Author's Homepage:   R. C. Hibbeler
Chapter 20: Three-Dimensional Kinematics of a Rigid Body
"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."