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Line across the Earth
0°
class=notpageimage|
Modern IERS Reference Meridian on Earth (interactive map)
Countries that touch the Equator (red) and that touch the Prime Meridian (blue)

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°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ellipse. This divides the body (e.g. 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]

Gerardus Mercator in his Atlas Cosmographicae (1595) used a prime meridian somewhere close to 25°W, passing just to the west of Santa Maria Island in the Azores in the Atlantic Ocean. His 180th meridian runs along the Strait of Anián (Bering Strait)

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

[edit]
See also: History of longitude
Ptolemy's 1st projection, redrawn under Maximus Planudes around 1300, using a prime meridian through the Canary Islands west of Africa, at the left-hand edge of the map (The obvious central line shown here is the junction of two sheets.)

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). Ptolemy (c. 90 – 168 CE) was the first geographer to use 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°W to 18°W), although his maps correspond more closely to the Cape Verde islands (22°W to 25°W). The main point is to be comfortably west of the western tip of Africa (17°30′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 / 28.900; 76.633 (Rohitaka (Rohtak))), a city near the Kurukshetra.[4][better source needed]

William Grigg's facsimile of the 1529 Spanish Padron Real, from the copy made by Diogo Ribeiro and held by the Vatican Library

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. 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 Spain and Portugal over newly discovered lands. The Tordesillas line was eventually settled at 370 leagues (about 2,190 kilometres (1,360 miles; 1,180 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°30′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]

1571 Africa map by Abraham Ortelius, with Cape Verde marking its prime meridian
1682 map of East Asia by Giacomo Cantelli, with Cape Verde originating its prime meridian; Japan is thus located around 180° E.

In 1541, Mercator produced his 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. 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, 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

[edit]

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

[edit]
The line of the Greenwich meridian at the Royal Observatory, Greenwich, England
Main article: Prime meridian (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 (47 yards) 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

[edit]
Main article: 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).[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 (inches); 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 (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).[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

[edit]
Map all coordinates using OpenStreetMap Download coordinates as KML

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

The prime meridian on a globe
The prime meridian sign in Parnay, Maine-et-Loire, France
Prime meridian sign near Somanya, Ghana
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) North Pole and 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 For about 432 km (268 mi), running through Cinkassé.
11°7′N 0°0′E / 11.117°N 0.000°E / 11.117; 0.000 (Togo)  Togo For about 3.4 km (2.1 mi)
11°6′N 0°0′E / 11.100°N 0.000°E / 11.100; 0.000 (Ghana)  Ghana For about 16 km (10 mi)
10°58′N 0°0′E / 10.967°N 0.000°E / 10.967; 0.000 (Togo)  Togo For about 39 km (24 mi)
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) International waters
0°0′N 0°0′E / 0.000°N 0.000°E / 0.000; 0.000 (Equator) 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) EEZ of Bouvet Island (Norway)
57°13′S 0°0′E / 57.217°S 0.000°E / -57.217; 0.000 (International waters) 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, South Pole

Prime meridian on other celestial bodies

[edit]
See also: Longitude (planets)
"Prime meridian (planets)" redirects here; not to be confused with Central meridian (planets).

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] 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 perihelion, giving it more sunlight).[30][31][32]
  • Defined in 1992,[33] the prime meridian of Venus passes through the central peak in the crater Ariadne, chosen arbitrarily.[34]
  • 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.[citation needed]
  • The prime meridian of Mars was established in 1971[35] 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.[36]
  • The prime meridian on Ceres runs through the Kait crater, which was arbitrarily chosen because it is near the equator (about 2° south).[37]
  • The prime meridian on 4 Vesta is 4 degrees east of the crater Claudia, chosen because it is sharply defined.[38]
  • 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.[39] 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.[40]
    • Europa's prime meridian is defined such that the crater Cilix is at 182° W.[29] 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).[41]
    • 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.[42]
    • Callisto's prime meridian is defined such that the crater Saga is at 326° W.[43]
  • Titan is the largest moon of Saturn 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.[citation needed]
  • 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.[44]
  • 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

