The earth wobbles in space. This has been known for over a century by astronomers, and thanks to global positioning system (GPS) technologies, this wobble has been tracked with a precision of a few millimeters over the last decade. Until now, there were good theories as to why this happens, but no one could really prove it.

Now, however, Geoff Blewitt, University of Nevada research geophysicist, has an explanation for this mysterious geo-wobble.

“The theory, which my colleagues and I have proven using GPS observations of the Earth, is that it’s likely to be caused by the surface matter being redistributed,” Blewitt said. Blewitt and his colleagues, Richard Gross, Jet Propulsion Laboratory; Peter Clarke, University of Newcastle-upon-Tyne, England; and David Lavallée, University of Colorado, Boulder; published their findings in an article published April 1 in the journal, Geophysical Research Letters.

To make the Earth wobble, large amounts of mass need to be moved from one place to another so that the Earth is “off balance,” according to NASA-funded researcher Blewitt, who said the North Pole then adjusts to a new position to compensate. Large amounts of water are displaced seasonally when glaciers and ice sheets melt in spring, for example. The mass shifts back when they refreeze in winter.

“We measured the earth’s shape directly,” he said. “It agrees with the wobble. What our measurements are showing is that everything is consistent — the earth is wobbling while it’s changing its shape,” he said.

“The Earth isn’t a perfect sphere,” Blewitt noted. “It bulges at the equator because it’s spinning. If the position and height of that bulge changes slightly — even a few millimeters — we can pick it up.”

Blewitt will present his findings on why the Earth wobbles in a lecture, “GPS as a global sensor of systems Earth,” at the 2004 Joint Assembly of the American Geophysical Union, the Canadian Geophysical Union, The Society of Exploration Geophysicists, and the Environmental and Engineering Geophysical Society, on Thursday, May 20, in the Palais des Congrès, Montreal.

Blewitt has contributed to four books and co-authored some 60 papers in journals including Nature and Science, as well as given presentations and workshops at some 75 scientific conferences worldwide. Blewitt has been at Nevada since 1999, with previous positions at the University of Newcastle upon Tyne, England, and the Jet Propulsion Laboratory at the California Institute of Technology, Pasadena.