Gyroscope, pronounced JY ruh skohp, also called a gyro, is a device that uses rotation to produce a stable direction in space. A basic gyroscope consists of a spinning wheel or ball, called the rotor, and a support system. Once the rotor is set in motion, the gyroscope resists any attempt to change its direction of rotation. Because of this property, gyroscopes are widely used in flight and navigation instruments. For example, a gyroscope is used to provide heading or course information that is unaffected by air turbulence or heavy seas.
Gyroscopic inertia is the tendency of a spinning body to resist any attempt to change the direction of its axis of rotation. For example, the earth spins around its axis, an imaginary line that connects the North and South poles. Because of gyroscopic inertia, the north axis of the earth continues to point to the North Star as the earth moves in its orbit around the sun.
Gyroscopic inertia enables the axis of a spinning gyroscope always to point in the same direction, no matter how the support of the gyroscope moves about. The magnitude (strength) of the inertia depends on the distribution of the weight of the rotor and the speed of its spin. Gyroscopes with most of their weight at the rotor's rim have the greatest amount of inertia. Thus, a bicycle wheel makes a good gyroscope, but a pencil spinning on its point does not. In addition, the faster the rotor spins, the more gyroscopic inertia it possesses.
Precession is the tendency of a spinning body to move at right angles to the direction of any force that tries to change its direction of rotation. You can use precession to guide a rolling hoop. When you roll the hoop, it will not fall down if you push from the side against the top. In this case, the hoop merely turns a corner. The hoop precesses, or turns at right angles to the force that you have applied against it. Similarly, a spinning gyroscope will move at right angles to any force that attempts to change the direction of its axis. The earth itself precesses slightly. This turn occurs because the pull of gravity by the sun and moon tends to tip it over.