[edit]
Locality Modern longitude Meridian name Image Comment
Bering Strait 168°30′ W
Line across the Earth
168°
class=notpageimage|
168th meridian west (interactive map)
 Offered in 1884 as possibility for a neutral prime meridian by Pierre Janssen at the International Meridian Conference[45]
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
Line across the Earth
77°
class=notpageimage|
77th meridian west (interactive map)
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
77°02′11.56299″ W (NAD 83),[46] 77°02′11.55811″ W (NAD 83),[47] 77°02′11.58325″ W (NAD 83)[48] (three different monuments originally intended to be on the White House meridian) White House meridian
77°00′32.6″ W (NAD 83) Capitol meridian
Philadelphia 75° 10′ 12″ W
Line across the Earth
75°
class=notpageimage|
75th meridian west (interactive map)
 [49][50]
Rio de Janeiro 43° 10′ 19″ W
Line across the Earth
43°
class=notpageimage|
43rd meridian west (interactive map)
 [51]
Azores 25° 40′ 32″ W
Line across the Earth
25°
class=notpageimage|
25th meridian west (interactive map)
 Proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference[52]
El Hierro (Ferro),
Canary Islands		 18° 03′ W,
later redefined as
17° 39′ 46″ W 		Ferro meridian
Line across the Earth
18°
class=notpageimage|
18th meridian west (interactive map)
 [53]
Tenerife 16°38′22″ W Tenerife meridian
Line across the Earth
16°
class=notpageimage|
16th meridian west (interactive map)
 Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian[54]
Lisbon 9° 07′ 54.862″ W
Line across the Earth
9°
class=notpageimage|
9th meridian west (interactive map)
 [55]
Csdiz 6° 17′ 35.4" W Cádiz meridian
Line across the Earth
6°
class=notpageimage|
6th meridian west (interactive map)
 Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy.[56][57]
Madrid 3° 41′ 16.58″ W
Line across the Earth
3°
class=notpageimage|
3rd meridian west (interactive map)
 [55]
Kew 0° 00′ 19.0″ W Prime Meridian (prior to Greenwich)
Line across the Earth
0°
class=notpageimage|
Prime meridian (interactive map)
 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
Line across the Earth
2°
class=notpageimage|
2nd meridian east (interactive map)
Brussels 4° 22′ 4.71″ E
Line across the Earth
4°
class=notpageimage|
4th meridian east (interactive map)
 [55]
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[58]
Pisa 10° 24′ E
Line across the Earth
10°
class=notpageimage|
10th meridian east (interactive map)
 [49]
Oslo (Kristiania) 10° 43′ 22.5″ E [49][50]
Florence 11°15′ E Florence meridian
Line across the Earth
11°
class=notpageimage|
11th meridian east (interactive map)
 Used in the Peters projection, 180° from a meridian running through the Bering Strait
Rome 12° 27′ 08.4″ E Meridian of Monte Mario
Line across the Earth
12°
class=notpageimage|
12th meridian east (interactive map)
 Used in Roma 40 Datum[59]
Copenhagen 12° 34′ 32.25″ E Rundetårn[60]
Naples 14° 15′ E
Line across the Earth
14°
class=notpageimage|
14th meridian east (interactive map)
 [52]
Pressburg 17° 06′ 03″ E Meridianus Posoniensis
Line across the Earth
17°
class=notpageimage|
17th meridian east (interactive map)
 Used by Sámuel Mikoviny
Stockholm 18° 03′ 29.8″ E
Line across the Earth
18°
class=notpageimage|
18th meridian east (interactive map)
 At the Stockholm Observatory[55]
Buda 19° 03′ 37″ E Meridianu(s) Budense
Line across the Earth
19°
class=notpageimage|
19th meridian east (interactive map)
 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
Line across the Earth
21°
class=notpageimage|
21st meridian east (interactive map)
 [55]
Várad 21° 55′ 16″ E Tabulae Varadienses
Line across the Earth
21°
class=notpageimage|
21st meridian east (interactive map)
 [64] Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea (Varadinum at the time) by Georg von Peuerbach.[65] 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.[66]
Alexandria 29° 53′ E Meridian of Alexandria
Line across the Earth
29°
class=notpageimage|
29th meridian east (interactive map)
 The meridian of Ptolemy's Almagest.
Saint Petersburg 30° 19′ 42.09″ E Pulkovo meridian
Line across the Earth
30°
class=notpageimage|
30th meridian east (interactive map)
Great Pyramid of Giza 31° 08′ 03.69″ E
Line across the Earth
31°
class=notpageimage|
31st meridian east (interactive map)
 1884[67]
Jerusalem 35° 13′ 47.1″ E
Line across the Earth
35°
class=notpageimage|
35th meridian east (interactive map)
 [50]
Mecca 39° 49′ 34″ E
Line across the Earth
39°
class=notpageimage|
39th meridian east (interactive map)
 See also Mecca Time
Approx. 59° E
Line across the Earth
59°
class=notpageimage|
59th meridian east (interactive map)
 Maimonides[68] calls this point (24 degrees east of Jerusalem) 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
Line across the Earth
75°
class=notpageimage|
75th meridian east (interactive map)
 Used from 4th century CE Indian astronomy and calendars(see also Time in India).[69]
Beijing 116° 24′ E
Line across the Earth
116°
class=notpageimage|
116th meridian east (interactive map)
 Used in Qing dynasty for astronomical[70][71] and cartographical[72] purposes.
Kyoto 136° 14′ E
Line across the Earth
136°
class=notpageimage|
136th meridian east (interactive map)
 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
180th meridian
 Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming[52]

