Prime meridian#History

{{See also|History of longitude}}

File:Ptolemy-World Vat Urb 82.jpg'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.{{nbsp}}276{{snd}}195{{nbsp}}BCE) in Alexandria, and Hipparchus (c.{{nbsp}}190{{snd}}120{{nbsp}}BCE) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63{{nbsp}}BCE{{snd}}c.{{nbsp}}24{{nbsp}}CE). Ptolemy (c.{{nbsp}}90{{snd}}168{{nbsp}}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.{{sfn|Norgate|Norgate|2006}} 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 ({{Coord|28|54|N|76|38|E|type:city|name=Rohitaka (Rohtak)}}), a city near the Kurukshetra.{{cite journal|last=Schmidt|first=Olaf H.|year=1944|title=The Computation of the Length of Daylight in Hindu Astronomy|url=https://www.jstor.org/stable/330729|url-status=live|archive-url=https://web.archive.org/web/20220126062053/https://www.jstor.org/stable/330729|archive-date=26 January 2022|access-date=|journal=Isis|volume=35|issue=3|pages=205–211|publisher=The University of Chicago Press|doi=10.1086/358709|jstor=330729|s2cid=145178197|url-access=subscription}}{{Better source needed|reason=Cited work discusses latitude not longitude.|date=July 2021}}

File:Propaganda Map.jpg'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. 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.{{efn| These figures use the legua náutica (nautical league) of four Roman miles totalling {{convert|5.926|km|abbr=on}}, which was used by Spain during the 15th, 16th, and 17th centuries for navigation.{{cite journal |first=Roland |last=Chardon |title=The linear league in North America |journal=Annals of the Association of American Geographers |volume=70 |issue=2 |year=1980 |pages=129–153 [pp. 142, 144, 151] |jstor=2562946 |doi=10.1111/j.1467-8306.1980.tb01304.x}} 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.{{cite book |first=Henry |last=Harrisse |url=https://books.google.com/books?id=7I4cAAAAMAAJ&pg=PA85 |title=The Diplomatic History of America: Its first chapter 1452—1493—1494 |location= London |publisher=Stevens |date=1897|pages=85–97, 176–190 |isbn=9780697000071 |oclc=1101220811}} Modern scholars agree that the geographic stade was the Roman or Italian stade, not any of several other Greek stades, supporting these figures.{{cite journal |first=Donald |last=Engels |title=The length of Eratosthenes' stade |journal=American Journal of Philology |volume=106 |issue=3 |year=1985 |pages=298–311 |jstor=295030 |doi=10.2307/295030}} Harrise is in the minority when he uses the stade of {{convert|192.27|m|abbr=on}} marked within the stadium at Olympia, Greece, resulting in a league (32 stades) of {{convert|6.153|km|abbr=on}}, 3.8% larger.}} 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.{{sfn|Hooker|2006}}

File:Atlas Ortelius KB PPN369376781-006av-006br.jpg, with Cape Verde as its prime meridian.]]

File:CEM-36-Regno-della-China-2355.jpg, with Cape Verde as 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.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 {{citation|title=The voyage of Don Felipe Gonzalez to Easter Island in 1770-1 |editor=Bolton Glanville Corney |date=1908 |page=3 |publisher=Hakluyt Society |url=https://archive.org/stream/voyagecaptaindo00unkngoog#page/n88/mode/2up |access-date=13 January 2013}} 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.Speech by Pierre Janssen, director of the Paris observatory, at the first session of the [https://www.gutenberg.org/files/17759/17759-h/17759-h.htm#Page_73 Meridian Conference.] {{Webarchive|url=https://web.archive.org/web/20211218082731/https://www.gutenberg.org/files/17759/17759-h/17759-h.htm#Page_73 |date=18 December 2021 }}

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.{{sfn|Sobel|Andrewes|1998|pp=110–115}}

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."{{sfn|Sobel|Andrewes|1998|pp=197–199}}

