Earth Orientation Parameters
The Earth's rotation is not even. Any motion in/on the Earth causes a slowdown or speedup of the rotation, or a change of rotation axis. Most of them can be ignored, but movements of very large mass, like sea current or tide can produce discernible changes and cause error to very precise astronomical observations.
A single parameter can be used to describe one phenomenon. The collection of Earth Orientation Parameters (EOP) is fitted to describe the rotation irregularities all together. Technically, they provide the rotation transforming the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time.
Universal time. Universal time (UT1) tracks the Earth's rotation in time, which performs one revolution in about 24 hours. The Earth's rotation is uneven, so UT is not linear with respect to atomic time. It is practically proportional to the sidereal time, which is also a direct measure of Earth rotation. The excess revolution time is called length of day (LOD).
Coordinates of the pole. Due to the very slow pole motion of the Earth, the Celestial Ephemeris Pole (CEP, or celestial pole) does not stay still on the surface of the Earth. The Celestial Ephemeris Pole is calculated from observation data, and is averaged, so it differs from the instantaneous rotation axis by quasi-diurnal terms, which are as small as under 0.01" (see ). In setting up a coordinate system, a static terrestrial point called the IERS Reference Pole, or IRP, is used as the origin; the x-axis is in the direction of IRM, the IERS Reference Meridian; the y-axis is in the direction 90 degrees West longitude. x and y are the coordinates of the CEP relative to the IRP.
Celestial pole offsets. Celestial pole offsets are described in the IAU Precession and Nutation models. The observed differences with respect to the conventional celestial pole position defined by the models are monitored and reported by the IERS.
- Seidelmann, P.K. 1982: Celest. Mech., 27, 79.