"You see, Casaubon, even the Pendulum is a false prophet. You look at it, you think it's the only fixed point in the cosmos, but if you detach it from the ceiling of the Conservatoire and hang it in a brothel, it works just the same. And there are other pendulums: there's one in New York, in the UN building, there's one in the science museum in San Francisco, and God knows how many others. Wherever you put it, Foucault's Pendulum swings from a motionless point while the earth rotates beneath it. Every point of the universe is a fixed point: all you have to do is hang the Pendulum from it."

Foucault's pendulum is an easy experiment demonstrating the Earth's rotation. When Léon Foucault first performed the experiment in 1851, the concept that the Earth revolves was nothing new or radical; the pendulum's accomplishment was to provide a proof that did not require minute observations of the stars or other objects far removed from Earth. Foucault's pendulum is a highly localized, easily prepared experiment whose result is clear, powerful, and accessible even to the non-scientist. In short, the pendulum provides everything a science teacher could ask for in an instructional experiment.

So how does it work? The elegant answer is that the pendulum swings in a fixed plane and the Earth rotates beneath it, but this explanation is misleading. At the north or south pole, the pendulum is moving in a fixed plane (if we disregard the fact that the Earth is also revolving through space), so the plane of the pendulum seems to rotate through 360° as the Earth makes one full rotation. At any other point on Earth, however, the point at which the pendulum is attached cannot be considered a "fixed point," because that point also moves as the Earth rotates. The plane in which the pendulum swings is similarly in motion. Because of this, the amount of time that it takes for the pendulum to make one full rotation (with respect to its surroundings) is equal to one sidereal day (23.93 hours) divided by the sine of the latitude of its location. Since sin(0)=0, the plane of a pendulum located at the equator will not appear to move at all.

In order for a pendulum experiment to be accurate, precautions must be made to assure that the pendulum is not acted upon by any outside forces other than gravity. For example, to start the pendulum moving, it is usually held at an angle by a string, which the experimenter then burns to release the pendulum. Letting the pendulum go from one's hands, or even cutting the string, could give the pendulum undesired momentum in a particular direction. A heavy pendulum on a long, rigid wire can continue oscillating for long periods of time, but eventually air resistance will cause the motion to lessen and stop. Museums will often use an electromagnetic drive to keep their pendula moving, because such a setup provides additional energy to the pendulum without affecting its direction of motion.

On Earth, we call the apparent force due to the Earth's rotation the Coriolis force, a force that is mainly responsible for weather patterns and ocean currents. Contrary to popular belief, the Coriolis force does not cause toilets to drain in opposite directions in the northern and southern hemispheres; in fact, this phenomenon is not even real. The direction in which a sink or toilet drains depends on many factors, including any preexisting rotational motion in the water, as well as forces applied when draining begins. In experiments in which the draining is done very carefully—and usually weeks after the water was initially poured—the direction of draining will correspond with the Coriolis force, but these conditions are certainly not present in everyday situations.