Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant. In physics, there are two types of precession: torque-free and torque-induced. In astronomy, "precession" is any of several slow changes in an astronomical body''s rotational or orbital parameters, and especially to the Earth''s precession of the equinoxes. See Precession.