# Prime meridian

A prime meridian is an arbitrarily-chosen meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides a spheroid, like Earth, into two hemispheres: the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). For Earth's prime meridian, various conventions have been used or advocated in different regions throughout history.[1] 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.[2]

A prime meridian for a planetary body not tidally locked (or at least not in synchronous rotation) is entirely arbitrary, unlike an equator, which is determined by the axis of rotation. However, for celestial objects that are tidally locked (more specifically, synchronous), their prime meridians are determined by the face always inward of the orbit (a planet facing its star, or a moon facing its planet), just as equators are determined by rotation.

Longitudes for the Earth and Moon are measured from their prime meridian (at 0°) to 180° east and west. For all other Solar System 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.[3]

## History

The notion of longitude for Greeks was developed by the Greek Eratosthenes (c. 276 – 195 BCE) in Alexandria, and Hipparchus (c. 190 – 120 BCE) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63 BCE – c. 24 CE). But it was Ptolemy (c. 90 – 168 CE) 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.[1] At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.

One of the earliest known descriptions of standard time in India appeared in the 4th century CE astronomical treatise Surya Siddhanta. Postulating a spherical earth, the book described the thousands years old customs of the prime meridian, or zero longitude, as passing through Avanti, the ancient name for the historic city of Ujjain, and Rohitaka, the ancient name for Rohtak (28°54′N 76°38′E﻿ / ﻿28.900°N 76.633°E), a city near the Kurukshetra.[4][better source needed]

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 (2,193 kilometers, 1,362 statute miles, or 1,184 nautical miles) west of Cape Verde.[a] This is shown in the copies of Spain's Padron Real made by Diogo Ribeiro in 1527 and 1529. 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 declination line did not follow a line of longitude.[8]

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.[9] In 1634, Cardinal Richelieu used the westernmost island of the Canaries, El Hierro, 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.[10]

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.[11]

In the 18th century most countries in Europe adapted their own prime meridian, usually through their capital, hence in France the Paris meridian was prime, in Prussia it was the Berlin meridian, in Denmark the Copenhagen meridian, and in United Kingdom the Greenwich meridian.

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."[12]

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.[13] 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.

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.[14] It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[15]

## 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 physically 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., United States to be the common zero of longitude and standard of time reckoning throughout the world.[16][b]

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.[18] 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 2 seconds of longitude.[19] 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.[20][21]

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.

### 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.[22] 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 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.[23] 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 transit circle drifts northeast about 2.5 centimetres 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).[24][25][c] 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 North Pole and heading south to the South Pole, the IERS Reference Meridian (as of 2016) passes through:

Co-ordinates
(approximate)
Country, territory or sea Notes
Arctic Ocean
Exclusive Economic Zone (EEZ) of Greenland (Denmark)
Greenland Sea
EEZ of Svalbard (Norway)
International waters
EEZ of Jan Mayen (Norway)
Norwegian Sea
International waters
EEZ of Norway
EEZ of Great Britain
North Sea
United Kingdom From Tunstall in East Riding to Peacehaven, passing through Greenwich
English Channel EEZ of Great Britain
English Channel EEZ of France
France From Villers-sur-Mer to Gavarnie
Spain From Cilindro de Marboré to Castellón de la Plana
Mediterranean Sea Gulf of Valencia; EEZ of Spain
Spain From El Verger to Calp
Mediterranean Sea EEZ of Spain
Mediterranean Sea EEZ of Algeria
Algeria From Stidia to Algeria-Mali border near Bordj Badji Mokhtar
Mali Passing through Gao
Burkina Faso
Ghana From the Togo-Ghana border near Bunkpurugu to Tema
Passing through Lake Volta at
Atlantic Ocean EEZ of Ghana
International waters
Passing through the Equator (see Null Island)
EEZ of Bouvet Island (Norway)
International waters
Southern Ocean International waters
Antarctica Queen Maud Land, claimed by  Norway
Antarctica Amundsen–Scott South Pole Station, South Pole

