‘MERIT’ Working Group Plans for a New Earth Rotation Service



During the past 80 years, classical astronomical techniques have been used to determine polar motion (the variation of the direction of the spin axis of the earth with respect to a coordinate system fixed in the earth). The motion, which includes an Eulerian motion of about 0.3″ amplitude with a 430-day period and a somewhat smaller seasonal variation, is also subject to effects of geophysical processes that cannot be detected reliably because of the 0.01–0.02″ uncertainty (averaged over 5 days) in the astronomical determinations of pole position. The rate of the earth's rotation about its spin axis, measured to about 1-ms accuracy by astronomical techniques, also has seasonal variations of tens of milliseconds, variations of a few milliseconds over time periods of 2–4 months because of changing wind patterns, and currently undetectable effects as a result of tectonic processes. Higher accuracy and higher frequency determinations of polar motion and the earth–s rotation are required if the data are to be used to learn more about the internal constitution of the earth and earthquake processes.