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TheIERS Reference Meridian (IRM), also called theInternational Reference Meridian, is theprime meridian (0° longitude) maintained by theInternational Earth Rotation and Reference Systems Service (IERS). It passes about 5.3arcseconds east ofGeorge Biddell Airy's 1851transit circle, and thus it differs slightly from the historicalGreenwich Meridian. At thelatitude of theRoyal Observatory, Greenwich the difference is 102 metres (335 ft).[1][2][a]
It is the reference meridian of theGlobal Positioning System (GPS) operated by theUnited States Space Force, and ofWGS 84 and its two formal versions, the idealInternational Terrestrial Reference System (ITRS) and its realization, theInternational Terrestrial Reference Frame (ITRF).
The most important reason for the 5.3 seconds of longitude offset between the IERS Reference Meridian and the Airy transit circle is that the observations with the transit circle were based on theastronomical longitude, while the IERS Reference is a geodetic-based longitude; that is, the plane of the meridian contains the center of figure of the Earth.[1] Their difference is caused by the east-west component of thevertical deflection, between the localgravityvertical direction and theellipsoidal normal.
TheInternational Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.[3] It was adopted for air navigation by theInternational Civil Aviation Organization on 3 March 1989.[4]Tectonic plates slowly move over the surface of Earth, so most countries have adopted for their maps an IRM version fixed relative to their own tectonic plate as it existed at the beginning of a specific year. Examples include the North American Datum 1983 (NAD83), the European Terrestrial Reference Frame 1989 (ETRF89), and the Geocentric Datum of Australia 1994 (GDA94). Versions fixed to a tectonic plate differ from the global version by at most a few centimetres.
The IERS system is not quite fixed to any point attached to the Earth. For example, all points on the European portion of the Eurasian plate, including the Royal Observatory, are moving northeast at about 2.5 cm per year relative to it. The IRM is the weighted average (in theleast squares sense) of the reference meridians of the hundreds of ground stations contributing to the IERS network. The network includes GPS/GNSS stations,satellite laser ranging (SLR) stations,lunar laser ranging (LLR) stations, and the highly accuratevery long baseline interferometry (VLBI) stations.[5] All stations' coordinates are reduced to a reference epoch (a fixed date/time) and adjusted annually to remove net rotation relative to the major tectonic plates.[6] If Earth had only two hemispherical plates moving relative to each other around any axis which intersects their centres or their junction, then the longitudes (around any other rotation axis) of any two, diametrically opposite, stations must move in opposite directions by the same amount.
The180th meridian (themeridian at 180° both east and west of thePrime Meridian) is opposite the IERS Reference Meridian and forms agreat ellipse with it dividing the earth intoWestern Hemisphere andEastern Hemisphere.
Universal Time is notionally based on the prime meridian.[7] Because of changes in the rate of Earth's rotation, standard international timeUTC can differ from the mean observed solar time at noon on the prime meridian by up to 0.9 of a second.Leap seconds are inserted from time to time, to keep UTC close to Earth's angular position relative to the Sun; seemean solar time.
Starting at theNorth Pole and heading south to theSouth Pole, the IERS Reference Meridian passes through eight countries and three oceans (Arctic Ocean, Atlantic Ocean and Southern Ocean):
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