Scientists no longer waver on Earth's wobble

The cause of the Chandler wobble, a disturbance of motion exhibited by Earth as it rotates on its axis, lies at the bottom of the ocean

July 18, 2000
Web posted at: 3:39 p.m. EDT (1939 GMT)

By Environmental News Network staff

The cause of the so-called "Chandler wobble," a disturbance of motion exhibited by Earth as it rotates on its axis, is no longer a mystery. The answer lies at the bottom of the ocean, reports a scientist from NASA's Jet Propulsion Laboratory in Pasadena, California.

The Chandler wobble, named for Seth Carlos Chandler Jr., the scientist who discovered it in 1891, is one of several motions exhibited by Earth as it rotates on its axis, much like a top wobbles as it spins. While many of the wobbles are explained, the cause of the Chandler wobble has vexed scientists for years.

The wobble, which has been under observation for more than a century, has a period of about 433 days. In other words, it takes 1.2 years to complete one wobble. The amplitude of the wobble amounts to about 20 feet at the North Pole.

It has been calculated that the wobble would dampen down and cease to exist in 68 years unless some force constantly acted on it to reinvigorate it.

Previous explanations of that force have ranged from changes in snow cover and river runoff to interaction at the boundary of Earth's core and its surrounding mantle to earthquakes. None of these hypotheses panned out.

Richard Gross, a NASA scientist, has a new theory. He applied recently available numerical models of the oceans to data on the Chandler wobble obtained from 1985 to 1995 to determine the cause of the wobble.

The cause, he reports in the Aug. 1 issue of Geophysical Research Letters is fluctuating pressure on the bottom of the ocean caused by temperature and salinity changes and wind-driven changes in the circulation of the oceans.

According to his calculations, two-thirds of the Chandler wobble is caused by ocean-bottom pressure changes. The remaining one-third is the result of fluctuations in atmospheric pressure. Gross says the effect of atmospheric winds and ocean currents on the wobble is minor.

Scientists are interested in the Chandler wobble because variations in latitude due to the wobble could throw off celestial navigation systems that are unaware of the wobble by a fifth of a mile.

One such system dependent on knowledge of the wobble, according to a report in The Straight Dope, is the program that guides U.S. Trident nuclear missiles.

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Geophysical Research Letters
The Straight Dope
International Earth Rotation Service

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