Monitoring The Change In The Earth

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July 19, 2000 - Issue #275


Scientists have known about a strange wobble in the Earth's rotation since it was discovered by astronomer Seth Carlo Chandler back in 1891, but they've only recently uncovered what causes it. It turns out that changes in pressure at the bottom of the ocean as well as fluctuations in the atmosphere are to blame. Any stress applied to the surface of the Earth affects its spin.

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American Geophysical Union

July 17, 2000

A mystery of Earth's wobble solved: it's the ocean

WASHINGTON - The century old mystery of Earth's "Chandler wobble" has been solved by a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. The Chandler wobble, named for its 1891 discoverer, Seth Carlo Chandler, Jr., an American businessman turned astronomer, is one of several wobbling motions exhibited by the Earth as it rotates on its axis, much as a top wobbles as it spins.

Scientists have been particularly intrigued by the Chandler wobble, since its cause has remained a mystery even though it has been under observation for over a century. Its period is only around 433 days, or just 1.2 years, meaning that it takes that amount of time to complete one wobble. The amplitude of the wobble amounts to about 20 feet at the North Pole. It has been calculated that the Chandler wobble would be damped down, or reduced to zero, in just 68 years, unless some force were constantly acting to reinvigorate it.

But what is that force, or excitation mechanism? Over the years, various hypotheses have been put forward, such as atmospheric phenomena, continental water storage (changes in snow cover, river runoff, lake levels, or reservoir capacities), interaction at the boundary of Earth's core and its surrounding mantle, and earthquakes.

Writing in the August 1 issue of Geophysical Research Letters, Richard S. Gross of NASA's Jet Propulsion Laboratory reports that the principal cause of the Chandler wobble 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. He determined this by applying numerical models of the oceans, which have only recently become available through the work of other researchers, to data on the Chandler wobble obtained during the years 1985-1995. Gross calculated that two-thirds of the Chandler wobble is caused by ocean-bottom pressure changes and the remaining one-third by fluctuations in atmospheric pressure. He says that the effect of atmospheric winds and ocean currents on the wobble was minor.

Gross credits the wide distribution of the data that underlay his calculations to the creation in 1988 of the International Earth Rotation Service, which is based in Paris, France. Through its various bureaus, he writes, IERS enables the kind of interdisciplinary research that led to his solution of the Chandler wobble mystery. Gross's research was supported by NASA's Office of Earth Science.

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