thumb|right|Laser projected from the [[Royal Observatory, Greenwich|Royal Observatory, marking the prime meridian of Greenwich]]

thumb|Tourists queuing to take pictures on the brass line marking the prime meridian at the [[Royal Observatory, Greenwich|alt=A group of people waiting in a line curving to the left on a cobblestone surface. Behind it is an ornate brick building with a red ball on top. The people at the end of the line, closest to the camera, are taking pictures of other people near a shiny metal monument on the right, under a tree. A line in the cobblestone connects them]]

The Greenwich meridian is a prime meridian, a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. This prime meridian (at the time, one of many) was first established by Sir George Airy (in 1851). In 1883, the International Geodetic Association formally recommended to governments that the meridian through Greenwich be adopted as the international standard prime meridian. In October of the following year, at the invitation of the President of the United States, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This inter-governmental conference selected the meridian passing through Greenwich as the world standard prime meridian. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades.

The plane of the prime meridian contains the local gravity vector at the Airy transit circle instrument ()<!-- The date of 1721 is not at all correct. That is when Flamsteed erected his transit instruments at the observatory but there is no evidence that he had any concept of a prime meridian. -->

In the 19th century, astronomers and geodesists were concerned with questions of longitude and time, because they were responsible for determining them scientifically and used them continually in their studies. The International Geodetic Association, which had covered Europe with a network of fundamental longitudes, took an interest in the question of an internationally-accepted prime meridian at its seventh general conference in Rome in 1883. Indeed, the Association was already providing administrations with the bases for topographical surveys, and engineers with the fundamental benchmarks for their levelling. It seemed natural that it should contribute to the achievement of significant progress in navigation, cartography and geography, as well as in the service of major communications institutions, railways and telegraphs. From a scientific point of view, to be a candidate for the status of international prime meridian, the proponent needed to satisfy three important criteria. According to the report by Carlos Ibáñez e Ibáñez de Ibero, it must have a first-rate astronomical observatory, be directly linked by astronomical observations to other nearby observatories, and be attached to a network of first-rate triangles in the surrounding country. Four major observatories could satisfy these requirements: Greenwich, Paris, Berlin and Washington. The conference concluded that Greenwich Observatory best corresponded to the geographical, nautical, astronomical and cartographic conditions that guided the choice of an international prime meridian, and recommended the governments should adopt it as the world standard. The Conference further hoped that, if the whole world agreed on the unification of longitudes and times by the Association's choosing the Greenwich meridian, Great Britain might respond in favour of the unification of weights and measures, by adhering to the Metre Convention.

In 1884, the International Meridian Conference (of government representatives) took place in Washington, D.C. to establish an internationally-recognised single meridian. The meridian chosen was that which passed through the Airy transit circle at Greenwich, and it became the prime meridian of the world for a century. In 1984 it was superseded in that role by the IERS Reference Meridian which, at this latitude, runs about 102 metres to the east of the Greenwich meridian.

At around the time of the 1884 conference, scientists were making measurements to determine the deflection of the vertical on a large scale. One might expect that plumb lines set up in various locations, if extended downward, would all pass through a single point, the centre of Earth, but this is not the case, primarily due to Earth being an ellipsoid, not a sphere. The downward extended plumb lines don't even all intersect the rotation axis of Earth; this much smaller effect is due to the uneven distribution of Earth's mass. To make computations feasible, scientists defined ellipsoids of revolution, more closely emulating the shape of Earth, modified for a particular zone; a published ellipsoid would be a good base line for measurements. The difference between the direction of a plumb line or vertical, and a line perpendicular to the surface of the ellipsoid of revolution a normal to said ellipsoid at a particular observatory, is the deflection of the vertical.

IERS Reference Meridian

thumb|A [[GPS receiver at the marking strip of the Greenwich Meridian in front of the Royal Observatory. The indicated longitude is not exactly zero because the geodetic zero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using the IERS Reference Meridian) is 102 metres east of this strip. are based on a failure of understanding. The explanation by Malys et al. on the other hand is more studied and correct.

Meridian today

thumb|right|Greenwich meridian and Earth

The Greenwich meridian passes through eight countries in Europe and Africa from north to south:

  1. United Kingdom (specifically, only England)
  2. France
  3. Spain
  4. Algeria
  5. Mali
  6. Burkina Faso
  7. Togo
  8. Ghana

It also passes through Antarctica, only touching Queen Maud Land, a territorial claim of Norway, on its way from the North Pole to the South Pole.

It crosses the maritime exclusive economic zones of:

  1. Greenland (Denmark)
  2. Norway (via proximity to Svalbard, Jan Mayen Island, and the Norwegian mainland in the North Atlantic, and Bouvet Island in the South Atlantic)

See also

  • Arc measurement of Delambre and Méchain
  • 180th meridian
  • Prime meridian
  • United Kingdom Ordnance Survey Zero Meridian

Notes

References

Sources

  • "Where the Earth's surface begins—and ends", Popular Mechanics, December 1930
  • A pictorial catalogue of meridian markers

<!-- The interwiki links shall refer to GREENWICH MERIDIAN, not to zero meridian or null meridian -->