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A prime meridian is a meridian (a line of ) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a 360-degree system) form a . This divides the body (e.g. ) into two hemispheres: the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). Unlike the , which also divides a spherical celestial body into two hemispheres, the prime meridian is astronomically arbitrary. For each of Earth's historic prime meridians, various conventions have been used or advocated in different regions throughout history, but none (unlike the prime meridian of Mars) have a basis in physical geography. Earth's current international standard prime meridian is the IERS Reference Meridian. It is derived, but differs slightly, from the Greenwich Meridian, the previous standard.

Longitudes for the Earth and are measured from their prime meridian (at 0°) to 180° east and west. For all other bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body is prograde (or 'direct', like Earth), meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.

History
The notion of longitude for Greeks was developed by the (c.276195BCE) in , and (c.190120BCE) in , and applied to a large number of cities by the (64/63BCEc.24CE). (c.90168CE) was the first geographer to use a consistent meridian for a world map, in his Geographia.

Ptolemy used as his basis the "", a group of islands in the , which are usually associated with the (13°W to 18°W), although his maps correspond more closely to the (22°W to 25°W). The main point is to be comfortably west of the western tip of (17°30′W) as negative numbers were not yet in use. His prime meridian corresponds to 18°40′ west of (about 20°W) today. At that time the chief method of determining longitude was by using the reported times of in different countries.

One of the earliest known descriptions of standard time in India appeared in the 4th century CE treatise . Postulating a , the book described the thousands years old customs of the , or zero longitude, as passing through Avanti, the ancient name for the historic city of in Central India, and Rohitaka, the ancient name for (), a city north.

Ptolemy's Geographia was first printed with maps at 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. In 1493, Christopher Columbus reported 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 and over newly discovered lands. The Tordesillas line was eventually settled at 370 leagues (about ) west of . This is shown in the copies of Spain's made by Diogo Ribeiro in 1527 and 1529. São Miguel Island (25°30′W) in the 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 declination line did not follow a line of longitude.

In 1541, Mercator produced his 41 cm terrestrial globe and drew his prime meridian precisely through (14°1′W) in the Canaries. His later maps used the Azores, following the magnetic hypothesis, but by the time that 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.e.g. in 1722 reported the longitude of as 268°45′ (starting from Fuerteventura) in the Extract from the Official log of Jacob Roggeveen reproduced in In 1634, Cardinal Richelieu used the westernmost island of the Canaries, , 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.Speech by Pierre Janssen, director of the Paris observatory, at the first session of the Meridian Conference.

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 . The development of accurate star charts, principally by the first British , between 1680 and 1719 and disseminated by his successor , enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and .

In the 18th century most countries in adapted their own prime meridian, usually through their capital, hence in the was prime, in Prussia it was the meridian, in the meridian, and in the Greenwich meridian.

Between 1765 and 1811, 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 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 , but eventually abstained and continued to use the until 1911.

The current international standard Prime Meridian is the IERS Reference Meridian. The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.  Section 2.4.4. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989. WGS 84 Implementation Manual page i, 1998

International prime meridian
Since 1984, the international standard for the Earth's prime meridian is the IERS Reference Meridian. Between 1884 and 1984, the meridian of Greenwich was the world standard. These meridians are very close to each other.

Prime meridian at Greenwich
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., to be the common zero of longitude and standard of time reckoning throughout the world.

The position of the historic prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. It was defined by the location of the Airy Transit Circle ever since the first observation he took with it.

(1975). 9780850660937, Taylor & Francis.
Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second , 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 (47 yards) to the west of the Airy Transit Circle, a distance equivalent to roughly 2 seconds of longitude. It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the 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 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 .

IERS Reference Meridian
Beginning in 1973 the International Time Bureau and later the International Earth Rotation and Reference Systems Service changed from reliance on optical instruments like the Airy Transit Circle to techniques such as lunar laser ranging, satellite laser ranging, and very-long-baseline interferometry. The new techniques resulted in the IERS Reference Meridian, the plane of which passes through the centre of mass of the Earth. This differs from the plane established by the Airy transit, which is affected by vertical deflection (the local vertical is affected by influences such as nearby mountains). The change from relying on the local vertical to using a meridian based on the centre of the Earth caused the modern prime meridian to be 5.3 east of the astronomic Greenwich prime meridian through the Airy Transit Circle. At the latitude of Greenwich, this amounts to 102 metres (112 yards). 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 International Terrestrial Reference Frames (ITRFs).

Due to the movement of Earth's , 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 IERS Reference Meridian was established and is commonly used to denote the 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 drifts northeast about 2.5 centimetres (1 inch) per year relative to this Earth-centred 0° longitude.

