Editing Prime meridian (section)

==Prime meridian on other celestial bodies{{anchor|Planets}}==
{{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 [[Solar coordinate systems#Heliographic|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]].<ref>{{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}}</ref> The second is the [[Stonyhurst Observatory#Stonyhurst heliographic coordinates|Stonyhurst heliographic coordinates]] system, originated at [[Stonyhurst Observatory]] in [[Lancashire]], England.
* In 1975 the prime meridian of [[Mercury (planet)|Mercury]] was defined<ref>[[Merton E. Davies]], "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, 10 June 1975</ref><ref>[[Merton E. Davies]], S. E. Dwornik, D. E. Gault, and R. G. Strom, NASA Atlas of Mercury, NASA Scientific and Technical Information Office, 1978.</ref> to be 20° east of the crater [[Hun Kal (crater)|Hun Kal]].<ref name="IAU09">{{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 <!--
| url = http://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf -->
| journal = Celestial Mechanics and Dynamical Astronomy
| volume = 109
| issue = 2
| pages = 101–135
| year = 2010
| last1 = Archinal
| first1 = Brent A.
| last2 = A'Hearn
| first2 = Michael F.
| last3 = Bowell
| first3 = Edward G.
| last4 = Conrad
| first4 = Albert R.
| last5 = Consolmagno
| first5 = Guy J.
| display-authors = 5
| 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 = Paul C.
| last17 = Williams
| first17 = Iwan P.
| bibcode = 2011CeMDA.109..101A
| s2cid = 189842666
| access-date = 26 September 2018
| archive-url = https://web.archive.org/web/20160304065344/http://astropedia.astrogeology.usgs.gov/alfresco/d/d/workspace/SpacesStore/28fd9e81-1964-44d6-a58b-fbbf61e64e15/WGCCRE2009reprint.pdf
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}}</ref> 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 [[Apparent retrograde motion|retrogrades]] at noon at this point during [[perihelion]], giving it more sunlight).<ref>Davies, M. E., "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, June 10, 1975.</ref><ref name="ArchinalA’Hearn2010">{{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 }}</ref><ref name="usgs">{{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}}</ref>
* Defined in 1992,<ref>[[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</ref> the prime meridian of [[Venus]] passes through the central peak in [[Ariadne (crater)|the crater Ariadne]], chosen arbitrarily.<ref>{{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 }}</ref>
* 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 (crater)|Bruce]].{{cn|date=March 2025}}
* The prime meridian of [[Mars]] was established in 1971<ref>[[Merton 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</ref> and passes through the center of the crater [[Airy-0]], although it is fixed by the longitude of the [[Viking 1#Lander|''Viking 1'' lander]], which is defined to be 47.95137°W.<ref>{{ 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 }}</ref>
*The prime meridian on [[Ceres (dwarf planet)|Ceres]] runs through the Kait crater, which was arbitrarily chosen because it is near the equator (about 2° south).<ref>{{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}}</ref>
*The prime meridian on [[4 Vesta]] is 4 degrees east of the crater [[Claudia (crater)|Claudia]], chosen because it is sharply defined.<ref name=IAUcoordinates>{{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}}</ref>
*[[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.<ref>{{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 }}</ref> 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.<ref>[[Merton E. Davies]], Thomas A. Hauge, et al.: Control Networks for the Galilean Satellites: November 1979 R-2532-JPL/NASA</ref>
**[[Europa (moon)|Europa]]'s prime meridian is defined such that [[Cilix (crater)|the crater Cilix]] is at 182° W.<ref name="IAU09"/> The 0° longitude runs through the middle of the face that is always turned towards Jupiter.
**[[Io (moon)|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).<ref name="Lopes2005">{{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 }}</ref>
**[[Ganymede (moon)|Ganymede]]'s prime meridian is defined such that [[Anat (crater)|the crater Anat]] is at 128° W, and the 0° longitude runs through the middle of the subjovian hemisphere.<ref name="iau.table2">{{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 }}</ref>
**[[Callisto (moon)|Callisto]]'s prime meridian is defined such that [[Saga (crater)|the crater Saga]] is at 326° W.<ref>''Satellites of Jupiter''.&nbsp;(1982:912).&nbsp;United States:&nbsp;University of Arizona Press.</ref>
*[[Titan (moon)|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 (moon)|Triton]], was established in 1991.<ref>[[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.</ref>
*[[Pluto]]'s prime meridian is defined as the meridian passing through the center of the face that is always towards [[Charon (moon)|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.