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.{{cite web |url=https://www.rmg.co.uk/stories/topics/what-prime-meridian-why-it-greenwich |title=What is the Prime Meridian - and why is it in Greenwich? {{!}} Who decided that the Prime Meridian should be in Greenwich? |publisher=Royal Museums Greenwich |date=n.d. |access-date=28 December 2021 |archive-date=5 January 2022 |archive-url=https://web.archive.org/web/20220105211313/https://www.rmg.co.uk/stories/topics/what-prime-meridian-why-it-greenwich |url-status=live }} 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.{{cite web |url= http://www.iho.shom.fr/publicat/free/files/S-51_Ed4-EN.pdf |title= A manual on the technical aspects of the United Nations Convention on the Law of the Sea – 1982 |access-date= 23 July 2008 |archive-date= 10 September 2008 |archive-url= https://web.archive.org/web/20080910223739/http://www.iho.shom.fr/publicat/free/files/S-51_Ed4-EN.pdf |url-status= live }} {{small|(4.89 MB)}} Section 2.4.4. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[http://www.icao.int/pbn/docs/eurocontrolwgsman24.pdf WGS 84 Implementation Manual] {{Webarchive|url=https://web.archive.org/web/20081003102629/http://www.icao.int/pbn/docs/eurocontrolwgsman24.pdf |date=3 October 2008 }} page i, 1998

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.

File:Prime meridian.jpg at the Royal Observatory, Greenwich, England]]

{{Main|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.{{cite book|title=International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Protocols of the proceedings|url=https://www.gutenberg.org/files/17759/17759-h/17759-h.htm|publisher=Project Gutenberg|access-date=30 November 2012|date=1884|archive-date=18 December 2021|archive-url=https://web.archive.org/web/20211218082731/https://www.gutenberg.org/files/17759/17759-h/17759-h.htm|url-status=live}}{{efn|Voting took place on 13 October and the resolutions were adopted on 22 October 1884.{{harvnb|Howse|1997|pp=12, 137}} The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian.}}

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.{{cite book |title=Greenwich Observatory ... the story of Britain's oldest scientific institution, the Royal Observatory at Greenwich and Herstmonceux, 1675–1975 |volume=1 |first=Eric Gray |last=Forbes |page=10 |publisher=Taylor & Francis |date=1975 |isbn=9780850660937}} 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.{{sfn|Dolan|2013a}} 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.{{cite book|

title=TIME from Earth Rotation to Atomic Physics

|last1=McCarthy

|first1=Dennis

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|date=2009}}{{cite web|url= http://www.rmg.co.uk/explore/astronomy-and-time/astronomy-facts/history/the-prime-meridian-at-greenwich|title= The Prime Meridian at Greenwich|author= ROG Learning Team|date= 23 August 2002|work= Royal Museums Greenwich|access-date= 14 June 2012|archive-date= 7 November 2015|archive-url= https://web.archive.org/web/20151107023957/http://www.rmg.co.uk/explore/astronomy-and-time/astronomy-facts/history/the-prime-meridian-at-greenwich|url-status= live}}

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.

{{Main|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{{pprime}} east of the astronomic Greenwich prime meridian through the Airy Transit Circle. At the latitude of Greenwich, this amounts to 102 metres (112 yards).{{cite journal| title = Why the Greenwich meridian moved | first1 = Stephen | last1 = Malys | first2 = John H. | last2 = Seago | first3 = Nikolaos K. | last3 = Palvis | first4 = P. Kenneth | last4 = Seidelmann | first5 = George H. | last5 = Kaplan | journal = Journal of Geodesy | volume = 89 | issue = 12 | pages = 1263 | date = 1 August 2015 | doi = 10.1007/s00190-015-0844-y|bibcode = 2015JGeod..89.1263M | doi-access = free }} 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.{{sfn|Dolan|2013b}}

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).{{Cite web|url=http://gpsinformation.net/main/greenwich.htm|title=Greenwich Meridan, Tracing its History|website=gpsinformation.net|access-date=29 November 2006|archive-date=19 December 2017|archive-url=https://web.archive.org/web/20171219193541/http://www.gpsinformation.net/main/greenwich.htm|url-status=live}}[http://www.gearthblog.com/images/images2006/primemeridian.jpg IRM on grounds of Royal Observatory from Google Earth] {{Webarchive|url=https://web.archive.org/web/20161014021413/http://www.gearthblog.com/images/images2006/primemeridian.jpg |date=14 October 2016 }} Accessed 30 March 2012{{efn|The astronomic latitude of the Royal Observatory is 51°28{{prime}}38{{pprime}}N whereas its latitude on the European Terrestrial Reference Frame (1989) datum is 51°28{{prime}}40.1247{{pprime}}N.}} A current convention on the Earth uses the line of longitude 180° opposite the IRM as the basis for the International Date Line.

