"When a rigid body rotates about a fixed point, the distance r from the point to a particle P located on the body is the same for any position of the body. Thus the path of motion for the particle lies on the surface of a sphere having a radius r and centered at the fixed point. Since motion along this path occurs only from a series of rotations made during a finite time interval, we will first develop a familiarity with some of the properties of rotational displacements." p515