Earth has been known as terra firma through the centuries, but scientists reported Thursday that a shifting atmosphere contributes to a slight wobbling of the planet as it rotates.

The wobble ranges up to only about 2 feet wide at the poles, but its existence will let scientists use the Earth as a means to studying the atmosphere, said researcher T. Marshall Eubanks.He said the phenomenon is like the quiver of a clothes washer with an off-center load, with the shifting air masses acting like the unbalanced laundry.

An analysis is published in Thursday's issue of the British journal Nature by Eubanks, of the U.S. Naval Observatory in Washington, and scientists at the Jet Propulsion Laboratory in Pasadena, Calif., and Atmospheric and Environmental Research Inc., of Cambridge, Mass.

They found that, over three years, the wobble was related to changes in air pressure around the globe. Air pressure changes are created by the shifting in the atmosphere, and Eubanks said fluctuations as slight as one-tenth percent to three-tenths percent of normal atmospheric pressure appeared to contribute to the wobble.

Scientists have long known that the Earth's axis displays regular oscillations lasting about 433 days and about a year and that the latter cycle is linked to the atmosphere.

The new paper focuses on recently discovered fluctuations with cycles of only two weeks to several months.

The atmosphere's shiftings influence the Earth through changes in air pressure at the planet's surface. Because the Earth bulges around its equator, the weight of the atmosphere pushes not exactly toward the center of the Earth but a little bit off-center, tending to nudge the Earth off its rotational axis, Eubanks said in a telephone interview.

So, for example, if pressure is high over North America, low over South America, high over Australia and low over Siberia, it would tend to rotate the Earth as if a huge hand on North America were pushing south, Eubanks said.