See also

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Notes

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References

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Works cited

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Prime meridian

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The Prime Meridian is the line of 0° , serving as the principal reference meridian in the global for measuring longitude east and west around . It divides the planet into the Eastern and Western Hemispheres and passes through the Royal Observatory in Greenwich, , , where it was historically defined by the Airy Transit Circle telescope installed in 1851. This meridian, approximately 20,000 kilometers long from pole to pole, mostly traverses oceans and crosses land in countries including the , , , , , , , , and . Historically, various nations adopted different prime meridians for navigation and mapping, such as those through , , or , leading to inconsistencies in and astronomy. The modern Prime Meridian was established at the held in , from October 1 to November 1, 1884, attended by delegates from 25 nations. The conference passed Resolution II, adopting the Greenwich meridian as the by a vote of 22 in favor, 1 against, and 2 abstentions, primarily because about two-thirds of the world's shipping already used Greenwich-based charts for calculations. This decision, proposed by U.S. President , aimed to standardize global navigation and timekeeping. The Prime Meridian plays a crucial role in establishing Coordinated Universal Time (UTC), the basis for the world's 24 standard time zones, each representing 15° of . It also defines the approximate location of the at 180° longitude on the opposite side of , where the date changes. In modern , a slightly offset International Earth Rotation and Reference Systems Service (IERS) Reference Meridian (about 102.5 meters east of the historic Greenwich line) is used for satellite-based systems like GPS to account for 's irregular shape and rotation. Today, the original Greenwich meridian is marked by a strip and illuminated laser at the Royal Observatory, symbolizing its enduring legacy in and global connectivity.

