Polar motion
Polar motion of the earth is the movement of Earth's rotational 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.
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[edit] Analysis
It consists of two quasi-periodic components and a gradual drift, mostly in the direction of the 80th meridian west, 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 corresponds to about 3 cm):
- polar motion from 2005 until the last week
- x coordinate from 1962 until the last week
- 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 off 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.
[edit] Causes
The slow 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 glaciers.[1] The drift is roughly along the 80th meridian west.
Major earthquakes cause abrupt polar motion by altering the volume distribution of the Earth's solid mass. These shifts, however, are quite small in magnitude relative to the long-term core/mantle and isostatic rebound components of polar motion.[2]
[edit] See also
[edit] Notes
- ^ Munk, Walter (2002).
- ^ [1]
[edit] 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.
- Polar motion (IERS)
- CNN. "Quake moved Japan coast 8 feet. ". Retrieved 2011-03-12.
