Not very far from The Curie Museum is the former church and now burial place for the great and good men (and one woman) of France: The Pantheon. Inside the Pantheon is the original Foucault's Pendulum.
The pendulum was first mounted in the Pantheon in 1851 to demonstrate that the Earth is rotating. The pendulum swings back and forth in the same plane, but the Earth moves. Relative to the floor (and to the convenient hour scale provided) the pendulum appears to rotate.
The pendulum is on a 67m long cable hanging from the roof of the Pantheon. The bob at the end of the cable weight 27kg. In the Pantheon the pendulum appears to rotate at 11 degrees per hour (which means it takes more than a day to return to its original position). If it were mounted at the North Pole it would 'rotate' once every 24 hours, the pendulum's period of rotation depends on the latitude diminishing to 0 degrees per hour at the equator (i.e. it doesn't 'rotate' at all).
If you take a look at the photograph above you can see that I was there just after 1200. The scale shows the current time measured by the pendulum.
The actual movement of the pendulum is only hard to understand because the common sense assumption is that the floor is not moving, but of course it is. It appears that what we are observing is a pendulum swinging above a fixed floor.
But the floor is actually moving because of the rotation of the Earth. That makes understanding the pendulum's motion harder. The important factor is the Coriolis Effect (sometimes erroneously called the Coriolis Force).
The simplest way to visualize the Coriolis Effect is to imagine firing a gun at the Equator straight northwards along a meridian. Because the Earth rotates the bullet will not land on the meridian, the Earth will have moved and the bullet will land to the west of the meridian. It looks as though a force has acted on the bullet to push it sideways. Of course, there's no actual force, it's just that the frame of reference (i.e. where the observer is) is not stationary.
Essentially the same thing happens with Foucault's Pendulum. The observer and the floor are not stationary and so the pendulum has an apparent motion.