The Chandler wobble is the change in spin of the Earth
on its axis
Originally discovered in the late 1800s by Seth Carlo Chandler, the oscillation has a period of around 433 days (or 1.2 years). This means it takes roughly that long to complete one 'wobble', and the amplitude is about 20 feet.
The Chandler wobble puzzled scientists at first: the motion it exhibited was much like that of a spinning top as it slows down. The oscillation period and amplitude would become greater as the speed slowed, until eventually it disappeared entirely. By this rational it was expected the Chandler wobble would disappear within a matter of decades. When this didn't happen, more serious study was directed at this phenomenon.
Until recently, it was thought that this oscillation was caused by changes in the magnetic coupling between the Earths core and mantle. A new theory has emerged, stating that long-term fluctuations in pressure at the ocean floor may be the driving force for the Chandler wobble.