{{geoGroup}}

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:

File:Meridian-International.svg

File:Panneau du méridien de Greenwich à Parnay .jpg, France.]]

File:Prime Meridian Line, Longitude 0.jpg, Ghana.]]

{{See also|Longitude (planets)}}

{{Redirect-distinguish|Prime meridian (planets)|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.{{Cite web|url=http://www.encyclopedia.com/doc/1O80-Carringtonheligrphccrdnts.html|title=Carrington heliographic coordinates|access-date=27 July 2009|archive-date=28 June 2011|archive-url=https://web.archive.org/web/20110628232407/http://www.encyclopedia.com/doc/1O80-Carringtonheligrphccrdnts.html|url-status=live}} The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory in Lancashire, England.
• In 1975 the prime meridian of Mercury was definedMerton E. Davies, "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, 10 June 1975Merton E. Davies, S. E. Dwornik, D. E. Gault, and R. G. Strom, NASA Atlas of Mercury, NASA Scientific and Technical Information Office, 1978. to be 20° east of the crater Hun Kal.{{Cite journal

| doi = 10.1007/s10569-010-9320-4

| title = Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009

| url = http://astropedia.astrogeology.usgs.gov/alfresco/d/d/workspace/SpacesStore/28fd9e81-1964-44d6-a58b-fbbf61e64e15/WGCCRE2009reprint.pdf

| journal = Celestial Mechanics and Dynamical Astronomy

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| last10 = Krasinsky

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| last11 = Neumann

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| last12 = Oberst

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| last14 = Stooke

| first14 = Philip J.

| last15 = Tholen

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| last16 = Thomas

| first16 = Paul C.

| last17 = Williams

| first17 = Iwan P.

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}} 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).Davies, M. E., "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, June 10, 1975.{{cite journal |last1=Archinal |first1=Brent A. |display-authors=4 |last2=A'Hearn |first2=Michael F. |last3=Bowell |first3=Edward L. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Courtin |first6=Régis |last7=Fukushima |first7=Toshio |last8=Hestroffer |first8=Daniel |last9=Hilton |first9=James L. |last10=Krasinsky |first10=George A. |last11=Neumann |first11=Gregory A. |last12=Oberst |first12=Jürgen |last13=Seidelmann |first13=P. Kenneth |last14=Stooke |first14=Philip J. |last15=Tholen |first15=David J. |last16=Thomas |first16=Peter C. |last17=Williams |first17=Iwan P. |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 |journal=Celestial Mechanics and Dynamical Astronomy |volume=109 |issue=2 |date=2010 |pages=101–135 |issn=0923-2958 |doi=10.1007/s10569-010-9320-4 |bibcode=2011CeMDA.109..101A|s2cid=189842666 }}{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=October 22, 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=October 24, 2011 |url-status=dead}}

