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geodesy Geodesy or geodetics is the science of measuring and representing the Figure of the Earth, geometry, Gravity of Earth, gravity, and Earth's rotation, spatial orientation of the Earth in Relative change, temporally varying Three-dimensional spac ...
and
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...
, earth orientation parameters (EOP) describe irregularities in the rotation of planet Earth. EOP provide the rotational transform from the
International Terrestrial Reference System The International Terrestrial Reference System (ITRS) describes procedures for creating frame of reference, reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a Standard (metr ...
(ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time. 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,
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s, or those resulting from
earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
s, 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.


Components


Universal time

Universal time Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wi ...
( 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 Sidereal time ("sidereal" pronounced ) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal t ...
, 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 Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...
and Lunar laser ranging, whereas LOD can be derived from satellite observations, such as GPS,
GLONASS GLONASS (, ; ) is a Russian satellite navigation system operating as part of a radionavigation-satellite service. It provides an alternative to Global Positioning System (GPS) and is the second navigational system in operation with global cove ...
,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
and
Satellite laser ranging Satellite laser ranging (SLR) is a method to measure the distance to satellites in a geocentric orbit. It consists of an astronomical observatory equipped with a laser that sends ultrashort pulses of light. The pulses hit the satellite and boun ...
to geodetic satellites. LOD is changing due to gravitational effects from external bodies and geophysical processes occurring in different Earth layers. Then, the LOD prediction is extremely difficult due to extreme events such as El Niño which demonstrated themselves in the LOD signals.


Coordinates of the pole

Due to the very slow pole motion of the Earth, the Celestial Ephemeris Pole (CEP, or
celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...
) 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. Pole coordinates can be determined using various space geodesy and satellite geodesy techniques, e.g.,
Satellite laser ranging Satellite laser ranging (SLR) is a method to measure the distance to satellites in a geocentric orbit. It consists of an astronomical observatory equipped with a laser that sends ultrashort pulses of light. The pulses hit the satellite and boun ...
,
Very Long Baseline Interferometry Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...
, however, the most accurate techniques are GPS,
GLONASS GLONASS (, ; ) is a Russian satellite navigation system operating as part of a radionavigation-satellite service. It provides an alternative to Global Positioning System (GPS) and is the second navigational system in operation with global cove ...
, and
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
.


Celestial pole offsets

Celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...
offsets are described in the
IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
Precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
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. Celestial pole offsets can only be obtained by the VLBI. The observed CPO can quantify the deficiencies of the IAU2006/2000A precession–nutation model, including the astronomically forced nutations and a component of nutation that is considered unpredictable. Some studies indicate that substantial FCN (Free Core Nutation) amplitude and phase disturbances occurred at the epochs close to the revealed GMJ (Geomagnetic Jerk) events.


References


External links

* http://hpiers.obspm.fr/eop-pc/ * http://www.usno.navy.mil/USNO/earth-orientation {{Webarchive, url=https://web.archive.org/web/20110514132451/http://www.usno.navy.mil/USNO/earth-orientation , date=2011-05-14 * https://web.archive.org/web/20130802032453/http://maia.usno.navy.mil/ * http://www.iers.org/ Dynamics of the Solar System
Rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
Astrometry Geodesy