Definition and Fundamentals

Geographical and Astronomical Definition

The is defined as the meridian of ° longitude, an arbitrarily chosen reference line that serves as the zero point for measuring east-west positions, or , on and other celestial bodies. This imaginary north-south line extends from the to the , dividing the planet into the Eastern and Western Hemispheres, with longitudes measured eastward or westward up to 180°. It intersects the at the coordinate point ° and ° , commonly referred to as , an imaginary location in the off the west coast of where no land exists but a NOAA weather buoy marks the site. Astronomically, the prime meridian relates to the through Earth's daily rotation on its axis, which produces the apparent motion of stars and other celestial objects across the sky. The Earth's equatorial plane projects onto the celestial sphere as the , a serving as the reference for (analogous to ), while a location on the prime meridian aligns with the observer's local celestial meridian there—the passing through the celestial poles and the . This alignment facilitates the measurement of , the celestial equivalent of , starting from the vernal equinox, and underscores the prime meridian's role in synchronizing terrestrial observations with celestial coordinates. The prime meridian can be distinguished as either geodetic or astronomical. The geodetic meridian follows the reference ellipsoid approximating Earth's shape, providing a mathematically consistent global framework, whereas the astronomical meridian is defined by the local plumb line direction of , which varies due to local mass distributions and results in slight deflections from the geodetic path. In the modern International Terrestrial Reference Frame (ITRF), the prime meridian is geodetically defined, with the coordinate system's origin at Earth's , the Z-axis aligned through the poles, and the X-axis intersecting the at 0° to ensure precise, Earth-fixed positioning for applications like .

Significance in Navigation and Timekeeping

The prime meridian functions as the primary reference for , dividing the into the (east of the meridian, with longitudes from 0° to 180° east) and the (west of the meridian, with longitudes from 0° to 180° west). This division provides a standardized framework for measuring east-west positions relative to the 0° line, essential for global locational accuracy. In conjunction with latitude lines, the prime meridian establishes the , forming a universal grid that underpins and contemporary technologies like the (GPS). This graticule enables precise plotting of positions on maps and real-time navigation, with GPS systems relying on the , closely aligned with but offset from the historic Greenwich meridian, as the zero baseline in the World Geodetic System 1984 (WGS84) datum. The prime meridian is foundational to international timekeeping, serving as the origin for (GMT), which represents the mean at 0° as observed from the Royal Observatory in Greenwich. GMT was adopted as the global time standard at the 1884 and remained in use until 1972, when it was replaced by (UTC) to incorporate atomic timekeeping for greater precision while staying within 0.9 seconds of GMT. The Earth's 24-hour rotation corresponds to 360° of , dividing the planet into 24 zones, each ideally spanning 15° of with central meridians spaced 15° apart, starting from the prime meridian, which facilitates worldwide synchronization for , telecommunications, and daily operations. Historically, the prime meridian's significance in navigation was amplified by the 18th-century solution to the longitude problem through marine chronometers developed by John Harrison. Harrison's H4 chronometer, tested successfully in 1761–1762, allowed sailors to ascertain longitude by comparing the time at sea to GMT at the prime meridian, calculating position via the Earth's rotational difference of 15° per hour and thereby preventing shipwrecks from positional errors that previously claimed thousands of lives.

