Prime Meridian
50225
225055280
2008-07-11T17:49:14Z
Bazonka
561069
/* Geography */ Norwegian Antarctic claim
[[Image:Primemeridian.jpg|180px|thumb|right|Location of the Prime Meridian]]
The '''Prime Meridian''' is the [[meridian (geography)|meridian]] (line of [[longitude]]) at which longitude is defined to be 0°.
The Prime Meridian and the opposite [[180th meridian]] (at 180° longitude), which the [[International Date Line]] generally follows, form a [[great circle]] that divides the Earth into the [[Eastern Hemisphere|Eastern]] and [[Western Hemisphere]]s.
Unlike the parallels of [[latitude]], which are defined by the rotational axis of the [[Earth]] (the [[Geographical pole|poles]] being 90° and the [[Equator]] 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian is one passing through [[Greenwich]], [[London]], [[United Kingdom]], known as the '''International Meridian''' or '''Greenwich Meridian'''. Historically, various meridians have been used, including four different ones through Greenwich.
==Geography==
Starting at the [[North Pole]] and heading south to the [[South Pole]], the Prime Meridian passes through:
:{| class="wikitable"
! Country, territory or sea
! Notes
|-
| style="background:#b0e0e6;" | [[Arctic Ocean]]
| style="background:#b0e0e6;" |
|-
| style="background:#b0e0e6;" | [[Norwegian Sea]]
| style="background:#b0e0e6;" |
|-
| style="background:#b0e0e6;" | [[North Sea]]
| style="background:#b0e0e6;" |
|-valign="top"
| {{UK}}
| [[England]] - the most northerly land on the meridian is the shore (53°45′34″N) southeast of the Sand-le-Mere caravan park east of [[Kingston upon Hull]]
|-
| style="background:#b0e0e6;" | [[English Channel]]
| style="background:#b0e0e6;" |
|-
| {{FRA}}
|
|-
| {{ESP}}
|
|-
| style="background:#b0e0e6;" | [[Mediterranean Sea]]
| style="background:#b0e0e6;" | [[Gulf of Valencia]]
|-
| {{ESP}}
|
|-
| style="background:#b0e0e6;" | [[Mediterranean Sea]]
| style="background:#b0e0e6;" |
|-
| {{DZA}}
|
|-
| {{MLI}}
|
|-
| {{BFA}}
|
|-
| {{TOG}}
|
|-
| {{GHA}}
|
|-
| {{TOG}}
|
|-
| {{GHA}}
| Including [[Lake Volta]]
|-
| style="background:#b0e0e6;" | [[Atlantic Ocean]]
| style="background:#b0e0e6;" |
|-
| style="background:#b0e0e6;" | [[Southern Ocean]]
| style="background:#b0e0e6;" |
|-
| [[Antarctica]]
| [[Queen Maud Land]], [[List of Antarctic territorial claims|claimed]] by {{NOR}}
|-
|}
==History==
[[Image:Royalobs.jpg|180px|thumb|right|The Royal Observatory, Greenwich (today a museum)]]
The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history:
* [[Berne]] (7° 26′ 22.5″ E)
* [[Brussels]] (4° 22′ 4.71″ E)
* [[Copenhagen]] (12° 34′ 32.25″ east of Greenwich; [[Rundetårn]])
* [[El Hierro|El Hierro (Ferro)]] (Canary Islands) ''Ancient, later redefined 17° 39′ 46″ W of Greenwich to be exactly 20° W of Paris. French "submarin" at Washington 1884.''
* [[Great Pyramid of Giza]] (1884) <ref>Wilcomb E. Washburn, "[http://muweb.millersville.edu/~columbus/data/geo/WASHBR04.GEO The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth]"</ref>
* [[Alexandria]], the meridian of [[Ptolemy]]'s [[Almagest]].
* [[Jerusalem]] (35° 13′ 46″ east of Greenwich for the large dome of the [[Church of the Holy Sepulchre]])
* [[Lisbon]] (9° 07′ 54.862″ west of Greenwich)
* [[Madrid]] (3° 41′ 16.58″ west of Greenwich)
* [[Mecca]] (39° 49′ E)
* [[Oradea]], [[Romania]] (21° 55′ 16″ east of Greenwich)
* [[Oslo|Oslo (Kristiania)]] (10° 43′ 22.5″ east of Greenwich)
* [[Paris]] (2° 20′ 14.025″ east of Greenwich; see [[Paris Meridian]])
* [[Philadelphia, Pennsylvania|Philadelphia]]
* [[Pisa]], [[Italy]] (10°24′ east of Greenwich)
* [[Rome]] (''meridian of Mount Mario'' 12° 27′ 08.4″ east of Greenwich)
* [[Saint Petersburg]] (''Pulkovo meridian'', 30° 19′ 42.09″ east of Greenwich)
* [[Stockholm]] (at the observatory, 18° 3′ 29.8″ east of Greenwich)
* [[Ujjain]] (75° 47′ east of Greenwich) Used in Indian astronomy and calendars.
