precession

Change of direction of the axis of rotation of a body caused by external forces. A familiar example is the slow rotation of the axis of a toy spinning-top that is leaning over. The external forces are the weight of the top, acting at the centre of gravity, and the equal upward reaction force at the point where the top rests on the ground. Such a pair of equal and oppositely directed forces is called a torque, or couple. The twisting effect of these forces would cause a stationary top to fall over; they cause a top which is spinning apparently to defy gravity and to move slowly around the vertical line through the point of support of the top. The faster the top spins, and the greater its mass, the more slowly the top moves around the vertical line, or precesses.

In astronomical contexts, precession is such a change in the movements of a rotating or orbiting body - notably, the revolution of the direction of the Earth's axis, which carries each celestial pole in a circle around the sky in a period of 25,800 years. Precession also occurs in the oscillations of atoms, electrons and molecules - for example, Larmor precession arises when a magnetic field is applied to any of these objects.