Prime meridian

A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides a spheroid, e.g., Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.

Atlas Cosmographicae (Mercator) 033
Gerardus Mercator in his Atlas Cosmographicae (1595) uses a prime meridian somewhere close to 25°W, passing just to the west of Santa Maria Island in the Atlantic. His 180th meridian runs along the Strait of Anián (Bering Strait)

A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation—and various conventions have been used or advocated in different regions and throughout history.[1] The most widely used modern meridian is the IERS Reference Meridian. It is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884.

Line across the Earth
Prime Meridian


Ptolemy-World Vat Urb 82
Ptolemy's 1st projection, redrawn under Maximus Planudes around 1300, using a prime meridian east of Africa

The notion of longitude was developed by the Greek Eratosthenes (c. 276 BC – c. 195 BC) in Alexandria, and Hipparchus (c. 190 BC – c. 120 BC) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63 BC – c. 24 AD). But it was Ptolemy (c. AD 90 – c. AD 168) who first used a consistent meridian for a world map in his Geographia.

Ptolemy used as his basis the "Fortunate Isles", a group of islands in the Atlantic which are usually associated with the Canary Islands (13° to 18°W), although his maps correspond more closely to the Cape Verde islands (22° to 25° W). The main point is to be comfortably west of the western tip of Africa (17.5° W) as negative numbers were not yet in use. His prime meridian corresponds to 18° 40' west of Winchester (about 20°W) today.[2] At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.

Map Diego Ribero 1529
Facsimile of Diego Ribeiro's map of 1529; the original is in 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 west of Cape Verde. This is shown in Diogo Ribeiro's 1529 map. São Miguel Island (25.5°W) in the Azores was still used for the same reason as late as 1594 by Christopher Saxton, although by then it had been shown that the zero magnetic deviation line did not follow a line of longitude.[3]

Atlas Ortelius KB PPN369376781-006av-006br
1571 Africa map by Abraham Ortelius, with Cape Verde as its prime meridian.

In 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuerteventura (14°1'W) in the Canaries. His later maps used the Azores, following the magnetic hypothesis. But by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as is usual today. This practice was followed by navigators well into the 18th century.[4] In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 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.[5]

In the early 18th century the battle was on to improve the determination of longitude at sea, leading to the development of the marine chronometer by John Harrison. But it was the development of accurate star charts, principally by the first British Astronomer Royal, John Flamsteed between 1680 and 1719 and disseminated by his successor Edmund Halley, that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley.[6] 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." [7]

In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich[8] meridian as the prime meridian of the world. 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.

