The Prime Meridian is the meridian (line of longitude) at which the longitude is defined to be 0°.
The Prime Meridian and its opposite the 180th meridian (at 180° longitude), which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.
An international conference in 1884 decided the modern Prime Meridian passes through the Royal Observatory, Greenwich in southeast London, United Kingdom,[1] known as the International Meridian or Greenwich Meridian, although the Prime Meridian is ultimately arbitrary unlike the parallels of latitude, which are defined by the rotational axis of the Earth with the Poles at 90° and the Equator at 0°.
Historically, various meridians have been used, including four different ones through Greenwich.
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Starting at the North Pole and heading south to the South Pole, the Prime Meridian passes through:
| Co-ordinates (approximate) |
Country, territory or sea | Notes |
|---|---|---|
| Arctic Ocean | ||
| Greenland Sea | ||
| Norwegian Sea | ||
| North Sea | ||
| United Kingdom | The northernmost land on this meridian is near Tunstall in East Riding,Yorkshire. The southernmost land in the UK is Peacehaven, East Sussex. |
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| English Channel | ||
| France | The northernmost point on this meridian is in Villers-sur-Mer, Calvados. The southernmost point is near Gavarnie. |
|
| Spain | Passing just west of Monte Perdido, in the Pyrenees | |
| Mediterranean Sea | Gulf of Valencia | |
| Spain | ||
| Mediterranean Sea | ||
| Algeria | ||
| Mali | ||
| Burkina Faso | ||
| Togo | For about 600 m | |
| Ghana | For about 16 km | |
| Togo | For about 39 km | |
| Ghana | Passing through Lake Volta at | |
| Atlantic Ocean | Passing through the Equator at | |
| Southern Ocean | ||
| Antarctica | Queen Maud Land, claimed by Norway |
The Prime Meridian is ultimately arbitrary—a matter of convention—and various conventions have been used or advocated in different regions and throughout history:
The modern Greenwich Meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their maps. In October of that year, at the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. This conference selected the Greenwich Meridian as the official Prime Meridian due to its popularity. However, France abstained from the vote and French maps continued to use the Paris Meridian for several decades.
The Greenwich Meridian passes through the Airy transit circle of the Greenwich observatory. It was long marked by a brass strip in the courtyard, now upgraded to stainless steel, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.
The International Earth Rotation and Reference Systems Service (IERS) maintains the IERS Reference Meridian (IRM), also called the International Reference Meridian, which is the reference meridian (Prime Meridian, 0° longitude) of the Global Positioning System operated by the United States Department of Defense. It is the reference meridian in WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).
The IRM is 5.31 arcseconds east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich.[7] This shift is a legacy of the first satellite navigation system, the Doppler based TRANSIT system.[8] TRANSIT was developed by the Applied Physics Laboratory of Johns Hopkins University. Its lab is located in Howard County, Maryland, which was the location of TRANSIT's first ground station. The station's surveyed coordinates in the North American Datum 1927 (NAD27) — a non-Earth centered ellipsoid — became its coordinates in an Earth-centered ellipsoid, such as the World Geodetic System. This shifted the coordinates of any other location on an Earth-centered ellipsoid, especially those far away.
When the antenna of a TRANSIT ground station was mounted directly above Airy's transit circle in June 1969, its longitude on an Earth-centered ellipsoid was 5.64 arcseconds west of TRANSIT's reference meridian.[8] Several small additional longitude shifts were created by further improvement in gravitational models such as the Earth Geopotential Model 1996 (EGM96), a dramatic increase in the number of ground stations from only four to over 500, and the use of time-based GPS.
The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.[9] The IRM was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[10] 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.[11] 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 Earth's rotation, the standard international time UTC can differ from the mean observed time on the meridian by up to 0.9 second (equivalent to about 260 metres at Greenwich). Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun — mean solar time.
The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the Airy meridian marked at Greenwich. When the first Ordnance Survey map was published in 1801, the official Prime Meridian of Great Britain was the one established by the third Astronomer Royal, James Bradley. When Airy's new Prime Meridian ("new" by virtue of Sir George Airy's instrument being placed in a room next to that housing James Bradley's instrument) superseded it fifty years later in 1851, the Ordnance Survey simply continued to use Bradley's.[12]
As on the Earth, prime meridians must be arbitrarily defined. Often a landmark such as a crater is used, other times a prime meridian is defined by reference to another celestial object, or by magnetic fields. The prime meridians of the following planetographic systems have been defined:
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