## Prime meridian on 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:

• 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 sunspots.[26] The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory in Lancashire, England.
• In 1975 the prime meridian of Mercury was defined[27][28] to be 20° east of the crater Hun Kal.[29]
• Defined[30] in 1992, the prime meridian of Venus passes through the central peak in the crater Ariadne.[31]
• 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 was established in 1971[32] 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.[33]
• 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.[34] 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 Galilean moons were established in 1979.[35]
• Titan is the largest moon of Saturn and, like the Earth's moon, always has the same face towards Saturn, and so the middle of that face is 0 longitude.
• Like Jupiter, Neptune is a gas giant, so any surface is obscured by clouds. The prime meridian of its largest moon, Triton, was established in 1991.[36]
• Pluto'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

Locality Modern longitude Meridian name Image Comment
Bering Strait 168°30 W
168°
168th meridian west
Offered in 1884 as possibility for a neutral prime meridian by Pierre Janssen at the International Meridian Conference[37]
Washington, D.C. 77°0356.07″ W (1897) or 77°0402.24″ W (NAD 27)[clarification needed] or 77°0401.16″ W (NAD 83) New Naval Observatory meridian
77°
77th meridian west
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),[38] 77°0211.55811″ W (NAD 83),[39] 77°0211.58325″ W (NAD 83)[40] (three different monuments originally intended to be on the White House meridian) White House meridian
77°0032.6″ W (NAD 83) Capitol meridian
75°
75th meridian west
[41][42]
Rio de Janeiro 43° 10 19″ W
43°
43rd meridian west
[43]
Fortunate Isles / Azores 25° 40 32″ W
25°
25th meridian west
Used until the Middle Ages, proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference[44]
El Hierro (Ferro),
Canary Islands
18° 03 W,
later redefined as
17° 39 46″ W
Ferro meridian
18°
18th meridian west
[45]
Tenerife 16°3822″ W Tenerife meridian
16°
16th meridian west
Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian[46]
Lisbon 9° 07 54.862″ W
9th meridian west
[47]
6th meridian west
Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy.[48][49]
3rd meridian west
[47]
Kew 0° 00 19.0″ W Prime Meridian (prior to Greenwich)
Prime meridian
Located at King George III's Kew Observatory
Greenwich 0° 00 05.33″ W United Kingdom Ordnance Survey Zero Meridian Bradley Meridian[19]
0° 00 05.3101″ W Greenwich meridian Airy Meridian[19]
0° 00 00.00″ IERS Reference Meridian
Paris 2° 20 14.025″ E Paris meridian
2nd meridian east
Brussels 4° 22 4.71″ E
4th meridian east
[47]
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[50]
Pisa 10° 24 E
10°
10th meridian east
[41]
Oslo (Kristiania) 10° 43 22.5″ E [41][42]
Florence 11°15 E Florence meridian
11°
11th meridian east
Used in the Peters projection, 180° from a meridian running through the Bering Strait
Rome 12° 27 08.4″ E Meridian of Monte Mario
12°
12th meridian east
Used in Roma 40 Datum[51]
Copenhagen 12° 34 32.25″ E Rundetårn[52]
Naples 14° 15 E
14°
14th meridian east
[44]
Pressburg 17° 06 03″ E Meridianus Posoniensis
17°
17th meridian east
Used by Sámuel Mikoviny
Stockholm 18° 03 29.8″ E
18°
18th meridian east
At the Stockholm Observatory[47]
Buda 19° 03 37″ E Meridianu(s) Budense
19°
19th meridian east
Used between 1469 and 1495; introduced by Regiomontanus, used by Marcin Bylica, Galeotto Marzio, Miklós Erdélyi (1423–1473), Johannes Tolhopff (c. 1445–1503), Johannes Muntz. Set in the royal castle (and observatory) of Buda.[d]
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
21°
21st meridian east
[47]
21°
21st meridian east
[56] Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea (Varadinum at the time) by Georg von Peuerbach.[57] 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. Amerigo Vespucci also recalled, how was he acquired the knowledge to calculate meridians by means of these tables.[58]
Alexandria 29° 53 E Meridian of Alexandria
29°
29th meridian east
The meridian of Ptolemy's Almagest.
Saint Petersburg 30° 19 42.09″ E Pulkovo meridian
30°
30th meridian east
Great Pyramid of Giza 31° 08 03.69″ E
31°
31st meridian east
1884[59]
Jerusalem 35° 13 47.1″ E
35°
35th meridian east
[42]
Mecca 39° 49 34″ E
39°
39th meridian east
Approx. 59° E
59°
59th meridian east
Maimonides[60] calls this point אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day.
Ujjain 75° 47 E
75°
75th meridian east
Used from 4th century CE Indian astronomy and calendars(see also Time in India).[61]
Kyoto 136° 14 E
136°
136th meridian east
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.[citation needed]
~ 180
180°
180th meridian
Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming[44]