List of prime meridians on Earth

Locality Modern longitude Meridian name Comment
Bering Strait 168°30′ W Offered in 1884 as possibility for a "neutral prime meridian" by Pierre Janssen at the International Meridian Conference [9]
Washington, D.C. 77°03′56.07″ W (1897) or 77°04′02.24″ W (NAD 27) or 77°04′01.16″ W (NAD 83) New Naval Observatory meridian
Washington, D.C. 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
Washington, D.C. 77°02′11.56299″ W (NAD 83),[10] 77°02′11.55811″ W (NAD 83),[11] 77°02′11.58325″ W (NAD 83)[12] (three different monuments originally intended to be on the White House meridian) White House meridian
Washington, D.C. 77°00′32.6″ W (NAD 83) Capitol meridian
Philadelphia 75° 10′ 12″ W [13][14]
Rio de Janeiro 43° 10′ 19″ W [15]
Fortunate Isles / Azores 25° 40′ 32″ W Used until the Middle Ages, proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference[16]
El Hierro (Ferro),
Canary Islands
18° 03′ W,
later redefined as
17° 39′ 46″ W
Ferro meridian [17]
Tenerife 16° 38' 22" W Tenerife meridian Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian[18]
Cadiz 6° 17' 35.4" W Cadiz meridian Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy.[19][20]
Lisbon 9° 07′ 54.862″ W [21]
Madrid 3° 41′ 16.58″ W [21]
Greenwich 0° 00′ 05.3101″ W Greenwich meridian Airy Meridian[22]
Greenwich 0° 00′ 05.33″ W United Kingdom Ordnance Survey Zero Meridian Bradley Meridian[22]
Greenwich 0° 00′ 00.00″ IERS Reference Meridian
Paris 2° 20′ 14.025″ E Paris meridian
Brussels 4° 22′ 4.71″ E [21]
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[23]
Bern 7° 26′ 22.5″ E
Pisa 10° 24′ E [13]
Oslo (Kristiania) 10° 43′ 22.5″ E [13][14]
Florence 11°15′ E Florence meridian used in the Peters projection, 180° from a meridian running through the Bering Strait
Rome 12° 27′ 08.4″ E meridian of Monte Mario Used in Roma 40 Datum [24]
Copenhagen 12° 34′ 32.25″ E Rundetårn[25]
Naples 14° 15′ E [16]
Pressburg 17° 06′ 03″ E Meridianus Posoniensis Used by Sámuel Mikoviny
Buda 19° 03′ 37″ E meridianu(s) Budense Used between 1469–1495; 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.[26]
Stockholm 18° 03′ 29.8″ E at the Stockholm Observatory[21]
Kraków 19° 57′ 21.43″ E Kraków meridian at the Old Kraków Observatory at the Śniadecki' College; mentioned also in Nicolaus Copernicus's work On the Revolutions of the Heavenly Spheres.
Warsaw 21° 00′ 42″ E Warsaw meridian [21]
Várad 21° 55′ 16″ E Tabulae Varadienses [27] Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea (Varadinum at the time) by Georg von Peuerbach.[28] In his logbook Columbus stated, he had one copy of Tabulae Varadienses 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.[29]
Alexandria 29° 53′ E Meridian of Alexandria The meridian of Ptolemy's Almagest.
Saint Petersburg 30° 19′ 42.09″ E Pulkovo meridian
Great Pyramid of Giza 31° 08′ 03.69″ E 1884 [30]
Jerusalem 35° 13′ 47.1″ E [14]
Mecca 39° 49′ 34″ E see also Mecca Time [31]
Ujjain 75° 47′ E Used from 4th century CE Indian astronomy and calendars(see also Time in India).[32]
Kyoto 136° 14′ E 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.
~ 180 Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming [16]

International prime meridian

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.[33][note 1] The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian and is the prime meridian that currently has the widest use.

Prime meridian at Greenwich

Prime meridian
Markings of the prime meridian at the Royal Observatory, Greenwich.

The modern prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851.[35]

The position of the Greenwich Meridian has been defined by the location of the Airy Transit Circle ever since the first observation was taken with it by Sir George Airy in 1851.[35] Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal, Edmond Halley in 1721. It was set up in the extreme north-west corner of the Observatory between Flamsteed House and the Western Summer House. This spot, now subsumed into Flamsteed House, is roughly 43 metres to the west of the Airy Transit Circle, a distance equivalent to roughly 0.15 seconds of time.[22] 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.[36][37]

All of these Greenwich meridians were located via an astronomic observation from the surface of the Earth, oriented via a plumb line along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via the lunar distance method, then by chronometers carried on ships, then via telegraph lines carried by submarine communications cables, then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States.

IERS Reference Meridian

Beginning in 1973 the International Time Bureau and later the International Earth Rotation and Reference Systems Service changed from reliance on optical instruments like the Airy Transit Circle to techniques such as lunar laser ranging, satellite laser ranging, and very-long-baseline interferometry. The new techniques resulted in the IERS Reference Meridian, the plane of which passes through the centre of mass of the Earth. This differs from the plane established by the Airy transit, which is affected by vertical deflection (the local vertical is affected by influences such as nearby mountains). The change from relying on the local vertical to using a meridian based on the centre of the Earth caused the modern prime meridian to be 5.3" east of the astronomic Greenwich prime meridian through the Airy Transit Circle. At the latitude of Greenwich, this amounts to 102 metres.[38] 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 ITRFs (International Terrestrial Reference Systems).

Due to the movement of Earth's tectonic plates, the line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres; that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map. With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning.[39] As a result, the International Reference Meridian was established and is commonly used to denote Earth's prime meridian (0° longitude) by the International Earth Rotation and Reference Systems Service, which defines and maintains the link between longitude and time. Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres per year relative to this Earth-centred 0° longitude.

It is also the reference meridian of the Global Positioning System operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).[40][41][42] A current convention on the Earth uses the opposite of the IRM as the basis for the International Date Line.

