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prime meridian

 
Dictionary: prime meridian
 
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prime meridian
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prime meridian
(Jerry Malone)

n.

The zero meridian (0°), used as a reference line from which longitude east and west is measured. It passes through Greenwich, England.


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Columbia Encyclopedia: prime meridian
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prime meridian, meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal time, the standard basis for determining time throughout the world, is civil time measured at the prime meridian.

 
Science Q&A: What is the prime meridian?
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The north-south lines on a map run from the North Pole to the South Pole and are called "meridians." The word "meridian" means "noon": When it is noon on one place on the line, it is noon at any other point as well. The lines are used to measure longitudes, or how far east or west a particular place might be, and they are 69 miles (111 kilometers) apart at the equator. The east-west lines are called parallels, and, unlike meridians, are all parallel to each other. They measure latitude, or how far north or south a particular place might be. There are 180 lines circling the Earth, one for each degree of latitude. The degrees of both latitude and longitude are divided into 60 minutes, which are then further divided into 60 seconds each.

The prime meridian is the meridian of 0 degrees longitude, used as the origin for measurement of longitude. The meridian of Greenwich, England, is used almost universally for this purpose.

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Geography: prime meridian
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The meridian at zero degrees longitude. All longitude is measured relative to the prime meridian, which passes through Greenwich, England.

 
Wikipedia: Prime Meridian
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Location of the Prime Meridian
The nations that touch the equator (red) and the prime meridian (blue).

The Prime Meridian is the 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 and Western Hemispheres.

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. By international convention, the modern Prime Meridian passes through the Royal Observatory, Greenwich, in east London, United Kingdom, known as the International Meridian or Greenwich Meridian.

Historically, various meridians have been used, including four different ones through Greenwich.

Contents

Geography

Starting at the North Pole and heading south to the South Pole, the Prime Meridian passes through:

lat Country, territory or sea Notes
90° 80° 75° Arctic Ocean
70° 65° Norwegian Sea
60° 55° North Sea
53° 52° 51°  United Kingdom 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
50° English Channel
49° 48° 47° 46° 45° 44° 43°  France
42° 41° 40°  Spain
39° 38° 37° 36° Mediterranean Sea Gulf of Valencia
35° 30° 25°  Algeria Sahara
20° 15°  Mali Sahara, Sahel
13°  Burkina Faso
11°  Togo
10°  Ghana Including Lake Volta
5° N 0° 5° S

10° 15° 20° 25° 30° 35° 40° 45° 50° 55°

 Atlantic Ocean
60° Southern Ocean
70° 80° 90° Antarctica Queen Maud Land, claimed by  Norway

History

The Royal Observatory, Greenwich (today a museum)
Prime Meridian in Greenwich

The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history:

Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian

The modern Greenwich Meridian, based at the Royal Observatory, Greenwich, 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 maps. In October of that year, 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. 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.

The Greenwich Meridian is now marked at night by a laser beam emitted northwards from the observatory.

IERS Reference Meridian

A GPS receiver at the Greenwich Meridian. This does not indicate a longitude of zero because the prime meridian defined by the WGS84 system is about 100 metres to the east.

The International Earth Rotation and Reference Systems Service (IERS) maintains the IERS Reference Meridian (IRM), also called the International Reference Meridian, which is the reference meridian (prime meridian, 0° longitude) of the Global Positioning System operated by the United States Department of Defense, and is the reference meridian in WGS84 and in its formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). It is 5.31″ east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich.[5] This shift is a legacy of the first satellite navigation system, the Doppler based TRANSIT system developed by the Applied Physics Laboratory of Johns Hopkins University in Howard County, Maryland. This was the location of its first ground station, so the surveyed coordinates of that ground station in the North American Datum 1927 (NAD27), a non-Earth centered ellipsoid, became its coordinates in an Earth-centered ellipsoid, such as the World Geodetic System. This caused the coordinates of any other location on an Earth-centered ellipsoid to shift, especially those far away. When the antenna of a TRANSIT ground station was mounted directly above Airy's transit instrument in June 1969, its longitude on an Earth-centered ellipsoid was 5.64″ west of TRANSIT's reference meridian.[6] Further improvement in gravitational models, such as the Earth Geopotential Model 1996 (EGM96), and a dramatic increase in the number of ground stations from only four to over 500, as well as the use of time based GPS caused a small additional shift before it reached its present position.

The International Hydrographic Organization adopted an early version in 1983 for all nautical charts.[7] It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[8] Because tectonic plates slowly move over the surface of the Earth, most countries have adopted a version of the IRM for their maps that is fixed relative to their own tectonic plate as it existed at the beginning of a specific year, such as the North American Datum 1983 (NAD83), the European Terrestrial Reference Frame 1989 (ETRF89), and the Geocentric Datum of Australia 1994 (GDA94). Versions fixed to a tectonic plate differ from the global version by at most a few centimetres.

However, the IRM is not fixed to any point on Earth. Instead, all points on the European portion of the Eurasian plate, including the Royal Observatory, are slowly moving northeast about 2.5 cm per year relative to it. This IRM is the weighted average in the least squares sense of the reference meridians of the hundreds of ground stations contributing to the IERS network, including GPS stations, Satellite Laser Ranging (SLR) stations, Lunar Laser Ranging (LLR) stations, and the highly accurate Very Long Baseline Interferometry (VLBI) stations.[9] The coordinates of all of these stations are adjusted annually until there is no net rotation relative to the major tectonic plates. If Earth had only two hemispherical plates moving relative to each other around any axis from that intersecting their centres to one intersecting their junction, the longitudes (around any other rotation axis) of any two stations diametrically opposite each other must move in opposite directions by the same amount.

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 0.9 second (equivalent to about 260 metres at Greenwich), because of changes in the Earth's rotation. Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun — mean solar time.

United Kingdom Ordnance Survey Zero Meridian

The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the Airy meridian marked at Greenwich. When the first Ordnance Survey map was published in 1801, the official Prime Meridian of Great Britain was the one established by the third Astronomer Royal, James Bradley. When Airy's new Prime Meridian superseded it fifty years later, the Ordnance Survey simply continued to use Bradley's.[10]

Other planetary bodies

Like on 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:

  • 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 is defined by the crater Airy-0.
  • The prime meridian of Venus passes through the center of a radar-bright spot on the center of the oval feature Eve, which lies to the south of Alpha Regio.
  • Heliographic coordinates are used on the Sun. The prime meridian passes through the center of the solar disk as seen from the Earth on the date 1853 November 9.9.
  • Jupiter has several coordinate systems because different latitudes rotate at different rates.[11] Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south equatorial belts. System II coordinates use Jupiter's magnetic field.
  • Titan, like the the earth's moon, always has the same face towards Saturn, and so that face is 0 longitude.

See also

References

  1. ^ 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.
  2. ^ The meridian of Ptolemy's Almagest.
  3. ^ Wilcomb E. Washburn, "The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth"
  4. ^ 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.
  5. ^ History of the Prime Meridian -Past and Present
  6. ^ G. Gebel and B. Matthews, "Navigation at the Prime Meridian", Navigation: Journal of the Institute of Navigation (Washington, DC) 18/2 (Summer 1971) 141-146.
  7. ^ A manual on the technical aspects of the United Nations Convention on the Law of the Sea - 1982PDF (4.89 MB) Section 2.4.4
  8. ^ WGS 84 Implementation Manual page i, 1998
  9. ^ IERS Conventions (2003): Conventional Terrestrial Reference System and FramePDF (419 KB)
  10. ^ Charles Jennings; "Greenwich: The Place Where Days Begin and End", Little, Brown; 1999; p. 181
  11. ^ "Planetographic Coordinates". http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html. Retrieved on 2009-06-19. 

External links

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Circles of latitude / Meridians
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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
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Geography. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Prime Meridian".  Read more

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