepted that the Earth rotated around its own axis, and that this movement  created the illusion of "sunrise" and "sunset". Rotation produced 12 hours of daylight for a stationary observer on the surface of the Earth facing the Sun, and 12 hours of darkness facing away from the Sun. However there was no terrestrial proof, or any satisfactory visual representation of the fact that the Earth rotated.

Although Foucault made several important contributions to the science of Optics, he is best remembered for the innovative use of a Pendulum to demonstrate the rotation of the Earth - to anyone willing to wait and watch for at least an hour. To track the Earth's rotation, a pendulum would need to remain in motion for a couple of hours at least, and do so undisturbed by air currents or breeze. He needed to create a long and heavy pendulum, and he needed to create it indoors. He chose the Pantheon in Paris as the site of his demonstration, because it had one of the highest indoor suspension points available in Paris - the apex of its dome. The weight of the pendulum would help it overcome air resistance, and its length would give it a large period and swing, making it easy to observe.

He suspended a heavy bob from a 67 m long wire suspended from the roof of the Pantheon's dome and carefully set the pendulum into motion. It was important to set the pendulum into motion without imparting even a hint of sideways motion or spin, because that would dilute the validity of the observation.

The shiny golden sphere is the pendulum bob of the Foucault pendulum at the Pantheon

Markers on the ground allowed observers to note the points between which the pendulum moved in a straight line. Amazingly, even after a comparatively short span of 30 minutes, everyone could see that the pendulum's line of movement had shifted away from the initial marker points. The pendulum now swung between two points that were a few degrees clockwise of the original two points. After several hours, it was even more obvious that the pendulum seemed to be drifting more and more in a clockwise manner - and over a period of 32.7 hours, it would sweep the 360 degrees of a circle. At the poles, this duration would be approximately 24 hours, in keeping with our conception of the length of a day.

The Foucault pendulum works because the plane of the pendulum's movement does not respond to the Earth's rotation. As the Earth rotates in an anti-clockwise direction in the above perspective, the pendulum simply remains in its own plane, not following the Earth's rotation as viewed from a distant celestial object. 
Experiments also established that the Foucault pendulum "precesses", or "drifts" and loses time at different rates, depending on the latitude. This was mainly because of the difference in the Coriolis force component that changes gradually from the Equator to the Poles.

This video gives us an idea of how slowly a 67 m long pendulum swings. Note the markers set up below the pendulum, which allow you to track how the Earth rotates, while the pendulum continues to swing in its own plane relative to the stars.


  1. What a terrific explanation of an even more awesome apparatus !!!

    Real Satisfaction to see new Physics BLOGGERS showin some of Luck mate !!!!

  2. Hi Arnab,

    Thanks a lot for your appreciation. Please stay tuned for more posts!

  3. its a great that some buddy is courging peoples about physics : its a nice post mate . keep the good work up