Historical Evolution

Ancient and Pre-Modern Concepts

In and , the concept of meridians emerged as great circles running north-south on the Earth's surface, serving as reference lines for locating celestial bodies and terrestrial positions. , a second-century BCE , advanced this by dividing the into a grid of parallels (latitudes) and meridians (longitudes), enabling systematic star catalogs and geographical coordinates; he is credited with introducing the 360-degree circle division, which facilitated precise measurements tied to local observatories like that in . , building on Hipparchus's framework in his second-century CE , employed a network of meridians and parallels to plot over 8,000 locations, measuring latitudes from the along meridian arcs and longitudes relative to a principal meridian through the or , emphasizing the meridian's role in projecting the onto maps. These "principal meridians" were inherently local, often aligned with prominent observatories to standardize observations for astronomy and early navigation. During the medieval , scholars refined meridian-based methods for geodetic and navigational purposes, integrating Greek ideas with empirical observations. , a Persian of the , calculated the to within 1% accuracy using trigonometric observations along a in modern-day , and he devised a method to determine differences between two locations by measuring the and known latitudes, forming a spherical with the meridian as a key side. To support this, constructed an instrument at Jurjaniyya for observing solar meridian transits, allowing precise timing of the sun's passage over the local meridian, which was crucial for in . These techniques influenced maritime practices in the , where Muslim traders applied meridian alignments and astrolabes to plot routes between ports like those in the and , enhancing positional accuracy for commerce and exploration. The Age of Exploration in the 15th and 16th centuries saw European powers adopt specific meridians for imperial navigation and territorial claims, with the Ferro Meridian—passing through in the —serving as a common prime reference for Iberian sailors due to its western position relative to . This meridian, approximately 18° west of Greenwich, was used on and Spanish portolan charts to measure longitudes during voyages to and the . In 1494, the invoked this system by establishing a north-south 370 leagues west of the Islands (effectively about 46° west of Ferro), dividing Spanish and claims to newly discovered lands east and west of the line, respectively, to resolve disputes following Columbus's voyages. The Ferro Meridian's selection reflected its practicality for Atlantic navigation, as it minimized errors in from known European ports. By the 17th and 18th centuries, national observatories formalized meridian choices to advance scientific , particularly for determining at . In France, the , founded in 1667 under , established the (about 2° east of Greenwich) as the prime line for national cartography and astronomy, with the —created in 1666—proposing its use for precise geodetic surveys, including arc measurements to refine Earth's shape. These efforts supported French naval expansion, as meridian-based observations aided in plotting routes to colonies in the and . Meanwhile, in Britain, the Royal Observatory at Greenwich, established in 1675 by royal warrant, focused on lunar observations and to solve the problem, with early 18th-century trials of John Harrison's marine timekeepers (H4 in 1761) conducted there to calibrate against the local meridian for naval use. This work enhanced British maritime supremacy, as Greenwich time signals via cannon and ball drops standardized ratings for fleet operations worldwide.

19th-Century Standardization

In the mid-19th century, the proliferation of international telegraph networks and transcontinental railways highlighted the need for a unified system of longitude and time reckoning to facilitate global commerce and navigation. Proposals emerged from major powers: Britain advocated for the Greenwich meridian, already in use by approximately two-thirds of international shipping; France pushed for the Paris meridian or a neutral line such as through the Azores; and the United States, facing chaos from over 100 local time standards on its railroads, sought a practical international standard based on its nascent adoption of Greenwich-referenced zones. These efforts culminated in the U.S. Congress authorizing an international conference in 1882, with invitations extended to 26 nations in late 1883. The International Meridian Conference convened in Washington, D.C., from October 1 to November 1, 1884, attended by 41 delegates representing 25 nations, including the , , , , and . Debates focused on selecting a single prime meridian, with vehemently opposing Greenwich due to national prestige and the historical significance of the , established in 1667—predating Greenwich's by eight years—and equally productive in astronomical publications. Some delegates proposed alternatives like a meridian through the for equidistance, but these gained little traction amid arguments emphasizing Greenwich's widespread nautical adoption by seven out of ten seafaring nations. On October 22, 1884, the conference passed key resolutions: adopting the Greenwich meridian as the global prime meridian, with measured positively eastward and negatively westward up to 180 degrees; establishing a universal day commencing at Greenwich mean midnight and reckoned from 0 to 24 hours; and recommending time zones at 15-degree intervals without mandating abandonment of local mean times. The pivotal vote on the Greenwich meridian passed 22 in favor, 1 against (the ), and 2 abstentions ( and ), reflecting broad consensus despite French reservations. To address opposition, the conference allowed a transitional period for ephemerides and maps, with full implementation targeted for 1890 in astronomical tables, though France delayed broader adoption until 1911. The resolutions prompted immediate revisions to international nautical almanacs starting in , standardizing references for maritime . In the United States, where railroads had preemptively aligned with Greenwich-based zones in 1883, the accelerated full governmental endorsement by 1884, streamlining telegraph operations and reducing scheduling errors across expanding networks. Globally, most nations incorporated the Greenwich meridian into maps and time systems by 1900, profoundly influencing synchronized railway timetables and international telegraphic communications, though holdouts like persisted in using Paris coordinates for domestic purposes until the early .