List of places

On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian (as of 2016) passes through:

Country, territory or sea Notes
90°0′N 0°0′E / 90.000°N 0.000°E Arctic Ocean
81°39′N 0°0′E / 81.650°N 0.000°E Greenland Sea
72°53′N 0°0′E / 72.883°N 0.000°E Norwegian Sea
61°0′N 0°0′E / 61.000°N 0.000°E North Sea
53°45′N 0°0′E / 53.750°N 0.000°E  United Kingdom From Tunstall in East Riding to Peacehaven, passing through Greenwich
50°47′N 0°0′E / 50.783°N 0.000°E English Channel
49°19′N 0°0′E / 49.317°N 0.000°E  France From Villers-sur-Mer to Gavarnie
42°41′N 0°0′E / 42.683°N 0.000°E  Spain From Cilindro de Marboré to Castellón de la Plana
39°56′N 0°0′E / 39.933°N 0.000°E Mediterranean Sea Gulf of Valencia
38°52′N 0°0′E / 38.867°N 0.000°E  Spain From El Verger to Calpe
38°38′N 0°0′E / 38.633°N 0.000°E Mediterranean Sea
35°50′N 0°0′E / 35.833°N 0.000°E  Algeria From Stidia to Algeria-Mali border near Bordj Mokhtar
21°50′N 0°0′E / 21.833°N 0.000°E  Mali
14°59′N 0°0′E / 14.983°N 0.000°E  Burkina Faso
11°6′N 0°0′E / 11.100°N 0.000°E  Togo For about 600 m
11°6′N 0°0′E / 11.100°N 0.000°E  Ghana For about 16 km
10°57′N 0°0′E / 10.950°N 0.000°E  Togo For about 39 km
10°36′N 0°0′E / 10.600°N 0.000°E  Ghana From the Togo-Ghana border near Bunkpurugu to Tema
Passing through Lake Volta at 7°48′N 0°0′E / 7.800°N 0.000°E
5°37′N 0°0′E / 5.617°N 0.000°E Atlantic Ocean Passing through the Equator at 0°0′N 0°0′E / 0.000°N 0.000°E ("Null Island")
60°0′S 0°0′E / 60.000°S 0.000°E Southern Ocean
68°54′S 0°0′E / 68.900°S 0.000°E Antarctica Queen Maud Land, claimed by  Norway
90°0′S 0°0′E / 90.000°S 0.000°E Antarctica Amundsen-Scott station, South Pole

Prime meridian on other planetary bodies

As on the Earth, prime meridians must be arbitrarily defined. Often a landmark such as a crater is used; other times a prime meridian is defined by reference to another celestial object, or by magnetic fields. The prime meridians of the following planetographic systems have been defined:

  • Two different heliographic coordinate systems are used on the Sun. The first is the Carrington heliographic coordinate system. In this system, the prime meridian passes through the center of the solar disk as seen from the Earth on 9 November 1853, which is when the English astronomer Richard Christopher Carrington started his observations of sunspots.[43] The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory.
  • In 1975 the prime meridian of Mercury was defined[44][45] to be 20° east of the crater Hun Kal.[46]
  • Defined[47] in 1992, the prime meridian of Venus passes through the central peak in the crater Ariadne.[48]
  • The prime meridian of the Moon lies directly in the middle of the face of the moon visible from Earth and passes near the crater Bruce.
  • The prime meridian of Mars was established in 1971[49] 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 west.[50]
  • 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.[51] 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.
  • Titan, like the Earth's moon, always has the same face towards Saturn, and so the middle of that face is 0 longitude.
  • Pluto's prime meridian is defined as the center of the face that is always pointed towards Charon, its largest moon, as the two are tidally locked towards each other.

See also


  1. ^ Voting took place on 13 October and the resolutions were adopted on 22 October 1884.[34]


