permanent tide

time-independent gravitational potential exerted on the Earth by the Sun, Moon, and planets.

Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)

Original Source: NGS Glossary

polar motion

the motion of the Earth’s pole with respect to the ITRS. The main components are the Chandlerian free motion with a period of approximately 430 days, and an annual motion. It also includes sub-daily variations caused by ocean tides and periodic motions driven by gravitational torques with periods less than two days. Sub-daily variations are not included in the values distributed by the IERS, and are therefore to be added, after interpolation to the date of interest, using a model provided by the IERS Conventions.

Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)

Original Source: IAU 2006 NFA Glossary

Pole angles

The pole angles X and Y are the components of the direction of the Celestial Intermediate Pole in the Geocentric Celestial Reference System (GCRS).

Related terms:

• Celestial Intermediate Pole

Related links:

• Data, Products & Tools

precession-nutation

the ensemble of effects of external torques on the motion in space of the rotation axis of a freely rotating body, or alternatively, the forced motion of the pole of rotation due to those external torques. In the case of the Earth, a practical definition consistent with the IAU 2000 resolutions is that precession-nutation is the motion of the CIP in the GCRS, including FCN and other corrections to the standard models: precession is the secular part of this motion plus the term of 26000-year period and nutation is that part of the CIP motion not classed as precession.

Source: IERS Technical Note 36: Glossary (not accessible) [PDF, 244 KB] of the IERS Conventions (2010)

Original Source: IAU 2006 NFA Glossary