## Notes

1. ^ These figures use the legua náutica (nautical league) of four Roman miles totalling 5.926 km (3.682 mi), which was used by Spain during the 15th, 16th, and 17th centuries for navigation.[5] In 1897 Henry Harrise noted that Jaime Ferrer, the expert consulted by King Ferdinand and Queen Isabella, stated that a league was four miles of six stades each.[6] Modern scholars agree that the geographic stade was the Roman or Italian stade, not any of several other Greek stades, supporting these figures.[7] Harrise is in the minority when he uses the stade of 192.27 m (630.8 ft) marked within the stadium at Olympia, Greece, resulting in a league (32 stades) of 6.153 km (3.823 mi), 3.8% larger.
2. ^ Voting took place on 13 October and the resolutions were adopted on 22 October 1884.[17] The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian.[13]
3. ^ The astronomic latitude of the Royal Observatory is 51°2838″N whereas its latitude on the European Terrestrial Reference Frame (1989) datum is 51°2840.1247″N.
4. ^ When Tolhopff handed over his book, titled Stellarium (1480),[53] to King Matthias Corvinus, he emphasized that he had used the meridian of Buda for his calculations. The German physician, Johannes Müntz used it the same way in his 1495 calendar. However, in the second edition, he used the Vienna meridian.[54][55]

## References

1. ^ a b
2. ^ "What is the Prime Meridian and why is it in Greenwich?". Royal Museums Greeenwich. Archived from the original on 13 December 2021. Retrieved 13 December 2021. The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0° longitude by international agreement. The IRM passes 102.5 metres to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian.
3. ^ Archimal, B. A. (2015), Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015 (PDF), p. 27 of 46, archived (PDF) from the original on 6 August 2019, retrieved 6 August 2019, The range of longitudes shall extend from 0° to 360°. Thus, west longitudes are used when the rotation is direct, and east longitudes are used when the rotation is retrograde. ... The Earth, Sun, and Moon do not traditionally follow this definition. Their rotations are direct and longitudes run both east and west 180°, or positive to the east 360°.
4. ^ Schmidt, Olaf H. (1944). "The Computation of the Length of Daylight in Hindu Astronomy". Isis. The University of Chicago Press. 35 (3): 205–211. doi:10.1086/358709. JSTOR 330729. S2CID 145178197. Archived from the original on 26 January 2022.
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9. ^ e.g. Jacob Roggeveen in 1722 reported the longitude of Easter Island as 268° 45' (starting from Fuerteventura) in the Extract from the Official log of Jacob Roggeveen reproduced in Bolton Glanville Corney, ed. (1908), The voyage of Don Felipe Gonzalez to Easter Island in 1770-1, Hakluyt Society, p. 3, retrieved 13 January 2013
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12. ^ Sobel & Andrewes 1998, pp. 197–199.
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37. ^ International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884 Archived 18 December 2021 at the Wayback Machine, pp. 43–51. Project Gutenberg
38. ^ NGS 2016, PID: HV1847.
39. ^ NGS 2016, PID: HV1846.
40. ^ NGS 2016, PID: AH7372.
41. ^ a b c Hooker 2006, introduction.
42. ^ a b c Oct. 13, 1884: Greenwich Resolves Subprime Meridian Crisis Archived 1 December 2018 at the Wayback Machine, WIRED, 13 October 2010.
43. ^ Atlas do Brazil Archived 16 June 2014 at the Wayback Machine, 1909, by Barão Homem de Mello e Francisco Homem de Mello, published in Rio de Janeiro by F. Briguiet & Cia.
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45. ^ Ancient, used in Ptolemy's Geographia. Later redefined 17° 39 46″ W of Greenwich to be exactly 20° W of Paris. French "submarin" at Washington 1884.
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