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). IRM on grounds of Royal Observatory from Google Earth Accessed 30 March 2012 A current convention on the Earth uses the line of longitude 180° opposite the IRM as the basis for the International Date Line.

List of places
On Earth, starting at the and heading south to the , the IERS Reference Meridian (as of 2016) passes through 8 countries, 4 seas, 3 oceans and 1 channel:

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! scope="row"From Tunstall in East Riding to , passing through
! scope="row" style="background:#b0e0e6;" of Great Britain
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! scope="row"From to
! scope="row"From Cilindro de Marboré to Castellón de la Plana
! scope="row" style="background:#b0e0e6;" Mediterranean Sea Gulf of Valencia; of
! scope="row"From to
! scope="row" style="background:#b0e0e6;" Mediterranean Sea of
! scope="row" style="background:#b0e0e6;" Mediterranean Sea of
! scope="row"From to Algeria-Mali border near Bordj Badji Mokhtar
! scope="row"Passing through
! scope="row"For about , running through Cinkassé.
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! scope="row"From the Togo-Ghana border near to
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! scope="row" Queen Maud Land, claimed by
! scope="row" Amundsen–Scott South Pole Station,

Prime meridian on other celestial 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 . The prime meridians of the following planetographic systems have been defined:

  • 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 the English astronomer Richard Christopher Carrington started his observations of . The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory in , England.
  • In 1975 the prime meridian of Mercury was definedMerton E. Davies, "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, 10 June 1975Merton E. Davies, S. E. Dwornik, D. E. Gault, and R. G. Strom, NASA Atlas of Mercury, NASA Scientific and Technical Information Office, 1978. to be 20° east of the crater Hun Kal. This meridian was chosen because it runs through the point on Mercury's equator where the average temperature is highest (due to the planet's rotation and orbit, the sun briefly retrogrades at noon at this point during , giving it more sunlight).Davies, M. E., "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, June 10, 1975.
  • Defined in 1992,Merton E. Davies; Colvin, T. R.; Rogers, P. G.; Chodas, P. G.; Sjogren, W. L.; Akim, W. L.; Stepanyantz, E. L.; Vlasova, Z. P.; and Zakharov, A. I.; "The Rotation Period, Direction of the North Pole, and Geodetic Control Network of Venus", Journal of Geophysical Research, vol. 97, no. 8, 1992, pp. 1–14, 151 the prime meridian of passes through the central peak in the crater Ariadne, chosen arbitrarily.
  • The prime meridian of the lies directly in the middle of the face of the Moon visible from Earth and passes near the crater Bruce.
  • The prime meridian of was established in 1971Merton E. Davies, and Berg, R. A.; "Preliminary Control Net of Mars", Journal of Geophysical Research, vol. 76, no. 2, 10 January 1971, pp. 373–393 and passes through the center of the crater Airy-0, although it is fixed by the longitude of the Viking 1 lander, which is defined to be 47.95137°W.
  • The prime meridian on Ceres runs through the Kait crater, which was arbitrarily chosen because it is near the equator (about 2° south).
  • The prime meridian on 4 Vesta is 4 degrees east of the crater Claudia, chosen because it is sharply defined.
  • 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. System I and System II coordinates are based on atmospheric rotation, and System III coordinates use Jupiter's magnetic field. The prime meridians of Jupiter's four were established in 1979.Merton E. Davies, Thomas A. Hauge, et al.: Control Networks for the Galilean Satellites: November 1979 R-2532-JPL/NASA
    • Europa's prime meridian is defined such that the crater Cilix is at 182° W. The 0° longitude runs through the middle of the face that is always turned towards Jupiter.
    • Io's prime meridian, like that of Earth's moon, is defined so that it runs through the middle of the face that is always turned towards Jupiter (the near side, known as the subjovian hemisphere).
    • Ganymede's prime meridian is defined such that the crater Anat is at 128° W, and the 0° longitude runs through the middle of the subjovian hemisphere.
    • Callisto's prime meridian is defined such that the crater Saga is at 326° W. Satellites of Jupiter. (1982:912). United States: University of Arizona Press.
  • Titan is the largest moon of and, like the Earth's moon, is tidally locked and always has the same face towards Saturn. The middle of that face is 0 longitude.
  • Like Jupiter, is a gas giant, so any surface is obscured by clouds. The prime meridian of its largest moon, Triton, was established in 1991.Merton E. Davies, P. G. Rogers, and T. R. Colvin, "A Control Network of Triton," Journal of Geophysical Research, Vo l. 96, E l, pp. 15,675-15,681, 1991.
  • 's prime meridian is defined as the meridian passing through the center of the face that is always towards Charon, its largest moon, as the two are tidally locked to each other. Charon's prime meridian is similarly defined as the meridian always facing directly toward Pluto.

