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Tuesday, 18 July, 2000, 13:38 GMT 14:38 UK
Earth throws a wobbly
Earth BBC
The Chandler Wobble was first detected in 1891
A mysterious wobble that shakes the Earth as it spins on its axis is caused by pressure changes at the bottom of the ocean, say scientists at Nasa's Jet Propulsion Laboratory.

The Chandler Wobble, as it is known, was first detected in 1891 by an American astronomer called Seth Carlo Chandler.

Similar to a wobble in a spinning top, it is one of a number of tremors exhibited by the Earth as it rotates.

The force of the wobble is such that it is capable of moving the North Pole about six metres (20 feet) from where it should be.

The Chandler Wobble period lasts around 433 days, or just 1.2 years.

Mysterious force

Scientists originally calculated that this phenomenon should naturally run out of steam after 68 years unless some force keeps activating it. And this is precisely what appears to happen.

Writing in the 1 August issue of Geophysical Research Letters, Nasa's Richard Gross says the principle causes of the wobble are fluctuating pressures on the bottom of the oceans.

These fluctuations are the result of changes in temperature, salinity and wind patterns.

Using data from International Earth Rotation Service, in Paris, Gross says that two thirds of the wobble can be attributed to these ocean bottom pressure changes and one third to alterations in atmospheric pressure.

Dr Michael Tsimplis, from the Southampton Oceanography Centre, UK, says that the Gross theory is plausible. "Any stress you apply to the surface of the Earth can affect its axis," he said.

According to Dr Tsimplis, the Chandler Wobble is a long-standing problem with many competing solutions. "It has been suggested that the seasonal movement of water from the southern to the northern oceans may have caused it. It is also said that large earthquakes have caused it by giving a big kick to the Earth."

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