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.