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0°
Prime Meridian
Nations that touch the Equator (red) and the Prime Meridian (blue)
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3 arcseconds east of George Biddell Airy's 1851 transit circle or 102 metres (335 ft) at the latitude of the Royal Observatory, Greenwich. It is also the reference meridian of the Global Positioning System (GPS) operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).
Location
The
reason for the 5.3 arcsecond offset between the IERS Reference Meridian
and the Airy transit circle is that the observations with the transit
circle were based on the local vertical, while the IERS Reference is a geodetic longitude, that is, the plane of the meridian contains the center of mass of the Earth.
The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts. The IRM was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989. 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.
However, the IRM is not fixed to any point on Earth. Instead, all
points on the European portion of the Eurasian plate, including the
Royal Observatory, are slowly moving northeast about 2.5 cm per year
relative to it. Thus this IRM is the weighted average (in the least squares
sense) of the reference meridians of the hundreds of ground stations
contributing to the IERS network. The network includes GPS stations, Satellite Laser Ranging (SLR) stations, Lunar Laser Ranging (LLR) stations, and the highly accurate Very Long Baseline Interferometry (VLBI) stations.
All stations' coordinates are adjusted annually to remove net rotation
relative to the major tectonic plates. 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. The 180th meridian is opposite the IERS Reference Meridian and forms a great circle with it dividing the earth into Western Hemisphere and Eastern Hemisphere.
Universal Time is notionally based on the WGS84 meridian. Because of changes in the rate of Earth's rotation, standard international time UTC can differ from the mean observed solar time at noon on the prime meridian by up to 0.9 second. Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun.
List of places
Starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian passes through 8 countries.