  1. ^ Prime Meridian,
  2. ^ Norgate & Norgate 2006
  3. ^ Hooker 2006
  4. ^ e.g. Jacob Roggeveen in 1722 reported the longitude of Easter Island as 268° 45' (starting from Fuerteventura) in the Extract from the Official log of Jacob Roggeveen reproduced in Bolton Glanville Corney, ed. (1908), The voyage of Don Felipe Gonzalez to Easter Island in 1770-1, Hakluyt Society, p. 3, retrieved 13 January 2013
  5. ^ Speech by Pierre Janssen, director of the Paris observatory, at the first session of the Meridian Conference.
  6. ^ Sobel & Andrewes 1998, pp. 110–115
  7. ^ Sobel & Andrewes 1998, pp. 197–199
  8. ^ "The Prime Meridian at Greenwich". Royal Museums Greenwich. n.d. Retrieved 28 March 2016.
  9. ^ International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Project Gutenberg
  10. ^ NGS 2016, PID: HV1847.
  11. ^ NGS 2016, PID: HV1846.
  12. ^ NGS 2016, PID: AH7372.
  13. ^ a b c Hooker (2006), introduction.
  14. ^ a b c Oct. 13, 1884: Greenwich Resolves Subprime Meridian Crisis, WIRED, 13 October 2010.
  15. ^ Atlas do Brazil, 1909, by Barão Homem de Mello e Francisco Homem de Mello, published in Rio de Janeiro by F. Briguiet & Cia.
  16. ^ a b c "The Project Gutenberg eBook of International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day". 12 February 2006. Retrieved 28 March 2016.
  17. ^ Ancient, used in Ptolemy's Geographia. Later redefined 17° 39′ 46″ W of Greenwich to be exactly 20° W of Paris. French "submarin" at Washington 1884.
  18. ^ A.R.T. Jonkers; Parallel meridians: Diffusion and change in early modern oceanic reckoning, in Noord-Zuid in Oostindisch perspectief, The Hague, 2005, p. 7. Retrieved 2 February 2015.
  19. ^ "In search of the lost meridian of Cadiz", El País, 23 December 2016. Retrieved 8 November 2018.
  20. ^ Antonio Lafuente and Manuel Sellés, El Observatorio de Cádiz (1753–1831), Ministerio de Defensa, 1988, p.144, ISBN 84-505-7563-X. (in Spanish)
  21. ^ a b c d e Bartky, Ian R. (2007). One Time Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 98. ISBN 978-0-8047-5642-6.
  22. ^ a b c Dolan 2013a.
  23. ^ (in Dutch)Eenheid van tijd in Nederland (Unity of time in the Netherlands), Utrecht University website, retrieved 28 August 2013.
  24. ^ Grids & Datums – Italian Republic,, Retrieved 10 December 2013.
  25. ^ meridian, article from Den Store Danske Encyklopædi
  26. ^ When Tolhopff handed over his book, titled Stellarium (1480)[1] to king Matthias Corvinus, he empasized that he had used the meridian of Buda for his calculations. The German phisician, Johannes Müntz used it the same way in his 1495 calendar. However, in the second edition, he had already introduced the Vienna meridian. Zsoldos, Endre – Zsupán, Edina: Stellarium – egy csillagászati kódex Mátyás könyvtárában. Orpheus Noster V. évf. 2013/4. 62–85.[2]; Szathmáry, László: Az asztrológia, alkémia és misztika Mátyás király udvarában. In: Ponticulus Hungaricus, VI. évfolyam 5. szám · 2002.[3]
  27. ^ Oradea, Tourism office website, retrieved 3 February 2015.
  28. ^ "Romanian astronaut makrsk 10th anniversary of Prime Meridian Astronomy Club". NineO' 2015. Retrieved 26 June 2017.
  29. ^ Meridian Zero csillagászklub access-date = 27 December 2018
  30. ^ Wilcomb E. Washburn, "The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth Archived 29 May 2007 at the Wayback Machine"
  31. ^ Maimonides, Hilchot Kiddush Hachodesh 11:17, calls this point אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day.
  32. ^ Burgess 1860
  33. ^ International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Protocols of the proceedings. Project Gutenberg. 1884. Retrieved 30 November 2012.
  34. ^ Howse 1997, pp. 12, 137
  35. ^ a b Greenwich Observatory ... the story of Britain's oldest scientific institution, the Royal Observatory at Greenwich and Herstmonceux, 1675–1975 p.10. Taylor & Francis, 1975
  36. ^ McCarthy, Dennis; Seidelmann, P. Kenneth (2009). TIME from Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH. pp. 244–5.
  37. ^ ROG Learning Team (23 August 2002). "The Prime Meridian at Greenwich". Royal Museums Greenwich. Royal Museums Greenwich. Retrieved 14 June 2012.
  38. ^ Malys, Stephen; Seago, John H.; Palvis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (1 August 2015). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263. Bibcode:2015JGeod..89.1263M. doi:10.1007/s00190-015-0844-y.
  39. ^ Dolan 2013b.
  40. ^ History of the Prime Meridian -Past and Present
  41. ^ IRM on grounds of Royal Observatory from Google Earth Accessed 30 March 2012
  42. ^ The astronomic latitude of the Royal Observatory is 51°28'38"N whereas its latitude on the European Terrestrial Reference Frame (1989) datum is 51°28'40.1247"N.
  43. ^ "Carrington heliographic coordinates".
  44. ^ Davies, M. E., "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, 10 June 1975
  45. ^ Davies, M. E., S. E. Dwornik, D. E. Gault, and R. G. Strom, NASA Atlas of Mercury, NASA Scientific and Technical Information Office, 1978.
  46. ^ Archinal, Brent A.; A'Hearn, Michael F.; Bowell, Edward G.; Conrad, Albert R.; Consolmagno, Guy J.; et al. (2010). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (PDF). Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4.
  47. ^ Davies, M. E.; 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
  48. ^ "USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE)". Retrieved 22 October 2009.
  49. ^ Davies, M. E., and Berg, R. A.; "Preliminary Control Net of Mars", Journal of Geophysical Research, vol. 76, no. 2, 10 January 1971, pp. 373–393
  50. ^ Archinal, Brent A.; Acton, C. H.; A’Hearn, Michael F.; Conrad, Albert R.; et al. (2018), "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015", Celestial Mechanics and Dynamical Astronomy, 130 (22), doi:10.1007/s10569-017-9805-5
  51. ^ "Planetographic Coordinates". Retrieved 24 May 2017.

