The action of a couple whose axis is perpendicular to the rotational axis (a torque) on a rotating body causing its axis of rotation to trace out a path about an average position, instead of having a constant alignment, i.e. the axis of rotation itself revolves conically about a central point. The Earth's axis of rotation precesses as a result of several forces, e.g. changes in mass distribution on its surface, changes in the gravitational field due to changes in the relative positions of the Moon, Sun, and planets, etc.
Subjects: Earth Sciences and Geography.