As the world wobbles ....

As the world wobbles...

The earth seems as stable as the weather is changeable, but in fact scientists have long known that movements of air masses can make the planet wobble on its axis for periods of a year or more. Now, researchers using very sensitive satellite and radio astronomy techniques have found "rapid" wobbling of the earth on a time scale of two weeks to several months, and have shown that the wobbling is at least partially caused by atmospheric changes.

When high- and low-pressure air masses move about the earth, the weight distribution of the atmpshere is changed. This can make the rotating earth wobble, just as moving the balancing weights on the wheel of a car can change the way the hub rotates. The effect was anticipated in 1862 by Lord Kelvin, and has since been observed to at least partially cause the earth's annual wobble and the 14-month Chandler wobble.

Improvements in wobble-monitoring systems and in detailed, worldwide collection of weather data allowed the discovery of the shorter-period wobbling and its correlation with changes in weather patterns, reports a team of scientists in the July 14 NATURE. The researchers, from the Jet Propulsion Laboratory in Pasadena, Calif., and Atmospheric and Environmental Research Inc. in Cambridge, Mass., used two methods involving extraterrestrial bodies to pinpoint to within 5 centimeters the two surface points of the earth's rotational axis.

The group obtained highly accurate measurements of the earth's movement with a technique called satellite laser ranging, which involves bouncing laser beams off the moon and/or an artificial satellite and measuring the time it takes to travel the distance there and back. They confirmed this information with very long baseline interferometry, in which a number of radiotelescopes on different continents observe a quasar at the same time and compare the signals to get information about the relative motion of the observatories.

Using these techniques, the scientists were able to observe the earth's axis of rotation moving 6 to 60 centimeters over these shorter periods.

The researchers are not sure whether all of this short-period wobbling is caused by atmospheric changes or if there are other possible reasons, says Richard Rosen of Atmospheric and Environmental Research Inc. This is because air-pressure changes over the ocean cause sea-level changes of about 3 to 4 centimeters, and the team is unsure of what effect that water movement has, he says.

"We get better correlations if we ignore air masses over the ocean," but then the mass changes aren't enough to account for all the wobble, Rosen says. Other factors might be earthquakes and the shifting of tectonic plates, wind patterns around the earth or redistribution of water in rivers and lakes, he says. "There are a lot of things that there just arenht good data for, so you can dream up all sorts of possibilities," he says.

The laser ranging and interferometry measurements used to spot the wobble can also be used to measure the movement of tectonic plates and local changes in the earth's gravitational field, says coauthor Jean Dickey of the Jet Propulsion Laboratory. For instance, comparison of satellite and radiotelescope measurements will reveal gravitational changes, she says, because the satellite is affected by these changes while the interferometry measurements are not. Tectonic motion can be detected by comparing motion measurements from different points on the earth.