Contact: Rosemary Sullivant (818) 354-0474

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

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.