A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its antimeridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides the sphere, e.g., Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.
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. The most widely used modern meridian is the IERS Reference Meridian. It is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884.
The notion of longitude was developed by the Greek Eratosthenes (c. 276 BC – c. 195 BC) in Alexandria, and Hipparchus (c. 190 BC – c. 120 BC) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63 BC – c. 24 AD). But it was Ptolemy (c. AD 90 – c. AD 168) who first used a consistent meridian for a world map in his Geographia.
Ptolemy used as his basis the "Fortunate Isles", a group of islands in the Atlantic which are usually associated with the Canary Islands (13° to 18°W), although his maps correspond more closely to the Cape Verde islands (22° to 25° W). The main point is to be comfortably west of the western tip of Africa (17.5° W) as negative numbers were not yet in use. His prime meridian corresponds to 18° 40' west of Winchester (about 20°W) today. At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.
Ptolemy's Geographia was first printed with maps at Bologna in 1477, and many early globes in the 16th century followed his lead. But there was still a hope that a "natural" basis for a prime meridian existed. Christopher Columbus reported (1493) that the compass pointed due north somewhere in mid-Atlantic, and this fact was used in the important Treaty of Tordesillas of 1494 which settled the territorial dispute between Spain and Portugal over newly discovered lands. The Tordesillas line was eventually settled at 370 leagues west of Cape Verde. This is shown in Diogo Ribeiro's 1529 map. São Miguel Island (25.5°W) in the Azores was still used for the same reason as late as 1594 by Christopher Saxton, although by then it had been shown that the zero magnetic deviation line did not follow a line of longitude.
In 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuerteventura (14°1'W) in the Canaries. His later maps used the Azores, following the magnetic hypothesis. But by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as is usual today. This practice was followed by navigators well into the 18th century. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 19° 55' west of Paris, as the choice of meridian. The geographer Delisle decided to round this off to 20°, so that it simply became the meridian of Paris disguised.
In the early 18th century the battle was on to improve the determination of longitude at sea, leading to the development of the marine chronometer by John Harrison. But it was the development of accurate star charts, principally by the first British Astronomer Royal, John Flamsteed between 1680 and 1719 and disseminated by his successor Edmund Halley, that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley. Between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich. "Maskelyne's tables not only made the lunar method practicable, they also made the Greenwich meridian the universal reference point. Even the French translations of the Nautical Almanac retained Maskelyne's calculations from Greenwich—in spite of the fact that every other table in the Connaissance des Temps considered the Paris meridian as the prime." 
In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich meridian as the prime meridian of the world. The French argued for a neutral line, mentioning the Azores and the Bering Strait, but eventually abstained and continued to use the Paris meridian until 1911.
|Locality||Modern longitude||Meridian name||Comment|
|Bering Strait||168°30′ W||Offered in 1884 as possibility for a "neutral prime meridian" by Pierre Janssen at the International Meridian Conference |
|Washington, D.C.||77°03′56.07″ W (1897) or 77°04′02.24″ W (NAD 27)[clarification needed] or 77°04′01.16″ W (NAD 83)||New Naval Observatory meridian|
|Washington, D.C.||77°02′48.0″ W, 77°03′02.3″, 77°03′06.119″ W or 77°03′06.276″ W (both presumably NAD 27). If NAD27, the latter would be 77°03′05.194″ W (NAD 83)||Old Naval Observatory meridian|
|Washington, D.C.||77°02′11.56299″ W (NAD 83), 77°02′11.55811″ W (NAD 83), 77°02′11.58325″ W (NAD 83) (three different monuments originally intended to be on the White House meridian)||White House meridian|
|Washington, D.C.||77°00′32.6″ W (NAD 83)||Capitol meridian|
|Philadelphia||75° 10′ 12″ W|||
|Rio de Janeiro||43° 10′ 19″ W|||
|Fortunate Isles / Azores||25° 40′ 32″ W||Used until the Middle Ages, proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference|
|El Hierro (Ferro),
|18° 03′ W,
later redefined as
17° 39′ 46″ W
|Tenerife||16° 38' 22" W||Tenerife meridian||Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian|
|Cadiz||6° 17' 35.4" W||Cadiz meridian||Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy.|
|Lisbon||9° 07′ 54.862″ W|||
|Madrid||3° 41′ 16.58″ W|||
|Greenwich||0° 00′ 05.3101″ W||Greenwich meridian||Airy Meridian|
|Greenwich||0° 00′ 05.33″ W||United Kingdom Ordnance Survey Zero Meridian||Bradley Meridian|
|Greenwich||0° 00′ 00.00″||IERS Reference Meridian|
|Paris||2° 20′ 14.025″ E||Paris meridian|
|Brussels||4° 22′ 4.71″ E|||
|Antwerp||4° 24′ E||Antwerp meridian|
