IERS Reference Meridian

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 about 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 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 the 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.

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; see mean solar time.

List of places

Starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian passes through:

Co-ordinates (approximate)	Country, territory or sea	Notes
	Arctic Ocean
	Greenland Sea
	Norwegian Sea
	North Sea
	United Kingdom	From Tunstall in East Riding to Peacehaven, passing through Greenwich
	English Channel
	France	From Villers-sur-Mer to Gavarnie
	Spain	From Cilindro de Marboré to Castellón de la Plana
	Mediterranean Sea	Gulf of Valencia
	Spain	From El Verger to Calpe
	Mediterranean Sea
	Algeria	From Stidia to Algeria-Mali border near Bordj Mokhtar
	Mali
	Burkina Faso
	Togo	For about 600 m
	Ghana	For about 16 km
	Togo	For about 39 km
	Ghana	From the Togo-Ghana border near Bunkpurugu to Tema Passing through Lake Volta at
	Atlantic Ocean	Passing through the Equator at
	Southern Ocean
	Antarctica	Queen Maud Land, claimed by Norway

See also

• 1st meridian east
• 1st meridian west
• 180th meridian
• Greenwich Meridian
• Prime meridian
• UTC