The gyroscope was named by Leon Foucault, a French physicist, in an attempt to demonstrate the earth's rotation. A freely rotating disk, called a rotor, was mounted on a spinning axis in the center of a larger, stable wheel. As the earth spun on its axis, the stable wheel rotated with it, but the rotor did not move. The movement of the mounted wheel followed the rotation of the earth, rotating around the center disk and demonstrating the earth's spin.
Usually, in modern gyroscopes, the rotor is constantly spinning. Constant spinning adds certain properties to the gyroscope and increases its uses. Just like a spinning top, which remains level on a tilted surface, the spinning center of a gyroscope does not change its orientation. The spinning of the rotor means that any change in orientation affects all points on the rotor equally, causing the rotor to spin on a fixed axis. This is called precession.
Precession creates a fixed orientation. The rotor spins on a fixed axis while the structure around it rotates or tilts. In space, where the four compass points are meaningless, the axis of the spinning rotor is used as a reference point for navigation.
In addition to the rotor, modern gyroscopes typically have two additional rings, called gimbals, in the center of a larger stable ring. The rotor spins on an axle connected to the smaller, inner gimbal. This gimbal rotates on a horizontal axis created by its connection to the larger, outer gimbal. The larger gimbal rotates vertically and spins on an axis connected to the stable outside ring.
Gyroscopes are in compasses for airplanes, spacecraft, and boats. In airplanes, the pitch and orientation of the airplane is measured against the steady spin of the gyroscope. In space, where there are few reference points to help navigate, the spinning center of the gyroscope is used as a point of orientation.
Massive gyroscopes are used to stabilize large boats and some satellites. They are also used in guidance systems in some missiles. They even make a fun children's toy.