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Friday, August 04, 2006

Weather makes Earth wobble

Scientists have confirmed that weather makes the planet wobble on its axis after exploiting a rare opportunity to detect and measure the most subtle shifts in the Earth's spin.

The wobbling at the poles was in the order of centimetres, from the size of a 25 cent piece to the size of a DVD, says astronomer Thomas Johnson of the US Naval Observatory.

"These loops are on the order of two or three days," says Johnson of the timeframe in which weather tugs and varies the direction Earth's axis is pointed in space.

To see the weather wobbles, Belgian researchers took advantage of an unusual period from November 2005 to February 2006 when two better known, larger components in Earth's wobble cancelled each other out and no longer drowned out the signal of the smaller wobbles.

The two larger components are a 433-day wobble thought to be caused by deep ocean current changes and annual wobble that corresponds to seasonal changes. These change the position of the poles on about the scale of a baseball diamond, says Johnson. Every 6.4 years they cancel each other out.

"It was basically now or never," says Sébastien Lambert of the Royal Observatory of Belgium of their well-timed measurements, which used GPS data to ferret out the weather effects."We would have to wait more than six years for another chance.

"Lambert and his colleague Véronique Dehant publish their findings in the journal Geophysical Research Letters.Lambert was also able to connect the specific wobbles seen during that period to atmospheric pressure systems over Asia and Europe. This connection makes it possible to use smaller wobbles is critical for navigational, timing and communication systems, says Richard Gross of NASA's Jet Propulsion Laboratory.

"We have to know how the Earth's rotation is changing in order to track a spacecraft," Gross explains. At distances of tens of millions of miles, a few centimetres on Earth can make the difference between communicating with the spacecraft and losing contact altogether. "It's most important when you are trying to land something on another planet," he says.

On Earth the weather wobbles are also important because they can throw off models that are used to predict and correct for the larger wobbles, multiplying errors in GPS and military navigational systems."The accuracy would be orders of magnitude worse" without these wobble corrections, says Johnson. "At the latitude of Reagan National Airport [near Washington DC], the variation could be the difference between a plane landing on the runway or hitting the Potomac River."The wobbles also have to be accounted for when finely tuning clocks and the timing of satellite communications."They all require Earth orientation data nowadays," says Johnson.

Larry O'Hanlon
Discovery News
Greener Newsroom

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