|
( 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. 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) 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.
| 