Current International System

Greenwich-Based Meridian

The Greenwich-based prime meridian passes through the Royal Observatory in Greenwich, , where it is precisely defined by the Airy Transit Circle telescope. Installed in 1850 and brought into use in 1851 by , this instrument marked the meridian through the alignment of its crosshairs with distant stars, establishing the line as the reference for zero degrees at the observatory. The Royal Observatory itself was founded in 1675 by King Charles II specifically for astronomical observations aimed at improving at sea, including the determination of . From 1767 onward, the observatory played a key role in publishing The Nautical Almanac, an annual compendium of celestial positions and calculations based on the Greenwich meridian that enabled sailors to compute their positions accurately during voyages. This meridian's adoption as the international prime meridian was formalized at the 1884 . Physically, the meridian is marked by a stainless steel line embedded in the pavement of the observatory's courtyard, allowing visitors to stand astride the divide between east and west; an extension of this line runs through the grounds for public access. Beyond the observatory, it traverses , crosses the River Thames near Greenwich Pier, and continues through rural areas of southern England before exiting the country. The site's enduring significance is preserved as part of the Maritime Greenwich , inscribed in for its architectural and historical value in maritime and scientific advancement. Visitor exhibits at the highlight the meridian's , including displays on the Airy Transit Circle and early navigation tools, underscoring its foundational role in global coordinate systems—though modern has introduced a slight offset in the for precision.

IERS Reference Meridian

The (IRM), also known as the , serves as the modern (0° ) in the International Terrestrial Reference System (ITRS), a geocentric realized through the International Terrestrial Reference Frame (ITRF). It was established in 1988 by the International Earth Rotation and Reference Systems Service (IERS) as a successor to the Bureau International de l'Heure's Terrestrial System of 1984 (BTS84), aligning with recommendations from the (IAU) to provide a stable reference for global . Defined as the meridian passing through the Earth's and oriented to minimize rotation relative to the major tectonic plates, the IRM is approximately 102 meters east of the historic Greenwich meridian at the of the Royal Observatory (51°28′38″N). This offset arose from the transition from astronomical (vertical deflection-based) to geodetic (satellite-based) coordinate definitions, accounting for local gravitational effects known as deflection of the vertical. The primary purpose of the IRM is to maintain a dynamically stable reference frame amid Earth's crustal deformations, enabling precise measurements for applications in , , and space tracking. It achieves this by incorporating data from space geodetic techniques, including (VLBI), (GPS), (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), which monitor station positions worldwide. The IERS updates the ITRF periodically—most recently with ITRF2020-u2024 in 2025—to incorporate new observations and correct for plate motions, ensuring the frame's origin remains at the geocenter and its scale is tied to extragalactic sources. These updates involve least-squares adjustments of global station networks, removing net rotation and translation to keep the IRM aligned with no-net-rotation (NNR) plate models like NNR-MORVEL16. As of 2025, the IRM follows a path closely paralleling the Greenwich meridian but shifted eastward, intersecting key global locations including the (east of the Royal Observatory in Greenwich, near 51°28′40″N 0°00′05″E in ITRF coordinates), (through ), (near ), (Sahara Desert), (near Tombouctou), , , (near ), and (Ellsworth Land). This alignment ensures the meridian divides the Earth's surface into equal halves by mass, facilitating consistent assignments in scientific datasets. Unlike the static Greenwich meridian used in historical , the IRM drifts relative to fixed landmarks at about 2.5 cm per year northeast in regions like the due to Eurasian plate , though updates mitigate this for long-term stability. It is primarily employed in scientific domains such as climate modeling, Earth orientation parameters, and Global Navigation Satellite Systems (GNSS) like GPS and Galileo, rather than consumer apps which often default to legacy Greenwich coordinates.

Applications on Other Celestial Bodies

Selection Criteria and Examples

The selection of prime meridians on non-Earth celestial bodies prioritizes scientific utility, ensuring a stable, observable reference for mapping, navigation, and data analysis across missions. Typically, the 0° is anchored to a prominent surface feature, such as a or basin edge, that is easily identifiable in imagery and resistant to geological change. These choices are formalized through (IAU) resolutions, which standardize coordinate systems to prevent inconsistencies; key early examples include the 1970 IAU General Assembly resolution establishing general principles for planetary cartographic coordinates, with body-specific adoptions like the in 1970 and Mars in 1973. For the Moon, the prime meridian follows the mean sub-Earth point on the lunar —the average position facing —allowing longitudes to increase eastward toward the near-side limb in the vicinity of , a distinct basaltic basin. This system was ratified by the IAU in 1970, incorporating photographic data from Apollo missions (e.g., and 12 in 1969) to refine the global control network and enable precise selenographic positioning. The coordinates remain essential for contemporary exploration, supporting NASA's in site selection, trajectory planning, and resource mapping for future landings. On Mars, the prime meridian was defined in 1973 by an IAU resolution to pass through the center of Airy-0, a small (approximately 500 m diameter) impact crater selected for its sharp visibility and central location near the equator. This replaced prior references like Sinus Meridiani, using Mariner 9 orbiter images for initial positioning and later validated with Viking mission data in the mid-1970s. The system has proven critical for rover operations, guiding paths for missions like NASA's Perseverance rover (2021 landing at 77.45° E, 18.44° N) to target geological features and avoid hazards. For Mercury, the prime meridian was established in 1975 such that the small Hun Kal (about 1.5 km ) lies at 20° W , chosen for its prominence and resolvability in low-resolution images. Based on flyby data from 1974–1975, this reference provides a fixed point for the planet's 3:2 spin-orbit , facilitating consistent mapping of thermal extremes and volcanic plains. The definition was refined in 2009 using higher-resolution orbiter imagery, enhancing accuracy for global geological studies without altering the core feature.

Implications for Planetary Mapping

The establishment of prime meridians on celestial bodies beyond facilitates the creation of standardized coordinate systems essential for interplanetary scientific endeavors. These systems enable precise global referencing for observations from telescopes, navigation of probes and rovers, and integration of datasets across international missions, promoting efficient and collaborative analysis. For instance, the (IAU) defines prime meridians relative to the J2000 inertial reference frame, ensuring consistency in latitude and longitude for all mapped solar system objects, which supports unified cartographic products and enhances mission planning accuracy. A key example is the Martian prime meridian, defined by the center of the small Airy-0, which has been instrumental in standardized mapping efforts. The mission utilized this reference to produce global digital models with resolutions up to 463 meters, enabling the creation of topographic s at scales suitable for planetary , such as identifying geological features and potential landing sites. This standardization allowed seamless integration of laser altimeter data with imaging from the Mars Orbiter Camera, achieving meter-level accuracy in coordinate tying for subsequent missions. However, applying prime meridians to irregular bodies like asteroids presents unique challenges, as their non-spherical shapes and variable rotation complicate traditional definitions. The IAU recommends adapting coordinates using principal axes of or observable surface features, such as craters or bright spots, rather than fixed rotational poles, to accommodate tumbling or asymmetric forms; this impacts resource identification and planning for close-approach missions. For tidally locked satellites like Europa, the prime meridian is aligned with the sub-Jovian point, influencing landing site selection in missions such as , where precise longitude referencing aids in targeting subsurface ocean access points amid Jupiter's gravitational influence. Looking ahead, prime meridians play a growing role in human exploration and studies, where they support detailed mapping for resource utilization, such as the Lunar Reconnaissance Orbiter's illumination models of the using the Earth-directed prime meridian to locate deposits in permanently shadowed craters. The IAU's for uniform systems across solar system bodies further enables comparative planetology, allowing researchers to analyze rotational dynamics, surface evolution, and geological processes in a consistent framework. In research, analogous references enhance interpretations of transit timing variations in multi-planet systems, aiding models of orbital resonances and atmospheric phase shifts.

Historic and Alternative Meridians on Earth

National and Regional Meridians

Several nations and regions established their own prime meridians prior to the widespread adoption of the Greenwich standard following the 1884 , often for navigational, cartographic, and political purposes tied to national observatories or capitals. These local meridians reflected geopolitical priorities, enabling independent mapping and timekeeping systems while fostering scientific advancements in astronomy and . Although many were eventually supplanted by the international system, their legacies influenced regional surveys and standards. The Paris Meridian, defined through the Paris Observatory established in 1667, served as France's primary reference line for over two centuries. This meridian, located approximately 2° 20' east of the Greenwich meridian, was used extensively in French cartography until 1911, when France transitioned to Greenwich for timekeeping and navigation purposes. Its establishment supported the Cassini family's comprehensive mapping of France during the 18th and 19th centuries, including Napoleonic military surveys that extended across Europe. The meridian also played a pivotal role in the development of the metric system, as the 1792–1798 arc measurement expedition by Jean-Baptiste Delambre and Pierre Méchain along this line from Dunkirk to Barcelona provided the basis for defining the meter as one ten-millionth of the Earth's quadrant distance. In the United States, the Washington Meridian, passing through the U.S. Naval Observatory in , was proposed as a national standard to assert independence from European references. President advocated for an American meridian during the planning of the in the late , viewing it as essential for domestic . Congress officially adopted it on September 28, 1850, for astronomical and geodetic purposes, including U.S. coastal and interior surveys conducted by the Coast and Geodetic Survey. This meridian influenced mapping efforts until the early 20th century, with full legal adoption of the Greenwich meridian occurring in 1912 to align with international standards. The Ferro Meridian, running through El Hierro (also known as Ferro) in the , was an early reference adopted by Spanish and Portuguese navigators during the Age of Exploration. Revived from Ptolemy's ancient system, it positioned El Hierro at 0° and was approximately 17° 40' west of Greenwich, facilitating transatlantic route calculations. This meridian gained formal recognition in the 1493 papal bull by , which used a line 100 leagues west of the (near Ferro) to divide territories between and , influencing Iberian colonial mapping for centuries. Other regional meridians included the Bogotá Meridian in , utilized by the 19th-century Chorographic Commission for topographic mapping of the newly independent republic. This line through as the prime meridian supported detailed surveys of South American terrain, emphasizing national sovereignty in post-colonial cartography. The Ancient Observatory, established in 1442, served as the imperial center for astronomical observations during the (1644–1912), guiding calendrical computations and celestial tracking aligned with the local meridian. Additional examples include the Meridian, used in Italian cartography through the 20th century, and the Rio de Janeiro Meridian, adopted for Brazilian mapping in the .

Obsolete Proposals and Their Legacy

In the 19th century, the meridian emerged as a proposal rooted in biblical and religious significance, positioning the as a neutral global reference point for mapping. This idea gained attention during the of 1884 in , where U.S. President received a formal communication advocating as the prime meridian to symbolize universal harmony and spiritual centrality. Although it received no substantive debate and was ultimately rejected in favor of the Greenwich meridian, the proposal reflected broader efforts to select a culturally resonant origin amid competing national interests. Its legacy persists in niche religious and historical analyses of meridian selection, underscoring the tension between scientific practicality and symbolic value. The legacies of these obsolete proposals extend beyond their technical failures, shaping cultural narratives and specialized applications. For instance, the —France's 19th-century rival to Greenwich—inspired Verne's 1872 novel Meridiana: The Adventures of Three Englishmen and Three Russians in , which fictionalizes 19th-century geodetic measurements of a in to determine Earth's curvature, blending scientific adventure with national pride. Similarly, echoes of the meridian appear in contemporary local GIS systems for historical overlays and in educational recreations, preserving their role in illustrating the evolution from localized to universal geographic standards. These remnants underscore how unadopted meridians influenced , heritage mapping, and the conceptual foundations of international .

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