# IERS Reference Meridian

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, and thus it differs slightly from the historical Greenwich Meridian. At the latitude of the Royal Observatory, Greenwich the difference is 102 metres (335 ft).
It is the reference meridian of the Global Positioning System (GPS) operated by the United States Space Force, and of WGS 84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).

## Location

The most important reason for the 5.3 seconds of longitude offset between the IERS Reference Meridian and the Airy transit circle is that the observations with the transit circle were based on the astronomical longitude, while the IERS Reference is a geodetic-based longitude, that is, the plane of the meridian contains the center of figure of the Earth. Their difference is caused by the east-west component of the vertical deflection, between the local gravity vertical direction and the ellipsoidal normal.
The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989. Tectonic plates slowly move over the surface of 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.
The IERS system is not quite fixed to any point attached to the Earth. For example, all points on the European portion of the Eurasian plate, including the Royal Observatory, are moving northeast at about 2.5 cm per year relative to it. The 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/GNSS 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 reduced to a reference epoch (a fixed date/time) and 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.
The 180th meridian (the meridian at 180° both east and west of the Prime Meridian) is opposite the IERS Reference Meridian and forms a great ellipse with it dividing the earth into Western Hemisphere and Eastern Hemisphere.
Universal Time is notionally based on the prime 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 of a second. Leap seconds are inserted from time to time, to keep UTC close to Earth's angular position relative to the Sun; see mean solar time.