* [[Warsaw]] (21° 00’ 42” east of Greenwich; see [[Warsaw meridian]])
* [[Washington, D.C.]] (77° 3′ 2.3″ west of Greenwich; see [[Washington meridian]])
* Approximately 59° east of Greenwich <ref>[[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.</ref>
[[Image:Prime meridian.jpg|180px|thumb|left|Prime Meridian in Greenwich]]
The Greenwich Meridian, based at the [[Royal Observatory, Greenwich]], was established by [[George Biddell Airy|Sir George Airy]] in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their maps. In October of that year, at the behest of [[President of the United States|U.S. President]] [[Chester A. Arthur]], 41 delegates from 25 nations met in [[Washington, D.C.]], USA, for the [[International Meridian Conference]]. This conference selected the Greenwich Meridian 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.
{{clr}}
==Precise Greenwich Meridian==
[[Image:Greenwichmer1.jpg|180px|thumb|right|A [[GPS]] receiver at the Greenwich Meridian]]
[[World Geodetic System|WGS84]] longitudes, which are used by [[satellite]] navigation systems, differ slightly from traditional longitudes. The WGS84 zero meridian is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.<ref>[http://gpsinformation.net/main/greenwich.htm History of the Prime Meridian -Past and Present]</ref>. The offset at other locations can be as much as 30″ east or west.
WGS84 uses the zero meridian as defined by the [[International Time Bureau|Bureau International de l'Heure]],<ref>[[EUROCONTROL|European Organisation for the Safety of Air Navigation]] and [[Institute of Geodesy and Navigation|IfEN]]: "[http://octopus.hit.bme.hu/tkatona/mirrors/WGS84/www.wgs84.com/files/wgsman24.pdf WGS 84 Implementation Manual]", page 13. 1998</ref> which was defined by compilation of star observations in different countries. The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a [[plumb-bob|plumb line]]) which points opposite to the direction of the [[zenith]], to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of [[vertical deflection]], is 5.31”. The WGS84 datum is an average of the various [[continental drift]]s. As a result, the astronomical meridian between the vertical crosshairs of Airy's transit telescope drifts toward the east as it is carried by the European portion of the Eurasian [[tectonic plate]], closer to the geodetic meridian, by about one centimetre per year.
[[Image:24 Hour Clock.JPG|thumb|left|The 24-hour clock at Greenwich]]
Curiously, whether by accident or design, the location of the WGS84 0° meridian is marked in Greenwich by the presence of a waste basket on the path leading more or less due east from the observatory containing the transit telescope.
The zero meridian used by the [[Ordnance Survey]] ([[OSGB36]] datum) is about six metres to the west of the line marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply continued to use it.
[[Image:Greenwich observatory laser.jpg|thumb|right|Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian]]
[[Universal Time]] is notionally based on the WGS84 meridian. However, the standard international time [[Coordinated Universal Time|UTC]] can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the Earth's rotation. [[Leap second]]s are inserted periodically to keep UTC in sync with the Earth.
The Greenwich Meridian is now marked at night by a [[laser]] beam emitted from the observatory.[http://jonathan.rawle.org/gallery/greenwich/laser/]
{{clr}}
==Other planetary bodies==
The prime meridians of the following bodies in the [[Solar System]] have been defined:
* The prime meridian of the [[Moon]] lies directly in the middle of the face of the moon visible from Earth and passes near the [[Bruce (crater)|crater Bruce]].
* The prime meridian of [[Mars]] is defined by the crater [[Airy-0]].
==References==
<references/>
== External links==
*{{gutenberg|no=17759|name=International Conference Held at Washington for the Purpose of Fixing a Prime Meridian}}
* [http://www.ucolick.org/~sla/leapsecs/scans-meridian.html scanned TIFFs of the conference proceedings]
* [http://wwp.greenwichmeantime.com/info/prime-meridian.htm Prime meridians in use in the 1880s, by country]
* [http://www.flickr.com/photos/20683202@N00/324876051/ Canadian Prime Meridian]
{{Time measurement and standards}}
[[Category:Time zones]]
[[Category:Greenwich]]
[[Category:Geodesy]]
[[Category:Lines of longitude]]
[[Category:1884 establishments]]
[[Category:English inventions]]
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