The Prime Meridian and the opposite 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.
Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian is one passing through Greenwich, London, United Kingdom, known as the International Meridian or Greenwich Meridian. Historically, various meridians have been used, including four different ones through Greenwich.
|lat||Country, territory or sea||Notes|
|90° 80° 75°||Arctic Ocean|
|70° 65°||Norwegian Sea|
|60° 55°||North Sea|
|53° 52° 51°||England - the most northerly land on the meridian is the shore (53°45′34″N) southeast of the Sand-le-Mere caravan park east of Kingston upon Hull|
|49° 48° 47° 46° 45° 44° 43°|
|42° 41° 40°|
|39° 38° 37° 36°||Mediterranean Sea||Gulf of Valencia|
|35° 30° 25°||Sahara|
|20° 15°||Sahara, Sahel|
|10°||Including Lake Volta|
|5° N 0° 5° S 10° 15° 20° 25° 30° 35° 40° 45° 50° 55°||Atlantic Ocean|
|70° 80° 90°||Antarctica||Queen Maud Land, claimed by|
The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history:
The 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 is now marked at night by a laser beam emitted from the observatory.
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, and is the reference meridian in WGS84 and in its formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). It is 5.31″ east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich. This shift is a legacy of the first satellite navigation system, the Doppler based TRANSIT system developed by the Applied Physics Laboratory of Johns Hopkins University in Howard County, Maryland. This was the location of its first ground station, so the surveyed coordinates of that ground station 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 caused the coordinates of any other location on an Earth-centered ellipsoid to shift, especially those far away. When the antenna of a TRANSIT ground station was mounted directly above Airy's transit instrument in June 1969, its longitude on an Earth-centered ellipsoid was 5.64″ west of TRANSIT's reference meridian. Further improvement in gravitational models, such as the Earth Geopotential Model 1996 (EGM96), and a dramatic increase in the number of ground stations from only four to over 500, as well as the use of time based GPS caused a small additional shift before it reached its present position.
The International Hydrographic Organization adopted an early version in 1983 for all nautical charts. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989. Because tectonic plates slowly move over the surface of the Earth, most countries have adopted a version of the IRM for their maps that is fixed relative to their own tectonic plate as it existed at the beginning of a specific year, such as 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. 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, including GPS stations, Satellite Laser Ranging (SLR) stations, Lunar Laser Ranging (LLR) stations, and the highly accurate Very Long Baseline Interferometry (VLBI) stations. The coordinates of all of these stations are adjusted annually until there is no net rotation relative to the major tectonic plates. If Earth had only two hemispherical plates moving relative to each other around any axis from that intersecting their centres to one intersecting their junction, the longitudes (around any other rotation axis) of any two stations diametrically opposite each other must move in opposite directions by the same amount.
Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the Earth's rotation. Leap seconds are inserted periodically to keep UTC in sync with the Earth.
The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the Airy meridian marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply continued to use it.
The prime meridians of the following bodies in the Solar System have been defined: