Chandler Wobble

This important oscillation was discovered in 1891 by S. C. Chandler. This motion, due to the dynamic flattening of the Earth, appears when the rotation axis does not coincide anymore with the polar main axes of inertia. Without any external torque, the total angular momentum remains constant in magnitude and direction, but the Earth twists so that related to its surface, the instantaneous rotation axis moves around the polar main inertia axis. For a rigid Earth, Euler showed that the pole displacement in the terrestrial frame produces a latitude variation with a period of 305 days. However, as the Earth is deformable, and because of the presence of the inelastic mantle, the oceans and the liquid core, the observed period is about 435 days. In space, this motion is a quasi-diurnal mode of which the period equals 1+(1/435) day.

This free oscillation is potentially excited by mass redistribution, in atmosphere, oceans and mantle (due to earthquakes). Those mass displacements produce small changes of the momentum and inertia products, which implies a displacement of the rotation axis by angular momentum budget equation. The problem of the Chandler wobble excitation has been discussed in a large number of papers and it seems today that a large part of the wobble may be due to atmospheric forcing.