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Tracking wiggles in Earth's wobble

IT began as a nuisance for astronomers. A slight wobbling of Earth's rotation axis translates into small shifts in the latitude of observatories. But in the nine decades that scientists have been tracking the wobble in earnest, this source of small ``errors'' has become a valuable tool for studying our planet. Thanks to modern electronic tracking, scientists now can follow the wiggly detail of the wobble 10 times as accurately as they could just a decade ago. This new precision has already enabled a research team to relate rapid motions of the North Pole - fluctuations on time scales of a fortnight to several months - to the restless shifting of restless atmosphere.

T. Marshall Eubanks of the US Naval Observatory and colleagues at the NASA Jet Propulsion Laboratory and Atmospheric and Environmental Research Inc. of Cambridge, Mass., published the research earlier this month in Nature. It's an example, they say, of how more-precise wobble tracking can be used to study the overall dynamics of the air and sea.

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Earth's wobble is like the nodding of a spinning top. Small asymmetries in the distribution of mass - shifts of air, water, land, or even annual growth and decay of vegetation - cause the spin axis to wander around an average position. In the course of a year or so, the North Pole wanders in a rough ellipse that doesn't quite close on itself. While the ellipses vary in size, they generally cover less area than a basketball court.

Studying the wobble helps scientists track major redistributions of mass on and within the planet. One of the most striking such connections is the shift in the North Pole's average position as Earth's outer layers that were displaced by Ice Age glaciers readjust to a largely ice-free state. The rotational North Pole coincided with the pole where longitude lines met in 1900. It has since drifted some 10 meters south of the geographic pole.

Scientists have long recognized two major rhythms in the wobble. There is an annual frequency driven mainly by seasonal shifts in the atmosphere's mass.

Then there is a roughly 14-month cycle. This is the so-called free mode in which the rotation axis would oscillate if set to wobbling and then left to itself. Since this free wobbling should die out within a few decades, its persistence means something is exciting it. But no one knows what that something is. Geophysicists have not yet been able to account for it in terms of such seemingly obvious factors as shifts of air and water masses or earthquakes.

While traditional latitude tracking using telescopic observations of stars picks out these major cycles, it can't resolve the wobble's finer details. But two electronic methods of latitude measurement have improved wobble tracking accuracy. One method involves laser beam tracking of satellites whose orbit is accurately known.

The other method is based on simultaneous observation of objects several billions of light- years distant by two or more widely separated radiotelescopes. Such techniques let analysts pin down the North Pole to a precision of about 5 centimeters.

It is this accuracy that enabled Eubanks and his colleagues to relate small (less than a meter), rapid wiggles of the pole to shifts in the global atmosphere. It's a foretaste of the insights into general workings of our planet that scientists should gain as they turn a former astronomical nuisance into a precision research asset.

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A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.