Prime Meridian

0°

Prime Meridian

Nations that touch the Equator (red) and the Prime Meridian (blue)

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, 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.

Geography

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
90°0′N 0°0′E	Arctic Ocean
81°39′N 0°0′E	Greenland Sea
72°53′N 0°0′E	Norwegian Sea
61°0′N 0°0′E	North Sea
53°45′N 0°0′E	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.
50°47′N 0°0′E	English Channel
49°19′N 0°0′E	France	The northernmost point on this meridian is in Villers-sur-Mer, Calvados. The southernmost point is near Gavarnie.
42°41′N 0°0′E	Spain	Passing just west of Monte Perdido, in the Pyrenees
39°56′N 0°0′E	Mediterranean Sea	Gulf of Valencia
38°52′N 0°0′E	Spain
38°38′N 0°0′E	Mediterranean Sea
35°50′N 0°0′E	Algeria
21°50′N 0°0′E	Mali
14°59′N 0°0′E	Burkina Faso
11°6′N 0°0′E	Togo	For about 600 m
11°6′N 0°0′E	Ghana	For about 16 km
10°57′N 0°0′E	Togo	For about 39 km
10°36′N 0°0′E	Ghana	Passing through Lake Volta at 7°48′N 0°0′E
5°37′N 0°0′E	Atlantic Ocean	Passing through the Equator at 0°0′N 0°0′E
60°0′S 0°0′E	Southern Ocean
68°54′S 0°0′E	Antarctica	Queen Maud Land, claimed by Norway

History

The Royal Observatory, Greenwich (today a museum)

Prime Meridian in Greenwich

Gerardus Mercator in his Atlas Cosmographicae (1595) uses a prime meridian in the Atlantic, intended to separate the Old World (Eurasia and Africa) and the New World (the Americas) into two hemispheres. Mercator's 180th meridian runs along the Strait of Anián (Bering strait), while his prime meridian corresponds to somewhere close to 25° W, passing just to the west of Santa Maria Island.

The Prime Meridian is ultimately arbitrary—a matter of convention—and various conventions have been used or advocated in different regions and throughout history:

• Washington, D.C. (77° 3' 2.3” W), see Washington meridian
• Philadelphia (75° 10' 12” W)
• Rio de Janeiro (43° 10' 19” W)
• El Hierro (Ferro), Canary Islands (18° 03' W, later redefined as 17° 39' 46”? W)
• Lisbon (9° 07' 54.862” W)
• Madrid (3° 41' 16.58” W)
• Paris (2° 20' 14.025” E), see Paris Meridian
• Brussels (4° 22' 4.71” E)
• Antwerp (4° 24' E), used by Mercator, see Meridian of Antwerp
• Bern (7° 26' 22.5” E)
• Pisa, Italy (10°24' E)
• Oslo (Kristiania) (10° 43' 22.5” E)
• Rome (12° 27' 08.4” E), meridian of Monte Mario
• Copenhagen (12° 34' 32.25” E) Rundetårn
• Stockholm (18° 3' 29.8” E), at the observatory
• Warsaw (21° 00’ 42” E), see Warsaw meridian
• Oradea, Romania (21° 55' 16” E)
• Alexandria (29° 53' E)
• Saint Petersburg (30° 19' 42.09” E), Pulkovo meridian
• Great Pyramid of Giza (31° 8' 3.69” E), 1884
• Jerusalem (35° 13' 47.1” E), for the small dome of the Church of the Holy Sepulchre
• Mecca (39° 49′ 34″ E), see Mecca Time
• Approximately 59° east of Greenwich
• Ujjain (75° 47' E), Used from 4th century CE Indian astronomy and calendars.
• Kyoto (135° 74' E), Used in 18th and 19th (officially 1779-1871) century Japanese maps. Exact place unknown, but in "Kairekisyo" in Nishigekkoutyou-town in Kyoto, then the capital.

Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian

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.

IERS Reference Meridian

A GPS receiver at the Greenwich Meridian. This does not indicate a longitude of zero because the GPS reference meridian is about 100 metres to the east.

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. This shift is a legacy of the first satellite navigation system, the Doppler based TRANSIT system. 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. 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. 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 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.

United Kingdom Ordnance Survey Zero Meridian

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.

Other planetary bodies

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:

• The Prime Meridian of the Moon lies directly in the middle of the face of the moon visible from Earth and passes near the crater Bruce.
• The Prime Meridian of Mars is defined by the crater Airy-0.
• The Prime Meridian of Venus passes through the central peak in the crater Ariadne.
• Two different heliographic coordinate systems are used on the Sun. The first is the Carrington heliographic coordinate system. In this system, the Prime Meridian passes through the center of the solar disk as seen from the Earth on 9 November 1853, which is when Richard Christopher Carrington started his observations of sunspots. The second is the Stonyhurst heliographic coordinates system.
• Jupiter has several coordinate systems because its cloud tops -- the only part of the planet visible from space -- rotate at different rates depending on latitude. It is unknown whether Jupiter has any internal solid surface that would enable a more Earth-like coordinate system. Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south Equatorial belts. System III coordinates use Jupiter's magnetic field.
• Titan, like the Earth's moon, always has the same face towards Saturn, and so that face is 0 longitude.

See also

• 1st meridian east
• 1st meridian west
• Greenwich Mean Time

External links

• The Prime Meridian line at The Royal Observatory in Greenwich
• "Where the Earth's surface begins—and ends", Popular Mechanics, December 1930
• International Conference Held at Washington for the Purpose of Fixing a Prime Meridian at Project Gutenberg
• scanned TIFFs of the conference proceedings
• Prime meridians in use in the 1880s, by country
• Canadian Prime Meridian
• A pictorial catalogue of meridian markers