True polar wander

True polar wander is a solid-body rotation (or reorientation) of a planet or moon with respect to its spin axis, causing the geographic locations of the north and south poles to change, or "wander". In rotational equilibrium, a planetary body has the largest moment of inertia axis aligned with the spin axis, with the smaller two moments of inertia axes lying in the plane of the equator. This is because planets are not rigid - they form a rotational bulge which affects the inertia tensor of the body. Internal or external processes that change the distribution of mass (internal or external loadings) disrupt the equilibrium and true polar wander will occur: the planet or moon will rotate as a rigid body (reorient in space) to realign the largest moment of inertia axis with the spin axis. Because stabilization of rotation by the rotational bulge is only transient,^[1] even relatively small loads can result in a significant reorientation (See Polhode § Description.)

If the body is near the steady state but with the angular momentum not exactly lined up with the largest moment of inertia axis, the pole position will oscillate (Chandler wobble). Weather and water movements can also induce small changes. These subjects are covered in the article Polar motion.