Foucault's Pendulum

Foucault's Pendulum in Griffith Observatory

In the main entrance to the Griffith Observatory is a Foucault's Pendulum. Besides being the title to a complex and fantastic book by Umberto Eco, a Foucault's Pendulum is a tool invented by Jean Bernard Leon Foucault to demonstrate the rotation of the earth. The original, (proving to the world once and for all that the earth does indeed spin) is hanging in the Pantheon in Paris. (It was on my list of things to see this past trip to France, but I missed it because we didn't have enough time.)
Anyway, how it works is that a heavy ball is suspended from a cable which is attached to the ceiling about 30-35 feet up. The cable is hung in such a way that keeps the pendulum form getting twisted up. Thus it can swing back and forth without experiencing a spinning component, or any additional forces. When one views the pendulum swinging over the floor (often numbered like a clock) it appears as though it is moving clockwise over the floor as it swings back and forth. BUT what is really happening is that the floor is spinning beneath the pendulum which is swinging back and forth tracing the exact same arc over and over!
You might be wondering how we could know that it's the earth turning and not the pendulum? Well, depending on where on the earth you set it up, the pendulum and it's rotation will change, and it does so in a calculable and predictable way. At the north pole the floor will make a 360 degree rotation in a day, the farther south you go (or north from the south pole) the less complete the rotation will be. At the latitude of San Francisco the floor rotates about 220 degrees. At the equator it doesn't appear to rotate at all. (This is because there are two components of motion one is rotating around a center (like a spinning ice skater), and the other is like riding along in a great circle (like two dancers waltzing around a giant circle) The combination if these forces determines the motion of the floor beneath the pendulum. Thus it is demonstrated that the earth is turning, and not the pendulum itself. Cool stuff hu?