EurekAlert: Geophysicist to speak in Montreal on why the Earth 'wobbles'

5/19/04
"University of Nevada research geophysicist Geoff Blewitt will present his findings on why the Earth wobbles in a lecture, 'GPS as a global sensor of systems Earth,' ...Astronomers have known for more than a century that the earth wobbles in space. 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, Blewitt 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. "
The article: Richard S. Gross, Geoffrey Blewitt, Peter J. Clarke, David Lavallée. "Degree-2 harmonics of the Earth's mass load estimated from GPS and Earth rotation data" GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L07601, doi:10.1029/2004GL019589, 2004 (full text for subscribers)
We tend to think of the earth as just a solid ball making an even, smooth trip around the sun while it rotates on its axis. It's actually a lot more complicated: the earth's center is liquid, the surface weight distribution shifts, the pole precesses and nutates, the earth is actually flattened, the orbit is eccentric... Satellites can observe some of this wierdness and are used to measure it. I learned a lot of this information in my required Celestial Navigation classes but I'm remembering it now because PSIgate just added a record for an MIT OpenCourseWare class on Modern Navigation. Looking at the lecture notes provides a nice review.