If you sit and watch the Foucault pendulum for an hour, you will "see" that the plane of the swing of the iron ball slowly shifts anti-clockwise by about 8.4 degrees per hour. This is an optical illusion. The building is actually shifting "under" the Foucault Pendulum!
The Foucault Pendulum (support + wire + iron ball) is attached to this building. The building is attached to the Planet Earth, which rotates on its own axis about once every 24 hours. The Earth also goes around the Sun, once every year. The Sun, in turn, goes around the centre of our galaxy, The Milky Way, once every 250 million years. These are all local motions.

The Foucault Pendulum somehow ignores all these local motions! The Foucault Pendulum somehow aligns itself with the Rest of The Universe.

Imagine that the Foucault Pendulum is set up at the Geographic South (or North) Pole (this is the simplest case - the motions are a little more complicated anywhere away from the poles). Imagine that you, very carefully, release the ball, and imagine that the wind resistance is so low that the ball will swing for days. Over the next week or so, the ball will keep on swinging in the same plane in which you first launched it. The Pendulum will swing in a plane that is fixed relative to the distant stars (or The Rest Of The Universe), and the Earth will rotate "under" the Pendulum. So over the time of a 24-hour day, the Foucault Pendulum will appear to gradually sweep out a 360 degree circle.

How does the Foucault Pendulum "know" to ignore local motions, and line itself up with the distant stars? Some reputable physicists say that we really don't know.

Perhaps it's a case of Newton's First Law of Motion "A body will try to keep on doing whatever it's doing, unless acted upon by an external force." So a body that is still will not move, unless a force tries to push it. In another example of Newton's First Law, a body that is moving will keep on moving, unless an external force tries to stop it. This desire of a body to keep on doing whatever it's doing, is called inertia. Nobody really understands what inertia is. The traditional explanations involve some circular reasoning. The reasoning goes like this.

A body will keep on doing whatever it's doing is because of its inertia. And inertia is the tendency of a body to keep on doing whatever it's doing. But why does it keep on doing what it's doing? Because of its inertia. But what is inertia? The tendency of a body to keep on doing whatever it's doing. And so on.

At the North and South Poles, the Foucault Pendulum will take 24 hours to sweep out a complete 360 degree circle. This motion is anticlockwise in the Southern Hemisphere, and clockwise in the Northern Hemisphere. At the Equator, it will take an infinite time to sweep out a complete 360 degree circle. At the latitude of Sydney, it will take about 43 hours to sweep out a complete 360 degree circle . This works out to about 8.4 degrees every hour, or about 42 degrees in 5 hours. If you sit and watch the Pendulum, you will soon see a change. In fact, you could draw on the ground a 43-hour clockface, and read the time off it.