Prime meridian (Greenwich)
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A prime meridian, based at the Royal Observatory, Greenwich, in London, 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 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 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 prime meridian passes through the Airy transit circle () of the Greenwich observatory. It was long marked by a brass strip in the courtyard, now replaced by stainless steel, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.
Global Positioning System (GPS) receivers show that the marked 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 metres east of this line). 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.
Before the establishment of a common meridian, most maritime countries established their own prime meridian, usually passing through the country in question. In 1721, Great Britain established its own meridian passing through an early transit circle at the newly established Royal Observatory at Greenwich. The meridian was moved around 10 metres or so east on three occasions as transit circles with newer and better instruments were built, on each occasion next door to the existing one. This was to allow uninterrupted observation during each new construction. The final meridian was established as an imaginary line from the north pole to the south pole passing through the Airy transit circle. This became Great Britain's meridian in 1851. For all practical purposes of the period the changes as the meridian was moved went un-noticed.
Transit instruments are installed to be perpendicular to the local level (which is a plane perpendicular to a plumb line). In 1884 the International Meridian Conference took place to establish an internationally recognised single meridan. The meridian chosen was that which passed through the Airy transit circle at Greenwich and it became the prime meridian.
At around the time of this 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 the Earth, but this is not the case, due to uneven distribution of the Earth's mass. To make computations feasible, scientists defined ellipsoids of revolution; a given ellipsoid would be a good compromise for measurements in a given area, such as a country or continent. The difference between the direction of a plumb line and a line perpendicular to the surface of the ellipsoid of revolution at a particular observatory is the deflection of the vertical. The International Meridian Conference did not take this into account.
The phenomenon meant that when the Airy transit circle was built, that because they used a plumb line to align the telescope to the perpendicular, the plumb line is deflected slightly from a line perpendicular to the modern ellipsoid used to define latitude and longitude, the International Terrestrial Reference Frame (which is nearly the same as the WGS-84 system used by GPS). This in turn meant that the Airy transit circle points very slightly to the east of the modern celestial meridian (the line in the sky directly above the prime meridian). As a result of this, any measurements of transit time across the view of the transit telescope occurs 0.352 seconds (or 0.353 sidereal seconds) before the transit across the intended meridian. The practical upshot of this is that the modern prime meridian does not correspond to the Airy transit, but to a meridian that is 102 metres to the east of the transit. For all practical navigational purposes of the time, the difference was insignificant, and sailors certainly would not have noticed.
A recent analysis by Malys et al. shows the offset between the Airy transit circle and the ITRF/WGS 84 meridians can be explained by deflection of the vertical alone; other possible sources of the offset that have been proposed in the past are smaller than the current uncertainty of the deflection of the vertical near the observatory.
The meridian today
The Greenwich meridian passes through:
- United Kingdom,
- Burkina Faso,
- Queen Maud Land in Antarctica.
- ROG Learing Team (23 August 2002). "The Prime Meridian at Greenwich". Royal Museums Greenwich. Royal Museums Greenwich. Retrieved 14 June 2012.
- Malachy Postlethwayt. (1774) Universal Dictionary of Trade and Commerce. (4th edition) London: W. Strahan, J. & F. Rivington. Vol. 1 "A New and Correct Map of the Coast of Africa"
- 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. doi:10.1007/s00190-015-0844-6.
- Dracup, Joseph F. (8 June 2006). "Geodetic Surveys in the United States, the Beginning and the Next 100 Years". NOAA History: A Science Oddesy.
- Geodesy for the Layman (PDF) (Technical report) (5th ed.). National Ocean Service. December 1983. pp. 6–10.
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