Polar motion?

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The Earth's rotation axis tends, like the axis of a gyroscope, to maintain its orientation in inertial space. However, due to external forces acting upon the Earth, this axis nevertheless exhibits a slow, large-scale motion, known as precession and nutation. Nutation is divided into a predictable part from a "nutation theory," a long series of trigonometric terms depending on time, and derived from motions of the Moon and models of the Earth, plus corrections called the Celestial Pole Offset, which are the measured departures (two-dimensional on the celestial sphere) of the celestial rotation pole from the theory. 1 These corrections, of order 0.03 seconds of arc, are of no importance to most users, but by monitoring them, a better theory of nutation may be developed in the future.

When using, instead of an inertial reference frame, a frame attached to the body of the solid Earth (a so-called Earth-centred, Earth-fixed or ECEF reference frame), the rotation axis also varies slightly. This variation, which is only a few metres measured on the Earth's surface, is called Polar motion.

It consists of two quasi-periodic components and a gradual drift, mostly westward, of the Earth's instantaneous rotational axis or North pole, from a conventionally defined reference axis, the CIO (Conventional International Origin), being the pole's average location over the year 1900.

The two periodic parts are a more or less circular motion called Chandler wobble with a period of about 435 days, and a yearly circular motion. There is also a slow drift which is less well known. These motions are illustrated on the polar motion map 2 from the International Earth Rotation and Reference Systems Service. On that figure, the Earth's angular momentum vector points at the cross-hairs or axis origin, while North pole positions relative to that are illustrated in contrasting ways. The trace of the mean pole position from 1900 to 1997 is the wiggly line. The variations from the mean are shown from 1995 to mid 1998 with a dotted line showing looping motions. The title is not refreshed as often as the figure itself, so dates in the figure are to be preferred over those above it. The axis extending downwards in the diagram basically corresponds to the Greenwich Meridian, and the axis going off to the left corresponds to the 90th meridian west (towards North America). The axes are marked in units of 0.1 arc second (approx. 3 meters).

One can see that the mean displacement far exceeds in magnitude the wobbles. This can lead to errors in software for Earth observing spacecraft, since analysts may read of a 5 meter circular motion and ignore it, while a 20 meter offset sits there, fouling the accuracy of the calculated latitude and longitude. The latter are determined based on the International Terrestrial Reference System, which follows the polar motion.

The slow westward drift, about 20 m since 1900, is partly due to motions in the Earth's core and mantle, and partly to the redistribution of water mass as the Greenland ice sheet melts, as well as to isostatic rebound, i.e. the slow rise of land that was formerly burdened with ice sheets or glaciers (Munk, 2002). The drift is roughly along the 80th meridian west, towards the eastern part of North America.

See also

• Precession

References

• Fisher, Rick (1996). "Earth Rotation and Equatorial Coordinates". Retrieved Jun. 5, 2005.
• Munk, Walter (2002). "Twentieth century sea level: An enigma". Proceedings of the National Academy of Sciences 99, #10. pp. 6550—6555.

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