Tracking Earth’s wobbles

Like a spinning top, Earth wobbles as it rotates on its axis. Several oscillations, whose periods range from a few minutes to billions of years, contribute to the motion. The two largest oscillations, the so-called Chandler wobble of 433 days and the annual wobble, are well studied, and together they can tilt Earth’s axis 300 milliarcseconds (10 meters at the poles) or more. Earth’s irregular, shorter-term wobbles have been more difficult to study, partly because their smaller motions are usually masked by those of more prominent wobbles. Taking advantage of a period of destructive interference between the annual and Chandler oscillations, scientists in Belgium and France have used data from November 2005 to February 2006—computed with centimeter precision at the Earth Orientation Centre, part of the International Earth Rotation and Reference Systems Service, and based primarily on global positioning system measurements—to study the short-term wobbles that occurred during that period. As plotted in the figure, the pole position traced small loops that ranged from the size of a compact disc down to that of a postage stamp. The scientists found that the pole displacements were almost fully attributable to oceanic and atmospheric pressure variations. Although such effects had been previously credited with maintaining the large Chandler wobble, this is the first time scientists have been able to demonstrate that day-to-day changes in atmospheric pressure produce a measurable effect on Earth’s rotation. (S. B. Lambert et al., Geophys. Res. Lett. 13 , L13303, 2006 http://dx.doi.org/10.1029/2006GL026422 .)