- published: 05 Jan 2015
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A prime meridian is a meridian, i.e. a line of longitude, at which longitude is defined to be 0°. A prime meridian and its opposite in a 360°-system, the 180th meridian (at 180° longitude), form a great circle.
This great circle divides the sphere, e.g. the Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere.
An international conference in 1884 decided the Prime Meridian passes through the Royal Observatory, Greenwich in southeast London, United Kingdom, known as the International Meridian or Greenwich meridian.
As of 2012 the most used prime meridian for the Earth is the IERS Reference Meridian (IRM). It passes 5.31 arcseconds east of Airy's transit circle or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich, London. A current convention on the earth uses the opposite of the IRM as the basis for the International Date Line.
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., 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.