| polar motion |
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A slow and very slight movement of the Earth's geographic poles relative to the surface of the Earth (not relative to the stars). It does not affect the celestial coordinates of a star, but does affect the reduction of positional measurements, for example those made with a transit circle. The origin is geophysical: principally that lack of exact coincidence between the Earth's axes of symmetry and rotation. The magnitude of the effect is typically 0.3 arc seconds, and there are periodicities of 433 days and one year. Much smaller variations also take place over short timescales, ranging between two weeks and three months, due to surface air pressure changes. See also: Chandler wobble. |