Works cited

External links

45×90 points

The 45×90 points are the four points on earth which are halfway between the geographical poles, the equator, the Prime Meridian, and the 180th meridian.


Airy-0 is a crater on Mars whose location defined the position of the prime meridian of that planet. It is about 0.5 kilometres (0.31 mi) across and lies within the larger crater Airy in the region Sinus Meridiani. The IAU Working Group on Cartographic Coordinates and Rotational Elements has now recommended setting the longitude of the Viking 1 lander (47°.95137 west) as the standard. This definition maintains the position of the center of Airy-0 at 0° longitude, within the tolerance of current cartographic uncertainties.Merton Davies tied this crater into an extensive geodetic control network of the planet Mars based on Mariner 9 and earlier photographs. The Mariner 9 Geodesy/Cartography Group proposed that the prime meridian of Mars be defined by the center of Airy-0, which was selected by Harold Masursky, Gerard de Vaucouleurs, and Merton Davies at a Group meeting on 14 August 1972.It was named in honor of the British Astronomer Royal Sir George Biddell Airy (1801-1892), who in 1850 built the transit circle telescope at Greenwich. The location of that telescope was subsequently chosen to define the location of Earth's prime meridian.

Airy (Martian crater)

Airy is an impact crater on Mars, named in honor of the British Astronomer, Royal Sir George Biddell Airy (1801–1892). The crater is approximately 43 kilometres (27 mi) in diameter and is located at 0.1°E 5.1°S in the Meridiani Planum region. The much smaller crater Airy-0, which defines the location of Mars' prime meridian, lies within it.

Beer (Martian crater)

Beer is a crater lying situated within the Margaritifer Sinus quadrangle (MC-19) region of the planet Mars, named in honor of the German astronomer, Wilhelm Beer. It is located at 14.4°S 351.8°E .

Beer and collaborator Johann Heinrich Mädler produced the first reasonably good maps of Mars in the early 1830s. When doing so, they selected a particular feature for the prime meridian of their charts. Their choice was strengthened when Giovanni Schiaparelli used the same location in 1877 for his more famous maps of Mars. The feature was later called Sinus Meridiani ("Middle Bay"), but following the landing of the NASA probe MER-B Opportunity in 2004 it is perhaps better known as Meridiani Planum. Currently the Martian prime meridian is the crater Airy-0.

Beer lies in the southwest of Meridiani Planum, about 8° from the prime meridian and about 10° west from the crater Mädler. Schiaparelli is also in the region.

Dante (crater)

Dante is a lunar impact crater that is located on the far side of the Moon. It lies in the northern hemisphere exactly opposite the prime meridian facing the Earth. The nearest craters of note are Larmor to the north and Morse to the southeast. To the southwest is the oddly shaped Buys-Ballot.

This crater is overlain by part of the ray system radiating from Larmor Q to the northwest. The rim of Dante is circular but somewhat eroded. The fresh crater Dante G is attached to the exterior along the east-southeastern rim. The interior floor of this crater is uneven and marked by several small impacts.