List of historic prime meridians on Earth
168°30′ W Offered in 1884 as possibility for a neutral prime meridian by at the International Meridian Conference International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884 , pp. 43–51. Project Gutenberg
77°0356.07 W (1897) or 77°0402.24 W (NAD 27) or 77°0401.16 W (NAD 83)New Naval Observatory meridian
77°0248.0 W, 77°0302.3, 77°0306.119 W or 77°0306.276 W (both presumably NAD 27). If NAD27, the latter would be 77°0305.194 W (NAD 83)Old Naval Observatory meridian
77°0211.56299 W (NAD 83), 77°0211.55811 W (NAD 83), 77°0211.58325 W (NAD 83) (three different monuments originally intended to be on the White House meridian)White House meridian
77°0032.6 W (NAD 83)
75° 10 12 W Oct. 13, 1884: Greenwich Resolves Subprime Meridian Crisis , WIRED, 13 October 2010.
43° 10 19 W Atlas do Brazil , 1909, by Barão Homem de Mello e Francisco Homem de Mello, published in Rio de Janeiro by F. Briguiet & Cia.
25° 40 32 W Proposed as one possible neutral meridian by at the International Meridian Conference
18° 03 W,

17° 39 46 W		 Ancient, used in 's . Later redefined 17° 39 46 W of Greenwich to be exactly 20° W of Paris. French "submarin" at Washington 1884.
16°3822 WTenerife meridian Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridianA.R.T. Jonkers; Parallel meridians: Diffusion and change in early modern oceanic reckoning , in Noord-Zuid in Oostindisch perspectief, The Hague, 2005, p. 7. Retrieved 2 February 2015.
9° 07 54.862 W
(2026). 9780804756426, Stanford University Press. .
6° 17 35.4" WCádiz meridian Royal Observatory in southeast tower of , used 1735–1850 by Spanish Navy. "In search of the lost meridian of Cadiz" , El País, 23 December 2016. Retrieved 8 November 2018.Antonio Lafuente and Manuel Sellés, El Observatorio de Cádiz (1753–1831) , Ministerio de Defensa, 1988, p.144, .
3° 41 16.58 W
0° 00 19.0 WPrime Meridian (prior to Greenwich) Located at King George III's
0° 00 05.33 WUnited Kingdom Ordnance Survey Zero Meridian Meridian
0° 00 05.3101 WGreenwich meridianAiry Meridian
0° 00 00.00IERS Reference Meridian
2° 20 14.025 E
4° 22 4.71 E
4° 24 E
4° 53 E Through the in Amsterdam; used to define the legal time in the Netherlands from 1909 to 1937 Eenheid van tijd in Nederland (Unity of time in the Netherlands) , Utrecht University website, retrieved 28 August 2013.
10° 24 E
10° 43 22.5 E
11°15 EFlorence meridian Used in the Peters projection, 180° from a meridian running through the
12° 27 08.4 EMeridian of Used in Roma 40 Datum Grids & Datums – Italian Republic , asprs.org, Retrieved 10 December 2013.
12° 34 32.25 E Rundetårn meridian , article from Den Store Danske Encyklopædi
14° 15 E
17° 06 03 EMeridianus Posoniensis Used by Sámuel Mikoviny
18° 03 29.8 E At the Stockholm Observatory
19° 03 37 EMeridianu(s) Budense Used between 1469 and 1495; introduced by , used by , , Miklós Erdélyi (1423–1473), Johannes Tolhopff (c. 1445–1503), Johannes Muntz. Set in the royal castle (and observatory) of Buda.
19° 57 21.43 EKraków meridianat the Old Kraków Observatory at the Śniadecki' College; mentioned also in Nicolaus Copernicus's work On the Revolutions of the Heavenly Spheres.
21° 00 42 E
21° 55 16 ETabulae Varadienses Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea ( Varadinum at the time) by Georg von Peuerbach. In his logbook Columbus stated, he had one copy of Tabulae Varadienses (Tabula Varadiensis or Tabulae directionum) on board to calculate the actual meridian based on the position of the Moon, in correlation to Várad. also recalled, how was he acquired the knowledge to calculate meridians by means of these tables.
29° 53 EMeridian of Alexandria The meridian of 's .
30° 19 42.09 E
31° 08 03.69 E 1884Wilcomb E. Washburn, " The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth "
35° 13 47.1 E
39° 49 34 E See also
Approx. 59° E Hilchot Kiddush Hachodesh 11:17 calls this point (24 degrees east of ) אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day.
75° 47 E Used from 4th century CE Indian astronomy and calendars(see also Time in India).
116° 24 E Used in for astronomical and cartographical purposes.
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

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