Polar Motion

Polar motion is a wobble and a gradual drift, mostly westard, of the Earth's geographic North pole from its rotational axis. (Motion of the latter are called precession, nutation, and pole offset (a correction to nutation).

There are two parts, a more or less circular wobble called Chandler wobble with a period of about 435 days, and a slow drift which is less well known. Both are illustrated on a page of the International Earth Rotation and Reference Systems Service at

• Polar Motion Map from the IERS (http://www.iers.org/map/)

On that figure, which is not terribly well explained, 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 mean pole position from 1900 to over a period from 1900 to 1997 (it says 1996, but it shows 1997!) is the wiggly line. The variations from the mean are shown as more or less loopy motions from 1995 (it says 1994) to 1997 (it says 1996, but it shows 1997!) .

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, of course, determined based on the International Terrestrial Reference System, which follows the polar motion.