Atmospheric and oceanic forces drive Earth's wobble.

Atmosphere Drives Earth's Tipsiness

Dick writes about Earth and planetary science for Science magazine.

For more than a century, geophysicists who track Earth's rotation have sensed a rhythmic unsteadiness about the planet, an ever so slight wobbling whose source remained frustratingly mysterious. But researchers have been homing in on the roots of the so-called Chandler wobble, and now a report in the 1 August issue of Geophysical Research Letters fingers the shifting pressures of the deep sea and ultimately the fickle winds of the atmosphere.

Although 18th century Swiss mathematician Leonhard Euler predicted that Earth should wobble on its axis at a pace of around once a year, it wasn't until 1891 that American businessman and amateur scientist Seth Carlo Chandler Jr. detected this wobble through analysis of stellar observations. Once every 14 months, Earth's spin axis wanders around the official geographic pole, describing a rough circle anywhere from 3 to 6 meters across. If random, any such wobble should fade away in a few decades. But this one has persisted for more than a century. Something keeps pumping energy into it, researchers knew, something that repeatedly pushes on the tilted, spinning Earth to set it wobbling like a top.

Candidates for the driver of the Chandler wobble abounded, but most eventually fell short. The jolts of great earthquakes come too infrequently. Wind blowing on mountains proved too feeble. That seemed to leave something in the ocean as the most likely candidate. To pin it down, geophysicist Richard Gross of the Jet Propulsion Laboratory in Pasadena, California, compared how Earth actually wobbled between 1985 and 1996 with how strongly the ocean and atmosphere, as simulated in sophisticated computer models, could have driven the Chandler wobble. The motion of winds and currents proved far too weak in themselves, but the varying pressure the water exerted on the sea floor accounted for two-thirds of the wobble. The shifting load of the atmosphere as weather systems cross the surface accounted for the other third.

Gross "has found the two biggest contributors" to the Chandler wobble, says geophysicist Clark Wilson of the University of Texas, Austin. And only one of these is in charge. "The oceans are wind-driven, so you have the atmosphere driving the whole thing," Wilson explains. Aside from satisfying geophysical curiosity, that insight could help fly spacecraft to the planets. Knowing a spacecraft's precise location is tricky to accomplish from an unpredictably wobbling platform like Earth, but that challenge might be lessened by understanding what's rocking the boat.

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