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Greenwich Meridian

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Location of the Prime Meridian
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Prime Meridian in Greenwich
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A GPS receiver at the Greenwich Meridian
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Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian
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The Royal Observatory, Greenwich (today a museum)
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The 24-hour clock at Greenwich
The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — it is the meridian at which longitude is defined to be 0 degrees. The prime meridian, and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, separate the Eastern and Western Hemispheres. The Prime Meridian and the 180th meridian form a great circle which encircles the Earth like a longitudinal equator.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator, 0°), the prime meridian is arbitrary, and multiple meridians have been used through history as the prime meridians of various mapmaking systems (including four different Greenwich meridians). The Greenwich Meridian established by Sir George Airy in 1851 was agreed upon as the international standard in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. France abstained when the vote was taken, and French maps continued to use the Paris Meridian for several decades.

Heading south from the North Pole, the Prime Meridian passes through the following countries: The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.[1]. WGS84 uses the zero meridian as defined by the Bureau International de l'Heure,[2] which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.[3] 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 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 drifts. 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.

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.

Universal Time is notionally based on the WGS84 meridian. However, the standard international 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 seconds 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.[1]

One degree of longitude is equal to (111.320 + 0.373sin²φ)cosφ km, where φ is latitude.[4]

Other reference meridians

The meridian through Greenwich was selected as the Prime Meridian because over two-thirds of all ships and tonnage already used it as the reference meridian on their maps in 1884. Other reference meridians used previously, or advocated, include:
WGS84 longitudes have supplanted traditional ones. The offset at other locations can be as much as 30″ east or west.

Other planetary bodies

The prime meridians of the following bodies in the Solar System have been defined:

References

1. ^ History of the Prime Meridian -Past and Present
2. ^ European Organisation for the Safety of Air Navigation and IfEN: "WGS 84 Implementation Manual", page 13. 1998
3. ^ National Maritime Museum: "The Longitude of Greenwich"
4. ^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valley, Cal.: University Science Books, 1992) page 700.
5. ^ Wilcomb E. Washburn, "The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth"
6. ^ 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.

External links

meridian (or line of longitude) is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations with a given longitude. The position of a point on the meridan is given by the latitude.
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Longitude is the east-west geographic coordinate measurement most commonly utilized in cartography and global navigation.
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Royal Observatory, Greenwich (formerly the Royal Greenwich Observatory or RGO) was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August.
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London
Canary Wharf is the centre of London's modern office towers
London shown within England
Coordinates:
Sovereign state United Kingdom
Constituent country England
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Longitude is the east-west geographic coordinate measurement most commonly utilized in cartography and global navigation.
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180th meridian or antimeridian is the meridian defined as 180° longitude. It is the line of longitude exactly opposite the Prime Meridian. It is common to both east longitude and west longitude.
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International Date Line (IDL), also known as just the Date Line, is an imaginary line on the surface of the Earth opposite the Prime Meridian which offsets the date as one travels east or west across it.
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Eastern Hemisphere, also Eastern hemisphere[1] or eastern hemisphere,[2] is a geographical term for the half of the Earth that is east of the Prime Meridian (which crosses Greenwich, England, United Kingdom) and west of the International Date
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Western Hemisphere, also Western hemisphere[1] or western hemisphere,[2] is a geographical term for the half of the Earth that lies west of the Prime Meridian (which crosses Greenwich in London, England, United Kingdom), the other half being the
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A great circle
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator.
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Please help recruit one or [ improve this article] yourself. See the talk page for details.
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equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
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Sir George Biddell Airy FRS (July 27, 1801–January 2, 1892) was an English mathematician and astronomer, Astronomer Royal from 1835 to 1881. His many achievements include work on planetary orbits, measuring the mean density of the Earth, a method of solution of
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18th century - 19th century - 20th century
1850s  1860s  1870s  - 1880s -  1890s  1900s  1910s
1881 1882 1883 - 1884 - 1885 1886 1887

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Chester Alan Arthur (October 5, 1829 – November 18, 1886) was an American politician who served as the twenty-first President of the United States. Arthur was a member of the Republican Party and worked as a lawyer before becoming the twentieth vice president under James
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Washington, D.C.

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International Meridian Conference was a conference held in October 1884 in Washington, D.C. in the United States to determine the Prime Meridian of the world. It was held at the request of U.S. President Chester A. Arthur. Twenty-five nations were represented by 41 delegates.
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Paris Meridian is a meridian line running through Paris, now longitude 2°20′14.025″ east. It was a long-standing rival to Greenwich as the prime meridian of the world, as was the Antwerp meridian in Flanders.
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North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets the Earth's surface.
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