The crater lies within the Freundlich-Sharonov Basin.

Davies (crater)

Davies is a crater on Mars located at 46°N 0°E on the fringe of Acidalia Planitia near Arabia Terra. It is approximately 48 km in diameter. The crater's name was formally approved by the IAU in 2006.It was named in honor of Merton Davies (1917-2001), a pioneer in the cartography of planetary bodies. An employee of the RAND Corporation, he assisted NASA in mapping Mars with colleagues Gérard de Vaucouleurs and Harold Masursky and defined the prime meridian of Mars as passing through the crater Airy-0. Davies Crater lies on the prime meridian, appropriate because Davies was responsible for its delineation.

Decimal degrees

Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.

Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.

Eastern Hemisphere

The Eastern Hemisphere is a geographical term for the half of Earth which is east of the prime meridian (which crosses Greenwich, London, UK) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to pole). It is also used to refer to Afro-Eurasia (Africa and Eurasia) and Australia, in contrast with the Western Hemisphere, which includes mainly North and South America. The Eastern Hemisphere may also be called the "Oriental Hemisphere". In addition, it may be used in a cultural or geopolitical sense as a synonym for the "Old World".

Hemispheres of Earth

In geography and cartography, the hemispheres of Earth refer to any division of the globe into two hemispheres (from Ancient Greek ἡμισφαίριον hēmisphairion, meaning "half of a sphere").

The most common such divisions are by latitudinal or longitudinal markers:


Northern Hemisphere, the half that lies north of the Equator

Southern Hemisphere, the half that lies south of the Equator


Eastern Hemisphere, the half that lies east of the prime meridian and west of the 180th meridian

Western Hemisphere, the half that lies west of the prime meridian and east of the 180th meridianThe East–West division can also be seen in a cultural sense, as a division into two cultural hemispheres.

However, other schemes have sought to divide the planet in a way that maximizes the preponderance of one geographic feature or another in each division:

Land–Water Land Hemisphere, the hemisphere on Earth containing the largest possible area of land

Water Hemisphere, the hemisphere on Earth containing the largest area of water

The Earth may also be split into hemispheres of day and night by the terrestrial terminator.

Hun Kal (crater)

Hun Kal is a small crater on Mercury that serves as the reference point for the planet's system of longitude. The longitude of Hun Kal's center is defined as being 20° W, thus establishing the planet's prime meridian.

Hun Kal was chosen as a reference point since the actual prime meridian was in shadow when Mariner 10 photographed the region, hiding any features near 0° longitude from view.

Hun Kal is about 1.5 km in diameter.The name "Hun Kal" means '20' in the language of the Maya.

IERS Reference Meridian

The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3 arcseconds east of George Biddell Airy's 1851 transit circle or 102 metres (335 ft) at the latitude of the Royal Observatory, Greenwich. It is also the reference meridian of the Global Positioning System (GPS) 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).


Longitude (, AU and UK also ), is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians (lines running from pole to pole) connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question. (This forms a right-handed coordinate system with the z-axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x-axis (right hand index finger) extending from the Earth's center through the Equator at the Prime Meridian.)

A location's north–south position along a meridian is given by its latitude, which is approximately the angle between the local vertical and the equatorial plane.

If the Earth were perfectly spherical and radially homogeneous, then the longitude at a point would be equal to the angle between a vertical north–south plane through that point and the plane of the Greenwich meridian. Everywhere on Earth the vertical north–south plane would contain the Earth's axis. But the Earth is not radially homogeneous and has rugged terrain, which affect gravity and so can shift the vertical plane away from the Earth's axis. The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations. The longitude shown on maps and GPS devices is the angle between the Greenwich plane and a not-quite-vertical plane through the point; the not-quite-vertical plane is perpendicular to the surface of the spheroid chosen to approximate the Earth's sea-level surface, rather than perpendicular to the sea-level surface itself.

Meridian (geography)

A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles).

Mädler (Martian crater)

Mädler is a crater on Mars named in honor of the German astronomer Johann Heinrich Mädler. It is located at 2.7°E 10.7°S.

Mädler and collaborator Wilhelm Beer produced the first reasonably good maps of Mars in the early 1830s. When doing so, they selected a particular feature for the prime meridian of their charts. Their choice was strengthened when Giovanni Schiaparelli used the same location in 1877 for his more famous maps of Mars. The feature was later called Sinus Meridiani ("Middle Bay" or "Bay of the Meridian"), but following the landing of the NASA probe MER-B Opportunity in 2004 is perhaps better known as Meridiani Planum.