## List of places

Download coordinates as:
* KML
* GPX (all coordinates)
* GPX (primary coordinates)
* GPX (secondary coordinates)
Starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian passes through eight countries and three oceans (Arctic Ocean, Atlantic Ocean and Southern Ocean):
| Co-ordinates (approximate) | Country, territory or sea | Notes |
| 90°0′N 0°0′E﻿ / ﻿90.000°N 0.000°E﻿ / 90.000; 0.000﻿ (North Pole) | Arctic Ocean | |
| 85°46′N 0°0′E﻿ / ﻿85.767°N 0.000°E﻿ / 85.767; 0.000﻿ (EEZ of Greenland (Denmark)) | Exclusive Economic Zone (EEZ) of Greenland (Denmark) | |
| 81°39′N 0°0′E﻿ / ﻿81.650°N 0.000°E﻿ / 81.650; 0.000﻿ (Greenland Sea) | Greenland Sea | |
| 80°29′N 0°0′E﻿ / ﻿80.483°N 0.000°E﻿ / 80.483; 0.000﻿ (EEZ of Svalbard (Norway)) | EEZ of Svalbard (Norway) | |
| 76°11′N 0°0′E﻿ / ﻿76.183°N 0.000°E﻿ / 76.183; 0.000﻿ (International waters) | International waters | |
| 73°44′N 0°0′E﻿ / ﻿73.733°N 0.000°E﻿ / 73.733; 0.000﻿ (EEZ of Jan Mayen) | EEZ of Jan Mayen (Norway) | |
| 72°53′N 0°0′E﻿ / ﻿72.883°N 0.000°E﻿ / 72.883; 0.000﻿ (Norwegian Sea) | Norwegian Sea | |
| 69°7′N 0°0′E﻿ / ﻿69.117°N 0.000°E﻿ / 69.117; 0.000﻿ (International waters) | International waters | |
| 64°42′N 0°0′E﻿ / ﻿64.700°N 0.000°E﻿ / 64.700; 0.000﻿ (EEZ of Norway) | EEZ of Norway | |
| 63°29′N 0°0′E﻿ / ﻿63.483°N 0.000°E﻿ / 63.483; 0.000﻿ (EEZ of Great Britain) | EEZ of Great Britain | |
| 61°0′N 0°0′E﻿ / ﻿61.000°N 0.000°E﻿ / 61.000; 0.000﻿ (North Sea) | North Sea | |
| 53°46′N 0°0′E﻿ / ﻿53.767°N 0.000°E﻿ / 53.767; 0.000﻿ (United Kingdom) | United Kingdom | From Tunstall in East Riding to Peacehaven, passing through Greenwich |
| 50°47′N 0°0′E﻿ / ﻿50.783°N 0.000°E﻿ / 50.783; 0.000﻿ (English Channel) | English Channel | EEZ of Great Britain |
| 50°14′N 0°0′E﻿ / ﻿50.233°N 0.000°E﻿ / 50.233; 0.000﻿ (EEZ of France) | English Channel | EEZ of France |
| 49°20′N 0°0′E﻿ / ﻿49.333°N 0.000°E﻿ / 49.333; 0.000﻿ (France) | France | From Villers-sur-Mer to Gavarnie |
| 42°41′N 0°0′E﻿ / ﻿42.683°N 0.000°E﻿ / 42.683; 0.000﻿ (Spain) | Spain | From Cilindro de Marboré to Castellón de la Plana |
| 39°56′N 0°0′E﻿ / ﻿39.933°N 0.000°E﻿ / 39.933; 0.000﻿ (Mediterranean Sea) | Mediterranean Sea | Gulf of Valencia; EEZ of Spain |
| 38°52′N 0°0′E﻿ / ﻿38.867°N 0.000°E﻿ / 38.867; 0.000﻿ (Spain) | Spain | From El Verger to Calp |
| 38°38′N 0°0′E﻿ / ﻿38.633°N 0.000°E﻿ / 38.633; 0.000﻿ (Mediterranean Sea) | Mediterranean Sea | EEZ of Spain |
| 37°1′N 0°0′E﻿ / ﻿37.017°N 0.000°E﻿ / 37.017; 0.000﻿ (EEZ of Algeria) | Mediterranean Sea | EEZ of Algeria |
| 35°50′N 0°0′E﻿ / ﻿35.833°N 0.000°E﻿ / 35.833; 0.000﻿ (Algeria) | Algeria | From Stidia to Algeria-Mali border near Bordj Badji Mokhtar |
| 21°52′N 0°0′E﻿ / ﻿21.867°N 0.000°E﻿ / 21.867; 0.000﻿ (Mali) | Mali | Passing through Gao |
| 15°00′N 0°0′E﻿ / ﻿15.000°N 0.000°E﻿ / 15.000; 0.000﻿ (Burkina Faso) | Burkina Faso | |
| 11°7′N 0°0′E﻿ / ﻿11.117°N 0.000°E﻿ / 11.117; 0.000﻿ (Togo) | Togo | For about 600 m |
| 11°6′N 0°0′E﻿ / ﻿11.100°N 0.000°E﻿ / 11.100; 0.000﻿ (Ghana) | Ghana | For about 16 km |
| 10°58′N 0°0′E﻿ / ﻿10.967°N 0.000°E﻿ / 10.967; 0.000﻿ (Togo) | Togo | For about 39 km |
| 10°37′N 0°0′E﻿ / ﻿10.617°N 0.000°E﻿ / 10.617; 0.000﻿ (Ghana) | Ghana | From the Togo-Ghana border near Bunkpurugu to Tema Passing through Lake Volta at 7°46′N 0°0′E﻿ / ﻿7.767°N 0.000°E﻿ / 7.767; 0.000﻿ (Lake Volta) |
| 5°37′N 0°0′E﻿ / ﻿5.617°N 0.000°E﻿ / 5.617; 0.000﻿ (EEZ of Ghana in Atlantic Ocean) | Atlantic Ocean | EEZ of Ghana |
| 1°58′N 0°0′E﻿ / ﻿1.967°N 0.000°E﻿ / 1.967; 0.000﻿ (International waters) | Atlantic Ocean | International waters |
| 0°0′N 0°0′E﻿ / ﻿0.000°N 0.000°E﻿ / 0.000; 0.000﻿ (Equator) | Atlantic Ocean | Passing through the Equator (see Null Island) |
| 51°43′S 0°0′E﻿ / ﻿51.717°S 0.000°E﻿ / -51.717; 0.000﻿ (EEZ of Bouvet Island) | Atlantic Ocean | EEZ of Bouvet Island (Norway) |
| 57°13′S 0°0′E﻿ / ﻿57.217°S 0.000°E﻿ / -57.217; 0.000﻿ (International waters) | Atlantic Ocean | International waters |
| 60°0′S 0°0′E﻿ / ﻿60.000°S 0.000°E﻿ / -60.000; 0.000﻿ (Southern Ocean) | Southern Ocean | International waters |
| 69°36′S 0°0′E﻿ / ﻿69.600°S 0.000°E﻿ / -69.600; 0.000﻿ (Antarctica) | Antarctica | Queen Maud Land, claimed by Norway |
| 90°0′S 0°0′E﻿ / ﻿90.000°S 0.000°E﻿ / -90.000; 0.000﻿ (Amundsen–Scott South Pole Station) | Antarctica | Amundsen–Scott South Pole Station, United States South Pole |