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“The Chandler wobble is an excited resonance of the Earth's rotation having a period of about 14 months. Although it has been under investigation for more than a century, its excitation mechanism has remained elusive.”
Source: Geophysical Research Letters
It amounts to change of about 9 metres (30 ft) in the point at which the axis intersects the Earth's surface.
“While it has to be maintained by changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble.”