Earth Orientation Parameters
Encyclopedia
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
(UT1) stands for the earth rotation, which performs one revolution in about 24h. The earth rotation is uneven, so UT is not linear with respect to atom 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 stick still on surface of earth. The Celestial Ephemeris Pole is calculated with past observation data, and is somehow 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 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
.
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
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
International Terrestrial Reference System
The International Terrestrial Reference System describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of...
(ITRS) to the International Celestial Reference System
International Celestial Reference System
The International Celestial Reference System is the current standard celestial reference system adopted by the International Astronomical Union . Its origin is at the barycenter of the solar system, with axes that are intended to be "fixed" with respect to space...
(ICRS), or vice versa, as a function of time.
Components
Universal time.Universal time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...
(UT1) stands for the earth rotation, which performs one revolution in about 24h. The earth rotation is uneven, so UT is not linear with respect to atom time. It is practically proportional to the sidereal time
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...
, 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
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
) does not stick still on surface of earth. The Celestial Ephemeris Pole is calculated with past observation data, and is somehow 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 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
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
offsets are described in the IAU
IAU
IAU may refer to:*International Astronomical Union*International American University*International American University College of Medicine*International Association of Universities*International Association of Ultrarunners...
Precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
and Nutation
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
models. The observed differences with respect to the conventional celestial pole position defined by the models are monitored and reported by the IERS
IERS
IERS may refer to:* International Earth Rotation and Reference Systems Service* Independent Electricity Retail Solutions Pty Ltd* Information Exchange Requirements - used within MODAF and DODAF as the OV-3 view - called Information Exchange Matrix....
.
External links
- http://hpiers.obspm.fr/eop-pc/
- http://www.usno.navy.mil/USNO/earth-orientation
- http://maia.usno.navy.mil/
- http://www.iers.org/