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What makes the world wobble

We think of Earth as a rotating sphere. It would be more accurate to consider it a wobbling top. Like a top spinning on a table, Earth's spin axis undergoes a complex mix of wobbles. Scientists have tracked the larger loops and dips for over a century. Thanks to the exquisite accuracy of the Global Positioning System (GPS) and a break from Mother Nature, they now have found patterns in which the axis traces small loops over an area no bigger than a sheet of typing paper every few days.

It's the latest result of an ongoing effort to understand how the planet interacts with shifts of mass in the air and sea and to internal factors that may include movements in its core. Push on a spinning top and it responds by wobbling. Stick a wad of gum on it, and the top will wobble. Mass shifts and changes in air and sea-bottom pressure can do the same to our planet. Working out these interactions is forcing geophysicists to try to gain a more comprehensive understanding of how our planet works.

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In 1765, Swiss mathematician Leonard Euler predicted Earth's wobbling. In 1891, American amateur astronomer Seth Carlo Chandler Jr. identified the first observed wobble pattern. Slight changes in the position of stars revealed changes in the orientation of astronomical telescopes due to the wobble. This "Chandler wobble" goes through a 14-month cycle. Later, other scientists found an annual cycle as well. Thanks to these two wobbles, the geographic north pole wanders over an area as wide as 20 meters across.

Other tilts and loops occurring over a week or less have been more elusive. Nature, however, gives scientists a break. The Chandler and annual wobbles cancel each other out every 6.4 years. This lets weaker short-term wobbles stand out. The satellite-based GPS, which pins down locations such as the North Pole with centimeter accuracy, can identify them. Sebastien Lambert at Belgium's Royal Observatory and colleagues took advantage of this for the period of November 2005 through February 2006 when the larger wobbles last canceled. During that time, the North Pole stayed within a one-meter square area, tracing loops that ranged in size from the palm of a small hand to a sheet of typing paper every few days.

What causes any of the wobbles is an only partly solved mystery. Once started, a wobble should die out unless something gives it an occasional kick. In its paper in Geophysical Research Letters, Dr. Lambert's team explains why they think the short-term wobbles they found are partly excited by Northern Hemisphere weather patterns. They were able to correlate atmospheric and sea-bottom pressure changes with the wobble motions. Other unidentified factors also may be at work.

Richard Gross at NASA's Jet Propulsion Laboratory in Pasadena, Calif., encountered similar uncertainty six years ago. He showed that the main cause of the 14-month Chandler wobble probably is change in sea-bottom pressures. Yet the report of a subsequent workshop on the subject, in which Dr. Gross participated, concluded that the mechanism behind the wobble "is still under investigation."

Meanwhile, if the stars seem a bit wonky when you look at them, pin it to the fact that you are standing on a wobbly top.