Earth Wobbles With the Weather

Aug. 2, 2006 — Scientists have exploited a rare opportunity to detect and measure the most subtle shifts in the Earth's spin. In the process they have confirmed that weather makes the planet wobble on its axis.

The weather-related wobbling at the poles was on the order of centimeters – from the size of a silver dollar to the size of a DVD, says astronomer Thomas Johnson of the U.S. Naval Observatory.

"These loops are on the order of two or three days," said Johnson of the timeframe in which weather tugs and varies the direction Earth’s axis is pointed in space.

To see the weather wobbles, Belgian researchers took advantage of an unusual period from Nov. 2006 to Feb. 2006 when two better known, larger components in Earth's wobble cancelled each other out and no longer drowned out the signal of the smaller wobbles.

The two larger components are a 433-day wobble thought to be caused by deep ocean current changes and annual wobble that corresponds to seasonal changes. These change the position of the poles on about the scale of a baseball diamond, said Johnson. Every 6.4 years they cancel each other out.

"It was basically now or never," said Sébastien Lambert of the Royal Observatory of Belgium of their well-timed measurements, which used GPS data to ferret out the weather effects. "We would have to wait more than six years for another chance."

Lambert and his colleague Véronique Dehant published their findings in the current issue of Geophysical Research Letters.

Something else Lambert was able to do was connect the specific wobbles seen during that period to atmospheric pressure systems over Asia and Europe. This connection makes it possible to use weather forecasts to also forecast the smaller variations in Earth’s wobble.

Measuring these smaller wobbles is critical for navigational, timing and communication systems, says Richard Gross of NASA’s Jet Propulsion Laboratory.

"We have to know how the Earth’s rotation is changing in order to track a spacecraft," Gross explained.

At distances of tens of millions of miles, a few centimeters on Earth can make the difference between communicating with the spacecraft and losing contact altogether. "It’s most important when you are trying to land something on another planet," he said.

On Earth the weather wobbles are also important because they can throw off models that are used to predict and correct for the larger wobbles, multiplying errors in the Global Positioning System and military navigational systems.

"The accuracy would be orders of magnitude worse" without these wobble corrections, said Johnson. "At the latitude of Reagan National Airport, the variation could be the difference between a plane landing on the runway or hitting the Potomac River."

The wobbles also have to be accounted for when finely tuning clocks and the timing of satellite communications. "They all require Earth orientation data nowadays," said Johnson.