The Earth's axis is tilted about 23.5 degrees to the orbital plane as it orbits the Sun. This daily planetary rotation creates a bulge at the equator away from the orbital plane, but the Sun and Moon's gravity tends to pull this bulge back in the direction of the orbital plane. In a planetary move called the precession the Earth pulls back on the bulge.

The Chandler Wobble

Named for its discoverer, Seth Carlo Chandler, Jr., in 1891, the Chandler wobble is a motion exhibited by Earth as it rotates on its axis. It is like a spinning top that is slowing down. Scientists have been observing the Chandler wobble for more than 100 years. It shifts from the North Pole by only 20 feet. It takes only around 433 days, or just over 1.2 years to complete one wobble. Scientists have long known that the oceanic tides are caused by the gravitational pull. But that is not all. Other factors such as continental water storage caused by changes in snow cover, or river runoff, and lake levels, affect the pull.

According to Richard Gross, a Jet Propulsion Laboratory geophysicist, the principal cause of the Chandler wobble comes from pressure variations found at the bottom of the oceans. These variations are caused by temperature and salinity changes as well as by changes due to wind in the circulation of the oceans. He calculated that the wobble was two-thirds the result of ocean-bottom pressure changes and one-third the result of atmospheric pressure changes. Hence as the tides shift geological shifts also occur.

Nutations

A nutation is a slight irregular motion in the axis of rotation of a planet, or gyroscope. A pure nutation is a movement of the rotation axis such that the first precession is constant. These wobbles are also known and they affect the planet differently. While a wobble is in one direction, a nutation can cause the wobble to alternate back and forth.

One nutation is caused by Earth's annual orbital path around the Sun. Another, longer nutation, involves the 18.6-year precession of the Moon's orbit. Both of these affect the precession cycle of the Earth. Hence it is difficult to measure the wobble directly when there are variations in the rotation. But now that has changed.

GPS and the Earth Wobble

Using GPS observations as reported by Sara Pratt in Geotimes on June 2007, "Why the Wobble?", Geologists believe that the Earth wobble is likely to be caused by the surface matter being redistributed. Scientists have long suspected this but the GPS finding is proof. To make the Earth wobble, large amounts of mass need to be moved from one place to another so that the Earth is currently out of kilter, so to speak. So much so that the North Pole then adjusts to a new position to compensate. It does this by a seasonal variation. Large amounts of water are displaced from winter to spring and back to winter again when glaciers and ice sheets melt in the spring. Subsequently in the winter the mass refreezes.

One of the longest unexplained phenomena on Earth, the wobble, has been solved using the GPS navigation system. Now scientists attribute the wobble to a shifting surface caused by seasonal variation. Even the nutations that may have interfered with accurate data collection do not interfere with the GPS observations.

Sources

The Chandler Wobble

http://www.jpl.nasa.gov/releases/2000/chandlerwobble.html

http://www.geocities.com/lkp550/space/chanwob.htm

http://www.technologyreview.com/blog/arxiv/24057/

Nutations

http://www.reference.com/browse/wiki/Chandler_wobble

GPS and the Earth Wobble

http://www.sciencedaily.com/releases/2004/05/040520065656.htmhttp://www.navigadget.com/index.php/2006/06/26/earths-wobble-tracked-to-centimeter-scale-using-gps