If you have ever been in a science museum (like the Astronaut Memorial Planetarium pictured below), it's likely that you've seen a Foucault pendulum. It's not a clever or obscure pendulum, but just a simple pendulum. It is typically very long and massive, suspended from a very high ceiling with clock-like markings in a circle at the base. The pendulum is named after
Jean Bernard Leon Foucault, a French scientist. He made significant contributions in many areas of science, but he is best known for illustrating the rotation of the earth using a simple pendulum.
While working on a lathe, Focault noticed an interesting effect from a metal rod that was held in the chuck of the lathe at one end, but free at the other. The rod was set vibrating in one direction (perhaps by accident?) and then the chuck was rotated. The plane of vibration of the rod stayed fixed!
From observing this, Focault's intuition was that if a pendulum were observed for a sufficiently long period of time, the plane of oscillation should appear to rotate. But that is from the point of view of the observer here on the rotating earth. In fact, the plane of rotation of the pendulum stays fixed and it is the earth that is rotating! The plane of rotation of the pendulum stays fixed relative to what? It stays fixed relative to a fixed inertial reference system. A good approximation of such a system would be fixed relative to the sun. (It was the German scientist Wilhelm Olbers, famous for Olbers' Paradox, that suggested the only fixed inertial reference frame would be relative to the distant stars.)
Viewed from the point of view of the earth, there seems to be a transverse force that pushes the pendulum perpenducular to the plane of rotation, causing the slow rotation of the plane. (This force is named after a French mathematician and is the same force that causes hurricanes!) But this force is really fictitious! It is the result of observing from a non-inertial reference frame. For example, if you toss a ball directly upwards from inside a car that is accelerating forward, there will appear (to you) to be a horizontal force that pushes the ball backwards over your head. (It's the same apparent force you feel pushing you back into your seat.) Observers outside the car understand that there is no horizontal force on the ball and it is you that is accelerating out from under the ball. This same phenomena is at the heart of the apparent motion of the Foucault pendulum. We are moving in a circle here on earth and hence, we our frame of reference has a centripetal acceleration. This acceleration is very small and we don't normally notice it. But if we watch a pendulum for long enough, we can see the effect.
The period of rotation depends upon your lattitude on earth. The period is shortest (fastest rotation) at the poles of the earth, where it is exactly 24 hours. Had Focault lived at the equator, he would not have been able to demonstrate this effect. The period of rotation at the equator is infinite; it doesn't rotate at all. Picture a pendulum at the pole and and at the equator and see if you can understand why this happens. There is a rather extensive Foucault pendulum site by Professor B. Nickel, Physics Department at the University of Guelph, that will provide many more details.
So, can we build a Foucault pendulum here at UVI? Yes, if you have the time and patience to do it. There are several factors that will work against you. First, there is simple air resistance. Although air resistance can be diminished by using a very massive ball and very low amplitude, even the big museum versions will come to rest after only about an hour. It is necesssary to create a carefully controlled "kick" system that replaces the lost energy. And, as if that wasn't enough, it is critical that the motion be in a plane. If the ball is released improperly, the pendulum will swing in an elliptical path. This will create a separate precession, like that for a spinning top, that can easily dominate the motion. Small vibrations of the support or asymmetry in the kick system can also introduce this problem.
The Queen Victoria Building (QVB) in Sydney has recently installed a Focault pendulum. They have a nice descriptive page that provides another fine general description of the Foucault pendulum, but also some specific details about their setup as well. It will give you some insights into the difficulty of maintaining a properly running Foucault pendulum. If you are undaunted by the challenges, building one could make for a very nice project ... perhaps a permanent display here at UVI. There are many recent articles describing how to build a modest-sized version. I'd be happy to help!
Questions:
What is the name of the fictitious force that causes the plane of oscillation of the Focault pendulum to precess?
How long was the pendulum that Foucault constructed at the Pantheon that created considerable interest at the 1851 Paris Exhibition?
What would be the rotational period of a Focault Pendulum here in St. Thomas? (We are at lattitude 18.3 o.)
It what direction (clockwise or counterclockwise as you look down on the pendulum) does the Foucault pendulum rotate in the Northern hemisphere?
How does the QVB keep its Foucault pendulum going?
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From observing this, Focault's intuition was that if a pendulum were observed for a sufficiently long period of time, the plane of oscillation should appear to rotate. But that is from the point of view of the observer here on the rotating earth. In fact, the plane of rotation of the pendulum stays fixed and it is the earth that is rotating! The plane of rotation of the pendulum stays fixed relative to what? It stays fixed relative to a fixed inertial reference system. A good approximation of such a system would be fixed relative to the sun. (It was the German scientist Wilhelm Olbers, famous for Olbers' Paradox, that suggested the only fixed inertial reference frame would be relative to the distant stars.) 
The period of rotation depends upon your lattitude on earth. The period is shortest (fastest rotation) at the poles of the earth, where it is exactly 24 hours. Had Focault lived at the equator, he would not have been able to demonstrate this effect. The period of rotation at the equator is infinite; it doesn't rotate at all. Picture a pendulum at the pole and and at the equator and see if you can understand why this happens. There is a rather extensive