Subject: A mystery of Earth's wobble solved: It's the ocean
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Date: Tue, 18 Jul 2000 09:13:12 -0700 (PDT)

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FOR IMMEDIATE RELEASE                         July 18, 2000

A MYSTERY OF EARTH'S WOBBLE SOLVED: IT'S THE OCEAN

     The century-old mystery of Earth's "Chandler wobble" has 
been solved by a scientist at NASA's Jet Propulsion Laboratory in 
Pasadena, Calif. 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 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 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 Gross, a JPL geophysicist, 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, the service 
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, Washington, D.C.

     JPL is a division of the California Institute of Technology 
in Pasadena.

                            #####
7/17/00 rs    
#2000-066

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