In geodesy, **earth orientation parameters** (**EOP**) are a collection of parameters that describe irregularities in the rotation of planet Earth.

Earth's rotational velocity is not constant over time. Any motion of mass in or on Earth causes a slowdown or speedup of the rotation speed, or a change of rotation axis. Small motions produce changes too small to be measured, but movements of very large mass, like sea currents, tides, or those resulting from earthquakes, can produce discernible changes in the rotation and can change very precise astronomical observations. Global simulations of atmosphere, ocean, and land dynamics are used to create effective angular momentum (EAM) functions that can be used to predict changes in EOP.^{[1]}

The collection of earth orientation parameters is fitted to describe the observed rotation irregularities. Technically, they provide the rotational transform from the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time.

## Components

### 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). The absolute value of UT1 can be determined using space geodetic observations, such as Very Long Baseline Interferometry and Lunar laser ranging, whereas LOD can be derived from satellite observations, such as GPS, GLONASS, Galileo^{[2]} and Satellite laser ranging to geodetic satellites.^{[3]}^{[4]}

### 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 ^{[5]}). 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. Pole coordinates can be determined using various space geodesy and satellite geodesy techniques, e.g., Satellite laser ranging, Very Long Baseline Interferometry, however, the most accurate techniques are GPS, GLONASS, and Galileo.^{[6]}

### 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.

## References

**^**Dobslaw, Henryk; Dill, Robert (February 2018). "Predicting Earth orientation changes from global forecasts of atmosphere-hydrosphere dynamics".*Advances in Space Research*.**61**(4): 1047–1054. Bibcode:2018AdSpR..61.1047D. doi:10.1016/j.asr.2017.11.044.**^**Zajdel, Radosław; Sośnica, Krzysztof; Bury, Grzegorz; Dach, Rolf; Prange, Lars (July 2020). "System-specific systematic errors in earth rotation parameters derived from GPS, GLONASS, and Galileo".*GPS Solutions*.**24**(3): 74. doi:10.1007/s10291-020-00989-w.**^**Zajdel, R.; Sośnica, K.; Drożdżewski, M.; Bury, G.; Strugarek, D. (November 2019). "Impact of network constraining on the terrestrial reference frame realization based on SLR observations to LAGEOS".*Journal of Geodesy*.**93**(11): 2293–2313. Bibcode:2019JGeod..93.2293Z. doi:10.1007/s00190-019-01307-0.**^**Sośnica, K.; Bury, G.; Zajdel, R. (16 March 2018). "Contribution of Multi‐GNSS Constellation to SLR‐Derived Terrestrial Reference Frame".*Geophysical Research Letters*.**45**(5): 2339–2348. Bibcode:2018GeoRL..45.2339S. doi:10.1002/2017GL076850.**^**Seidelmann, P.K. 1982: Celest. Mech., 27, 79.**^**Zajdel, Radosław; Sośnica, Krzysztof; Bury, Grzegorz; Dach, Rolf; Prange, Lars; Kazmierski, Kamil (January 2021). "Sub-daily polar motion from GPS, GLONASS, and Galileo".*Journal of Geodesy*.**95**(1): 3. doi:10.1007/s00190-020-01453-w.