Polar motion

Polar motion is the movement of Earth's rotation axis across its surface. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called "Earth-centered, Earth-fixed" or ECEF reference frame). This variation is only a few meters.

Analysis

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 Earth Orientation Center of the International Earth Rotation and Reference Systems Service (mas = milliarcseconds):
* [http://hpiers.obspm.fr/eop-pc/products/combined/C04plot.php?date=2&dimx=600&dimy=450&graphe=12&year1=2005&month1=1&day1=1&year2=2027&month2=12&day2=31&langue=1&SUBMIT=Submit+request polar motion from 2005 until the last week]
* [http://hpiers.obspm.fr/eop-pc/products/combined/C04plot.php?date=2&dimx=600&dimy=450&graphe=1&year1=1962&month1=1&day1=1&year2=2027&month2=12&day2=31&langue=1&SUBMIT=Submit+request x coordinate from 1962 until the last week]
* [http://hpiers.obspm.fr/eop-pc/products/combined/C04plot.php?date=2&dimx=600&dimy=450&graphe=2&year1=1962&month1=1&day1=1&year2=2027&month2=12&day2=31&langue=1&SUBMIT=Submit+request y coordinate from 1962 until the last week]

The mean displacement far exceeds the magnitude of 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 exists, 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.

Causes

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, and to isostatic rebound, i.e. the slow rise of land that was formerly burdened with ice sheets or glaciersMunk, Walter (2002).] . The drift is roughly along the 80th meridian west.

Notes

References

* Fisher, Rick (1996). " [http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html Earth Rotation and Equatorial Coordinates] ". Retrieved Jun. 5, 2005.
* Munk, Walter (2002). " [http://www.pnas.org/cgi/content/full/99/10/6550 Twentieth century sea level: An enigma] ". "Proceedings of the National Academy of Sciences" 99, #10. pp. 6550—6555.