The Foucault pendulum ball swinging in the pit below you demonstrates the earth's
rotation.
The pendulum is suspended two and a half stories above you at the center of the sky dome. It swings back and forth always in the same plane. The earth however is rotating. So it appears as if the pendulum is crossing different parts of the world map in the pit.
The earth rotates once in every twenty-four hours, the pendulum's plane of swing covers the whole map on the floor below it in just about twelve hours. Each swing of the pendulum covers a slice of both sides of the map, so it only takes 12 hours rather than 24 to cover the whole map. In this sense it keeps good time... But in 24 hours the earth has moved 1/365 of its orbit around the sun and so the earth takes about an extra 4 minutes to get the sun back to the same place in the sky. Our 24 hour time is based on the sun. The pendulum's plane, however, is fixed, and independent of not only the rotation of the earth but also of the earth's rotation around the sun, so the plane of the pendulum swing really covers the whole map in about 2 minutes less than 12 hours. This is called astronomical time or star time, the technical term is sidereal time.
Because of these seasonal changes we can't mark the edge of the pit with time marks because midnight (and all the other times) move throughout the year.
The map of the earth on the floor of the pit is made of small tiles imported from Italy. It is a mosaic designed by our staff to represent a rough (artistic) map of the earth. The position at the center marks the north polar axis. The numbers around the side indicate standard longitude markings. The circular black line about half way in represents the equator. The longitude markings are correctly oriented with the earth, so if you want to look in the direction of your home and you know it's longitude, just stand exactly on the other side of the pendulum pit from your home, using the center to align yourself, and you will be looking directly towards it. All directions from here are South. We use longitude values to point in a particular direction.
From as far back as we have records, man has wondered about the sun, planets and stars. If archeologists are to be believed, then early man had three major preoccupations: hunting, procreation, and the stars. Little is known about the way early man hunted or what he ate. The guess at procreation as a major concern comes from the observation of modern people, and the fact that statues of pregnant females and large penii are relatively common.
The least likely concern is the astronomical one. By measuring angles and guessing heights most ancient monumental art has been related to risings and settings of bright starts and the sun and moon. The connections are statistical and somewhat suspect. Because of the presumption of an early religion and the alignments with the sky of these monuments one draws the conclusion that astronomy was a major concern.
The presumption of star worship is because the motions of the planets and moon are somewhat erratic and therefore they are not suitable with which to align monuments. It took man until the Renaissance fourteen centuries after Christ for man to correctly understand the motions of the planets in the sky. Before that planets, sun, and moon were associated with deities. Because of the monument alignments, archeologists, assume that the earliest religions were fixed star based.
The science of the Foucault Pendulum once understood provides an operating model of the mysteries and beauty of celestial mechanics. It is a beautiful object in itself.
Several visitors have asked how the pendulum keeps going without coming to rest at the center of the circle. The answer lies far in the dome overhead. There is an electromagnet arranged in a circle around the top of the pendulum. A magnet field is turned off and on to give the pendulum a little pull on each swing. This is just enough to compensate for the friction of the pendulum ball moving through the air.
The ball at the end of the pendulum weights 1000 kilo and is made of lead covered with a layer of gold, 1/100 of a millimeter thick. The great weight of the ball maintains the stability of the swing and helps to keep the pendulum swinging in the same plane. It is so heavy that to start the pendulum it took five staff members to push the ball into its starting position. There it was held back by a cord and left to rest a day. When all vibrations were damped out, the ball was released by burning the cord at the attachment point which smoothly released it to swing in exactly one plane. We used laser tracking to establish that the pendulum was working correctly. It took us three tries to get it running right.
