Axis of Rotation, Instantaneous

A rigid body that has a fixed point (a gyroscope, for example) may be displaced from a given position to a position infinitesimally close to it by means of a rotation about an axis passing through this point; such an axis is called an instantaneous axis of rotation. The motion of the body after a finite period of time may be considered as a succession of rotations about instantaneous axes that are continually changing their direction in space.

The motion of a free rigid body in the general case may be considered to be the sum of a translation of the center of mass (or of another arbitrarily chosen point called a pole) of the body and pure rotations about instantaneous axes of rotation that pass through the center of mass or the pole.