Mädler lies in the south of Meridiani Planum, close to the prime meridian and about 10° east of Beer. Schiaparelli is also in the region.

Null Island

Null Island is a name for the area around the point where the prime meridian and the equator cross, located in the Gulf of Guinea (Atlantic Ocean) off the west African coast. In the WGS84 datum, this is at zero degrees latitude and longitude (0°N 0°E), and is the location of a buoy. The name 'Null Island' serves as both a joke based around the suppositional existence of an island there, and also as a name to which coordinates erroneously set to 0,0 are assigned in placenames databases in order to more easily find and fix them. The nearest land is Annobón island, part of Equatorial Guinea, at 1°25′S 5°38′E.

Prime meridian (Greenwich)

A prime meridian, based at the Royal Observatory, Greenwich, in London, England, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of US President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the official prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.The plane of the prime meridian is parallel to the local gravity vector at the Airy transit circle (51°28′40.1″N 0°0′5.3″W) of the Greenwich observatory. The prime meridian was therefore long symbolised by a brass strip in the courtyard, now replaced by stainless steel, and since 16 December 1999, it has been marked by a powerful green laser shining north across the London night sky.

Global Positioning System (GPS) receivers show that the marking strip for the prime meridian at Greenwich is not exactly at zero degrees, zero minutes, and zero seconds but at approximately 5.3 seconds of arc to the west of the meridian (meaning that the meridian appears to be 102.478 metres east). In the past, this offset has been attributed to the establishment of reference meridians for space-based location systems such as WGS 84 (which GPS relies on) or that errors gradually crept into the International Time Bureau timekeeping process. The actual reason for the discrepancy is that the difference between precise GNSS coordinates and astronomically determined coordinates everywhere remains a localized gravity effect due to the deflection of the vertical; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.

Royal Borough of Greenwich

The Royal Borough of Greenwich ( (listen), , or ) is a London borough in south-east London, England. Taking its name from the historic town of Greenwich, the London Borough of Greenwich was formed in 1965 by the amalgamation of the former area of the Metropolitan Borough of Greenwich with part of the Metropolitan Borough of Woolwich to the east. The local council is Greenwich London Borough Council which meets in Woolwich Town Hall. The council's offices are also based in Woolwich, the main urban centre in the borough.

Greenwich is world-famous as the traditional location of the Prime Meridian, on which all Coordinated Universal Time is based. The Prime Meridian running through Greenwich and the Greenwich Observatory is where the designation Greenwich Mean Time, or GMT began, and on which all world times are based. In 2012, Greenwich was listed as a top ten global destination by Frommer's – the only UK destination to be listed.

Greenwich was one of six host boroughs for the 2012 London Olympics and events were held at the Royal Artillery Barracks (shooting), Greenwich Park (equestrianism) and The O2 – the former Millennium Dome (gymnastics and basketball). It is also the home borough of professional football club Charlton Athletic.

To mark the Diamond Jubilee of Elizabeth II, Greenwich became a Royal Borough on 3 February 2012, due in part to its historic links with the Royal Family, and to its UNESCO World Heritage Site status as home of the Prime Meridian.

Selenographic coordinates

Selenographic coordinates are used to refer to locations on the surface of Earth's moon. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth. The longitude gives the position east or west of the Moon's prime meridian, which is the line passing from the lunar north pole through the point on the lunar surface directly facing Earth to the lunar south pole. (See also Earth's prime meridian.) This can be thought of as the midpoint of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar equator. Both of these coordinates are given in degrees.

Astronomers defined the fundamental location in the selenographic coordinate system by the small, bowl-shaped satellite crater 'Mösting A'. The coordinates of this crater are defined as:

The coordinate system has become precisely defined due to the Lunar Laser Ranging Experiment.

Anything past 90°E or 90°W would not be seen from Earth, except for libration, which makes 59% of the Moon visible.

Western Hemisphere

The Western Hemisphere is a geographical term for the half of Earth which lies west of the prime meridian (which crosses Greenwich, London, United Kingdom) and east of the antimeridian. The other half is called the Eastern Hemisphere.

International standards
Obsolete standards
Time in physics
Archaeology and geology
Astronomical chronology
Other units of time
Related topics

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.