The mystery of "Chandler's wobble" has been solved. The wobble, named for its 1891 discoverer, Seth Carlo Chandler, Jr., an American businessman turned astronomer, is a wobbling motion exhibited by Earth as it rotates on its axis.

Scientists have had the Chandler wobble under observation for over a century. Its period is only around 433 days, or just 1.2 years, meaning that it takes that amount of time to complete one wobble. The wobble amounts to about 20 feet at the North Pole.

But what is that force? Over the years, things like atmospheric phenomena, continental water storage (changes in snow cover, river runoff, lake levels, or reservoir capacities), interaction at the boundary of Earth's core and its surrounding mantle, and earthquakes have benn believed to be the cause.

Richard Gross, a Jet Propulsion Laboratory geophysicist, reports that the principal cause of the Chandler wobble is fluctuating pressure on the bottom of the ocean, caused by temperature and salinity changes and wind-driven changes in the circulation of the oceans. Gross calculated that two-thirds of the Chandler wobble is caused by ocean-bottom pressure changes and the remaining one-third by fluctuations in atmospheric pressure.