Since, our childhood we have been taught that the Earth is moving and is not only revolving around the Sun but also around its own axis. By watching how Sun appears to be moving throughout the day, you can quickly conclude that at least something is in motion. This apparent movement of the sun is what prompted ancient observers to conclude that Sun along with other heavenly bodies are revolving around our Earth and hence Earth is the center of our universe. Now we know that Earth is revolving around the Sun and also around its own axis.
Despite the fact that Earth is moving, you probably think that you didn’t feel like you are moving. You may wonder how can you find out and convince yourself on your own if our Earth is moving?
There are several methods from which we can find out about Earth’s motion. These include Stellar Parallax, Doppler Effect, Coriolis Effect, Aberration of Light etc. However most of the methods are fairly advanced and require sophisticated instruments before we conclude Earth’s motion.
But there exist a simple experiment with which we can demonstrate the rotation of Earth. All we need is a long pendulum and long patience to witness the Earth’s rotation. This experiment was first performed by French physicist Leon Foucault in 1851. Therefore it is sometimes called as Foucault’s Experiment.
The basic idea behind the experiment is that as the Earth rotates, the floor beneath the pendulum rotates while the plane of pendulum remains fixed which gives an appearance that the pendulums plane is rotating relative to ground whereas it is actually an effect due to Earth’s rotation.
In how much time, will pendulum plane makes full rotation depends upon your location in Earth.
If you are at North pole, it will take 24 hours to complete one full rotation of pendulum plane and the direction would be clockwise. This effect will be reversed at South pole, that is full rotation of the pendulum will still take 24 hours however the direction would be counter-clockwise.
At Earth’s equator, there is no apparent motion of swing plane. Hence you won’t find the effect of Earth’s rotation at equator. This is not because that Earth is not rotating but its mainly due to the limitation of the experiment involving pendulum.
At intermediate latitudes, the rotation rate is slower and depends on the sine of the latitude. Here is the formula to calculate the rate of rotation of pendulum plane:
For example, if we are performing this experiment in Paris (where it was originally performed by Foucault) then the latitude of Paris is 49o N so rate of rotation of pendulum plane is
This mean the pendulum will take full rotation in about 32 hours at Paris having latitude of 49o N.
Foucault’s Experiment
The full Foucault’s Experiment goes like this:
- First you need long and heavy pendulum suspended from a fixed point in the ceiling such that its free to move in vertical plane. The pendulum should be at least 3-4 meters long so observe the effect of Earth’s motion.
- Make sure that the pivot point around which pendulum is rotated is created in such way that it allows minimum friction so that pendulum can swing for longer periods.
- Now if you give an initial swing to the pendulum and it starts swinging back and forth. Knowing the latitude of your location and using the equation mentioned above, you can found out how long it will take your pendulum to make one full rotation.
This experiment provides a clear confirmation of Earth’s rotation. The predictable change in swing plane rotation is aligned with the expected effects of Earth’s rotation.
To find out more about Foucault’s experiment, you can watch the video here: