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Aprime meridian is an arbitrarily chosenmeridian (a line oflongitude) in ageographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the180th meridian in a360°-system) form agreat ellipse. This divides the body (e.g.Earth) intotwo hemispheres: theEastern Hemisphere and theWestern 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 currentinternational standard prime meridian is theIERS Reference Meridian. It is derived, but differs slightly, from theGreenwich Meridian, the previous standard.[2]
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Longitudes for the Earth andMoon are measured from their prime meridian (at 0°) to 180° east and west. For all otherSolar System bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body isprograde (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 isretrograde.[3]
The notion of longitude for Greeks was developed by theGreekEratosthenes (c. 276 – 195 BCE) inAlexandria, andHipparchus (c. 190 – 120 BCE) inRhodes, and applied to a large number of cities by thegeographerStrabo (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 hisGeographia.
Ptolemy used as his basis the "Fortunate Isles", a group of islands in theAtlantic, which are usually associated with theCanary Islands (13°W to 18°W), although his maps correspond more closely to theCape Verde islands (22°W to 25°W). The main point is to be comfortably west of the western tip ofAfrica (17°30′W) as negative numbers were not yet in use. His prime meridian corresponds to 18°40′ west ofWinchester (about 20°W) today.[1] At that time the chief method of determining longitude was by using the reported times oflunar eclipses in different countries.
One of the earliest known descriptions ofstandard time in India appeared in the 4th century CEastronomical treatiseSurya Siddhanta. Postulating aspherical Earth, the book described the thousands years old customs of theprime meridian, or zero longitude, as passing throughAvanti, the ancient name for the historic city ofUjjain, andRohitaka, the ancient name forRohtak (28°54′N76°38′E / 28.900°N 76.633°E /28.900; 76.633 (Rohitaka (Rohtak))), a city near theKurukshetra.[4][better source needed]
Ptolemy'sGeographia was first printed with maps atBologna 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 importantTreaty of Tordesillas of 1494, which settled the territorial dispute betweenSpain andPortugal over newly discovered lands. The Tordesillas line was eventually settled at 370leagues (2,193 kilometers, 1,362 statute miles, or 1,184 nautical miles) west ofCape Verde.[a] This is shown in the copies of Spain'sPadron Real made byDiogo Ribeiro in 1527 and 1529.São Miguel Island (25°30′W) in theAzores was still used for the same reason as late as 1594 byChristopher Saxton, although by then it had been shown that the zeromagnetic declination line did not follow a line of longitude.[8]
In 1541,Mercator produced his 41 cm terrestrial globe and drew his prime meridian precisely throughFuerteventura (14°1′W) in the Canaries. His later maps used the Azores, following the magnetic hypothesis, but by the time thatOrtelius 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 geographerDelisle 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 themarine chronometer byJohn Harrison. The development of accurate star charts, principally by the first BritishAstronomer Royal,John Flamsteed between 1680 and 1719 and disseminated by his successorEdmund Halley, enabled navigators to use thelunar method of determining longitude more accurately using theoctant developed byThomas Godfrey andJohn Hadley.[11]
In the 18th century most countries inEurope adapted their ownprime meridian, usually through their capital, hence inFrance theParis meridian was prime, inPrussia it was theBerlin meridian, inDenmark theCopenhagen meridian, and inUnited Kingdom theGreenwich meridian.
Between 1765 and 1811,Nevil Maskelyne published 49 issues of theNautical Almanac based on the meridian of theRoyal Observatory, Greenwich. "Maskelyne's tables not only made the lunar method practicable, they also made theGreenwich meridian the universal reference point. Even the French translations of theNautical Almanac retained Maskelyne's calculations from Greenwich – in spite of the fact that every other table in theConnaissance des Temps considered theParis meridian as the prime."[12]
In 1884, at theInternational Meridian Conference inWashington, 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 theBering Strait, but eventually abstained and continued to use theParis meridian until 1911.
The current international standard Prime Meridian is theIERS Reference Meridian. TheInternational Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.[14] It was adopted for air navigation by theInternational Civil Aviation Organization on 3 March 1989.[15]
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.
In October 1884 the Greenwich Meridian was selected by delegates (forty-one delegates representing twenty-five nations) to theInternational Meridian Conference held inWashington, 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 theRoyal Observatory, Greenwich, was established bySir George Airy in 1851. It was defined by the location of theAiry 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 secondAstronomer 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 theParis 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 aplumb line along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via thelunar distance method, then by chronometers carried on ships, then via telegraph lines carried bysubmarine communications cables, then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods was that of theNorth American Datum 1927 or NAD27, an ellipsoid whose surface best matchesmean sea level under theUnited States.
Beginning in 1973 theInternational Time Bureau and later theInternational Earth Rotation and Reference Systems Service changed from reliance on optical instruments like the Airy Transit Circle to techniques such aslunar laser ranging,satellite laser ranging, andvery-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 byvertical 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 be5.3″ 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 theBureau International de l'Heure (BIH) in 1984 via its BTS84 (BIH Terrestrial System) that later became WGS84 (World Geodetic System 1984) and the variousInternational Terrestrial Reference Frames (ITRFs).
Due to the movement of Earth'stectonic 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, theIERS Reference Meridian was established and is commonly used todenote the Earth's prime meridian (0° longitude) by theInternational 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'stransit 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 theGlobal Positioning System operated by theUnited States Department of Defense, and ofWGS84 and its two formal versions, the idealInternational Terrestrial Reference System (ITRS) and its realization, theInternational 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 theInternational Date Line.
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On Earth, starting at theNorth Pole and heading south to theSouth Pole, the IERS Reference Meridian (as of 2016) passes through 8 countries, 4 seas, 3 oceans and 1 channel:
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 bymagnetic fields. The prime meridians of the following planetographic systems have been defined:
| 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 byPierre 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 | 77° 77th meridian west | |
| 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 | 75° 75th meridian west | [49][50] | |
| Rio de Janeiro | 43° 10′ 19″ W | 43° 43rd meridian west | [51] | |
| Fortunate Isles / Azores | 25° 40′ 32″ W | 25° 25th meridian west | Used until the Middle Ages, proposed as one possible neutral meridian byPierre Janssen at the International Meridian Conference[52] | |
| El Hierro (Ferro), Canary Islands | 18° 03′ W, later redefined as 17° 39′ 46″ W | Ferro meridian | 18° 18th meridian west | [53] |
| Tenerife | 16°38′22″ W | Tenerife meridian | 16° 16th meridian west | Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian[54] |
| Lisbon | 9° 07′ 54.862″ W | 9° 9th meridian west | [55] | |
| Cadiz | 6° 17′ 35.4" W | Cadiz meridian | 6° 6th meridian west | Royal Observatory in southeast tower ofCastillo de la Villa, used 1735–1850 by Spanish Navy.[56][57] |
| Madrid | 3° 41′ 16.58″ W | 3° 3rd meridian west | [55] | |
| Kew | 0° 00′ 19.0″ W | Prime Meridian (prior to Greenwich) | 0° Prime meridian | Located at King George III'sKew 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 | 2° 2nd meridian east | |
| Brussels | 4° 22′ 4.71″ E | 4° 4th meridian east | [55] | |
| Antwerp | 4° 24′ E | Antwerp meridian | ||
| Amsterdam | 4° 53′ E | Through theWesterkerk in Amsterdam; used to define the legal time in the Netherlands from 1909 to 1937[58] | ||
| Pisa | 10° 24′ E | 10° 10th meridian east | [49] | |
| Oslo (Kristiania) | 10° 43′ 22.5″ E | [49][50] | ||
| Florence | 11°15′ E | Florence meridian | 11° 11th meridian east | Used in thePeters projection, 180° from a meridian running through theBering Strait |
| Rome | 12° 27′ 08.4″ E | Meridian ofMonte Mario | 12° 12th meridian east | Used in Roma 40 Datum[59] |
| Copenhagen | 12° 34′ 32.25″ E | Rundetårn[60] | ||
| Naples | 14° 15′ E | 14° 14th meridian east | [52] | |
| Pressburg | 17° 06′ 03″ E | Meridianus Posoniensis | 17° 17th meridian east | Used bySámuel Mikoviny |
| Stockholm | 18° 03′ 29.8″ E | 18° 18th meridian east | At theStockholm Observatory[55] | |
| Buda | 19° 03′ 37″ E | Meridianu(s) Budense | 19° 19th meridian east | Used between 1469 and 1495; introduced byRegiomontanus, used byMarcin 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 theŚniadecki' College; mentioned also inNicolaus Copernicus's workOn the Revolutions of the Heavenly Spheres. | |
| Warsaw | 21° 00′ 42″ E | Warsaw meridian | 21° 21st meridian east | [55] |
| Várad | 21° 55′ 16″ E | Tabulae Varadienses | 21° 21st meridian east | [64] Between 1464 and 1667, aprime meridian was set in the Fortress of Oradea (Varadinum at the time) byGeorg von Peuerbach.[65] In his logbookColumbus stated, he had one copy of Tabulae Varadienses (Tabula Varadiensis orTabulae 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 | 29° 29th meridian east | The meridian ofPtolemy'sAlmagest. |
| 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[67] | |
| Jerusalem | 35° 13′ 47.1″ E | 35° 35th meridian east | [50] | |
| Mecca | 39° 49′ 34″ E | 39° 39th meridian east | See alsoMecca Time | |
| Approx. 59° E | 59° 59th meridian east | Maimonides[68] calls this point (24 degrees east ofJerusalem) 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 alsoTime in India).[69] | |
| Beijing | 116° 24′ E | 116° 116th meridian east | Used inQing Dynasty for astronomical[70][71] and cartographical[72] purposes. | |
| 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 |  | Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference bySandford Fleming[52] |
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 (112 yards) 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.
[W]est 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 conform to this definition. Their rotations are direct and longitudes run both east and west 180°, or positive to the east 360°. [footnotes omitted]
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