|Amsterdam||4° 53′ E||through the Westerkerk in Amsterdam; used to define the legal time in the Netherlands from 1909 to 1937|
|Bern||7° 26′ 22.5″ E|||
|Pisa||10° 24′ E|||
|Oslo (Kristiania)||10° 43′ 22.5″ E|||
|Florence||11°15′ E||Florence meridian||used in the Peters projection, 180° from a meridian running through the Bering Strait|
|Rome||12° 27′ 08.4″ E||meridian of Monte Mario||Used in Roma 40 Datum |
|Copenhagen||12° 34′ 32.25″ E||Rundetårn|
|Naples||14° 15′ E|||
|Bratislava||17° 06′ 03″ E||Meridianus Posoniensis||Used by Sámuel Mikoviny|
|Stockholm||18° 03′ 29.8″ E||at the Stockholm Observatory|
|Kraków||19° 57′ 21.43″ E||Kraków meridian||at the Old Kraków Observatory at the Śniadecki' College; mentioned also in Nicolaus Copernicus's work On the Revolutions of the Heavenly Spheres.|
|Warsaw||21° 00′ 42″ E||Warsaw meridian|||
|Oradea||21° 55′ 16″ E|| Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea (Varadinum at the time) by Georg von Peuerbach.|
|Alexandria||29° 53′ E||Meridian of Alexandria||The meridian of Ptolemy's Almagest.|
|Saint Petersburg||30° 19′ 42.09″ E||Pulkovo meridian|
|Great Pyramid of Giza||31° 08′ 03.69″ E||1884 |
|Jerusalem||35° 13′ 47.1″ E|||
|Mecca||39° 49′ 34″ E||see also Mecca Time |
|Ujjain||75° 47′ E||Used from 4th century CE Indian astronomy and calendars(see also Time in India).|
|Kyoto||136° 14′ 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.|
|~ 180||Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming |
In October 1884 the Greenwich Meridian was selected by delegates (forty-one delegates representing twenty-five nations) to the International Meridian Conference held in Washington, D.C., United States to be the common zero of longitude and standard of time reckoning throughout the world.[note 1] The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian and is the prime meridian that currently has the widest use.
The position of the Greenwich Meridian has been defined by the location of the Airy Transit Circle ever since the first observation was taken with it by Sir George Airy in 1851. Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal, Edmond Halley in 1721. It was set up in the extreme north-west corner of the Observatory between Flamsteed House and the Western Summer House. This spot, now subsumed into Flamsteed House, is roughly 43 metres to the west of the Airy Transit Circle, a distance equivalent to roughly 0.15 seconds of time. It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the Paris meridian abstaining) as the Prime Meridian of the world at the 1884 International Meridian Conference.
All of these Greenwich meridians were located via an astronomic observation from the surface of the Earth, oriented via a plumb line along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via the lunar distance method, then by chronometers carried on ships, then via telegraph lines carried by submarine communications cables, then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States.
Satellites changed the reference from the surface of the Earth to its centre of mass around which all satellites orbit regardless of surface irregularities. The requirement that satellite-based geodetic reference systems be centred on the centre of mass of the earth caused the modern prime meridian to be 5.3" east of the astronomic Greenwich prime meridian through the Airy Transit Circle[clarification needed]. At the latitude of Greenwich, this amounts to 102 metres. This was officially accepted by the Bureau International de l'Heure (BIH) in 1984 via its BTS84 (BIH Terrestrial System) that later became WGS84 (World Geodetic System 1984) and the various ITRFs (International Terrestrial Reference Systems).
Due to the movement of Earth's tectonic plates, the line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres; that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map. With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning. As a result, the International Reference Meridian was established and is commonly used to denote Earth's prime meridian (0° longitude) by the International Earth Rotation and Reference Systems Service, which defines and maintains the link between longitude and time. Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres per year relative to this Earth-centred 0° longitude. Circa 1999 the international reference meridian (IRM) passed 5.31 arcseconds east of Airy's meridian or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich, London. 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). A current convention on the Earth uses the opposite of the IRM as the basis for the International Date Line.
|Country, territory or sea||Notes|
|United Kingdom||From Tunstall in East Riding to Peacehaven, passing through Greenwich|
|France||From Villers-sur-Mer to Gavarnie|
|Spain||From Cilindro de Marboré to Castellón de la Plana|
|Merranean Sea||Gulf of Valencia|
|Spain||From El Verger to Calpe|
|Algeria||From Stidia to Algeria-Mali border near Bordj Mokhtar|
|Togo||For about 600 m|
|Ghana||For about 16 km|
|Togo||For about 39 km|
|Ghana||From the Togo-Ghana border near Bunkpurugu to Tema|
Passing through Lake Volta at
|Atlantic Ocean||Passing through the Equator at ("Null Island")|
|Antarctica||Queen Maud Land, claimed by Norway|
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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