• Defined in 1992,Merton E. Davies; Colvin, T. R.; Rogers, P. G.; Chodas, P. G.; Sjogren, W. L.; Akim, W. L.; Stepanyantz, E. L.; Vlasova, Z. P.; and Zakharov, A. I.; "The Rotation Period, Direction of the North Pole, and Geodetic Control Network of Venus", Journal of Geophysical Research, vol. 97, no. 8, 1992, pp. 1–14, 151 the prime meridian of Venus passes through the central peak in the crater Ariadne, chosen arbitrarily.{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=22 October 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=24 October 2011 |url-status=dead }}
• 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.{{cn|date=March 2025}}
• The prime meridian of Mars was established in 1971Merton E. Davies, and Berg, R. A.; "Preliminary Control Net of Mars", Journal of Geophysical Research, vol. 76, no. 2, 10 January 1971, pp. 373–393 and passes through the center of the crater Airy-0, although it is fixed by the longitude of the Viking 1 lander, which is defined to be 47.95137°W.{{ Citation | last1 = Archinal | first1 = Brent A. | last2 = Acton | first2 = C. H. | last3 = A'Hearn | first3 = Michael F. | author3-link = Michael A'Hearn | last4 = Conrad | first4 = Albert R. |display-authors=etal| date = 2018 | title = Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015 | journal = Celestial Mechanics and Dynamical Astronomy | volume = 130 | issue = 22 | pages = 22 | doi = 10.1007/s10569-017-9805-5 | bibcode = 2018CeMDA.130...22A | s2cid = 189844155 }}
• The prime meridian on Ceres runs through the Kait crater, which was arbitrarily chosen because it is near the equator (about 2° south).{{cite web|title= New Maps of Ceres Reveal Topography Surrounding Mysterious 'Bright Spots'|author= Marc Reyman|publisher=NASA|date=30 October 2015|url=https://www.jpl.nasa.gov/blog/2015/10/new-maps-of-ceres-reveal-topography-surrounding-mysterious-bright-spots|accessdate=13 September 2022}}
• The prime meridian on 4 Vesta is 4 degrees east of the crater Claudia, chosen because it is sharply defined.{{cite web|url=https://astrogeology.usgs.gov/search/details/Docs/WGCCRE/IAU-WGCCRE-Coordinate-System-for-Vesta/pdf |title=IAU WGCCRE Coordinate System for Vesta {{pipe}} USGS Astrogeology Science Center |publisher=Astrogeology.usgs.gov |date=2013-11-15 |access-date=2014-06-25}}
• 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.{{cite web |url=https://reference.wolfram.com/legacy/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html |title=Planetographic Coordinates |access-date=24 May 2017 |archive-date=15 April 2012 |archive-url=https://web.archive.org/web/20120415120251/http://reference.wolfram.com/legacy/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html |url-status=live }} 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.Merton E. Davies, Thomas A. Hauge, et al.: Control Networks for the Galilean Satellites: November 1979 R-2532-JPL/NASA
• Europa's prime meridian is defined such that the crater Cilix is at 182° W. The 0° longitude runs through the middle of the face that is always turned towards Jupiter.
• Io's prime meridian, like that of Earth's moon, is defined so that it runs through the middle of the face that is always turned towards Jupiter (the near side, known as the subjovian hemisphere).{{cite journal |title=Io after Galileo |journal=Reports on Progress in Physics |last1=Lopes |first1=R. M. C. |author1-link=Rosaly Lopes |first2=D. A. |last2=Williams |pages=303–340 |volume=68 |issue=2 |date=2005 |doi=10.1088/0034-4885/68/2/R02 |bibcode=2005RPPh...68..303L|s2cid=44208045 }}
• 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.{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table2.html |title=USGS Astrogeology: Rotation and pole position for planetary satellites (IAU WGCCRE) |access-date=August 28, 2017 |archive-url=https://web.archive.org/web/20111024101848/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table2.html |archive-date=October 24, 2011 |url-status=dead }}
• Callisto's prime meridian is defined such that the crater Saga is at 326° W.Satellites of Jupiter. (1982:912). United States: University of Arizona Press.
• Titan is the largest moon of 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.{{cn|date=March 2025}}
• 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.Merton E. Davies, P. G. Rogers, and T. R. Colvin, "A Control Network of Triton," Journal of Geophysical Research, Vo l. 96, E l, pp. 15,675-15,681, 1991.
• 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.

• {{Annotated link |1st meridian east}}
• {{Annotated link |1st meridian west}}
• {{Annotated link |180th meridian}}
• {{Annotated link |Null Island}}
• {{Annotated link |Geographical center of Earth}}

{{notelist}}

{{Reflist|30em}}

{{refbegin|30em|indent=yes}}

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}}

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{{refend}}

{{commons category|Prime meridian}}

• [https://books.google.com/books?id=qOIDAAAAMBAJ&pg=PA927 "Where the Earth's surface begins—and ends"], Popular Mechanics, December 1930
• [http://www.ucolick.org/~sla/leapsecs/scans-meridian.html scanned TIFFs of the conference proceedings]
• [https://web.archive.org/web/20061029234822/http://wwp.greenwichmeantime.com/info/prime-meridian.htm Prime meridians in use in the 1880s, by country]

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Category:Geodesy

Category:Meridians (geography)

Category:Cardinal Richelieu