It is now a given that the Earth spins on its axis, completing a rotation once every 24 hours. We learn it in school and we take it for granted based on the day-night cycle that we experience everyday. But for the longest of times, we only had astronomical evidence to back this claim. Based on watching how the stars circle in the sky, the conclusion regarding Earth’s rotation had been made.
However, there had been no obvious visual demonstration to support the same. As early as Galileo’s time, scientists had tried to drop objects from towers, attempting to measure the drift eastward based on where it landed, as the planet spun beneath it. This crude method proved to be a failure as the object dropped far too quickly and there were too many interfering factors.
It wasn’t until 1851 that there was a conclusive visual demonstration to show the rotation of the Earth. And the credit to it goes to Jean Bernard Leon Foucault, a French physicist who is best remembered for this demonstration.
Foucault’s insight
According to his journals, at about 2 A.M. on a night towards the end of the first week of January in 1851, Foucault had an insight. He realised that he would be able to see the effect of the Earth’s rotation if he were to devise a way to hang a pendulum from the ceiling such that it was free to swing in any direction.
As the Earth turns beneath the pendulum, it would appear that the pendulum’s path was shifting slowly, while in reality the plane of oscillation of the pendulum would remain fixed.
Foucault knew that there was no room for error. His pendulum had to be perfect with a symmetrical bob. As the slightest push would ruin the demo, the release too had to be done accurately.
Success in his basement
The first demonstration took place in his basement. With a 5kg pendulum hanging from a 2m cable, Foucault achieved the success he was seeking. Foucault observed a small clockwise motion of the pendulum’s apparent plane of oscillation, while in fact it was the Earth that was moving beneath the pendulum’s fixed plane of oscillation.
For his arranged demonstration for the scientists of Paris on February 3, 1851, Foucault was bold enough to send out an invite that read “You are invited to see the Earth turn.” The assembled scientists did indeed witness the Earth turn as Foucault’s pendulum suspended from a 11m cable in the Meridian Room of the Paris Observatory displayed the expected results.
Sine law
Foucault further refined his apparatus and also arrived at a simple equation, known as his sine law. The equation, which stated that the angular speed in clockwise degrees per sidereal day is 360 times the sine of the latitude, showed the influence that latitude had on how much a free-swinging pendulum would move. This meant that while the plane of oscillation would never move in the equator (sine of zero degrees is zero), the plane of the pendulum would complete a 360 degree rotation in 24 hours at the North Pole (sine of 90 degrees is one).
Public fascinated
With the backing of French President Louis Napoleon, also a science buff, Foucault performed his public demonstration of the same experiment on March 31, 1851. Under the huge roof of the Pantheon in Paris, Foucault hung a 28 kg brass bob that was 38 cm in diameter on a 67 m long wire. A wood platform with a thin layer of sand was placed in the marble flooring. As the pendulum moved, the pointer attached to the bottom of the sphere traced out patterns in the sand, indicating the movement of Earth.
The striking demonstration was a huge hit with the gathered public. And to this day, Foucault’s pendulums remain a popular exhibit in science museums throughout the world.
