It has been suggested that IERS Reference Meridian be merged into this article or section. (Discuss) Proposed since April 2012.

Zero meridian and longitude 0° redirect here.

0°

Prime Meridian

A prime meridian is a meridian, i.e. a line of longitude, at which longitude is defined to be 0°. A prime meridian and its opposite in a 360°-system, the 180th meridian (at 180° longitude), form a great circle.

This great circle divides the sphere, e.g. the Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere.

An international conference in 1884 decided the prime meridian passes through the Royal Observatory, Greenwich in southeast London, United Kingdom,^[1] known simply as the prime meridian^[2] but also sometimes referred to as the International Meridian or Greenwich meridian^{[citation needed]}.

As of 2012 the most used^{[clarification needed]} prime meridian for the Earth is the IERS Reference Meridian (IRM). It passes 5.31 arcseconds east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich, London.^[3][4][5] A current convention on the earth uses the opposite of the IRM as the basis for the International Date Line.

  Prime meridian at Greenwich

  Markings of the prime meridian at the Royal Observatory, 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 modern prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851.^[6] 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., for the International Meridian Conference. This conference selected the Greenwich meridian as the official prime meridian due to its popularity.^[1] However, France abstained from the vote and French maps continued to use the Paris meridian for several decades.

  IERS Reference Meridian

The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the reference meridian used as the prime meridian (0° longitude) by the International Earth Rotation and Reference Systems Service (IERS). It passes 5.31 arcseconds east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich, London.^[3][4][5] It is also the reference meridian of the Global Positioning System 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).

  From Pole to Pole

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

Co-ordinates (approximate)	Country, territory or sea	Notes
90°0′N 0°0′E / 90°N 0°E / 90; 0 (North Pole)	Arctic Ocean
81°39′N 0°0′E / 81.65°N 0°E / 81.65; 0 (Greenland Sea)	Greenland Sea
72°53′N 0°0′E / 72.883°N 0°E / 72.883; 0 (Norwegian Sea)	Norwegian Sea
61°0′N 0°0′E / 61°N 0°E / 61; 0 (North Sea)	North Sea
53°45′N 0°0′E / 53.75°N 0°E / 53.75; 0 (United Kingdom)	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 / 50.783°N 0°E / 50.783; 0 (English Channel)	English Channel
49°19′N 0°0′E / 49.317°N 0°E / 49.317; 0 (France)	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 / 42.683°N 0°E / 42.683; 0 (Spain)	Spain	Passing just west of Monte Perdido, in the Pyrenees
39°56′N 0°0′E / 39.933°N 0°E / 39.933; 0 (Mediterranean Sea)	Mediterranean Sea	Gulf of Valencia
38°52′N 0°0′E / 38.867°N 0°E / 38.867; 0 (Spain)	Spain
38°38′N 0°0′E / 38.633°N 0°E / 38.633; 0 (Mediterranean Sea)	Mediterranean Sea
35°50′N 0°0′E / 35.833°N 0°E / 35.833; 0 (Algeria)	Algeria
21°50′N 0°0′E / 21.833°N 0°E / 21.833; 0 (Mali)	Mali	This part of Mali has declared independence as  Azawad
14°59′N 0°0′E / 14.983°N 0°E / 14.983; 0 (Burkina Faso)	Burkina Faso
11°6′N 0°0′E / 11.1°N 0°E / 11.1; 0 (Togo)	Togo	For about 600 m
11°6′N 0°0′E / 11.1°N 0°E / 11.1; 0 (Ghana)	Ghana	For about 16 km
10°57′N 0°0′E / 10.95°N 0°E / 10.95; 0 (Togo)	Togo	For about 39 km
10°36′N 0°0′E / 10.6°N 0°E / 10.6; 0 (Ghana)	Ghana	Passing through Lake Volta at 7°48′N 0°0′E / 7.8°N 0°E / 7.8; 0 (Lake Volta)
5°37′N 0°0′E / 5.617°N 0°E / 5.617; 0 (Atlantic Ocean)	Atlantic Ocean	Passing through the Equator at 0°0′N 0°0′E / 0°N 0°E / 0; 0 (Equator)
60°0′S 0°0′E / 60°S 0°E / -60; 0 (Southern Ocean)	Southern Ocean
68°54′S 0°0′E / 68.9°S 0°E / -68.9; 0 (Antarctica)	Antarctica	Queen Maud Land, claimed by  Norway

  History

A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation—and various conventions have been used or advocated in different regions and throughout history.

  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.^[17]
• 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.^[18] 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.^[19] 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.

  References

  External links

Wikimedia Commons has media related to: Prime meridian

• "Where the Earth's surface begins—and ends", Popular Mechanics, December 1930
• scanned TIFFs of the conference proceedings
• Prime meridians in use in the 1880s, by country
• Canadian Prime Meridian

Circles of latitude / Meridians Equator Tropic of Cancer Tropic of Capricorn Arctic Circle Antarctic Circle Equator Tropic of Cancer Tropic of Capricorn Arctic Circle Antarctic Circle Equator Tropic of Cancer Tropic of Capricorn Arctic Circle Antarctic Circle W 0° E 30° 60° 90° 120° 150° 180° 30° 60° 90° 120° 150° 180° 5° 15° 25° 35° 45° 55° 65° 75° 85° 95° 105° 115° 125° 135° 145° 155° 165° 175° 5° 15° 25° 35° 45° 55° 65° 75° 85° 95° 105° 115° 125° 135° 145° 155° 165° 175° 10° 20° 40° 50° 70° 80° 100° 110° 130° 140° 160° 170° 10° 20° 40° 50° 70° 80° 100° 110° 130° 140° 160° 170° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 10° 20° 30° 40° 50° 60° 70° 80° 90° 5° N 15° 25° 35° 45° 55° 65° 75° 85° 5° S 15° 25° 35° 45° 55° 65° 75° 85° 45x90 45x90 45x90 45x90