The free oscillation of the Earth's pole of rotation. This wobble in the Earth's rotation has a 435-day periodicity and appears to have a decay time of the order of 40 years. The cause (excitation) is unknown: atmospheric effects appear to be on too short a time-scale; earthquake activity has been proposed, but has not been established. The major cause appears to be related to the Earth's core and its magnetic coupling with the lower mantle.
Subjects: Earth Sciences and Geography — Astronomy and Astrophysics.