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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.
A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation—and various conventions have been used or advocated in different regions and throughout history.
The prime 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 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.
The prime meridian passes through the Airy transit circle (51°28′40.1247″N 0°0′5.3101″W / 51.477812417°N 0.001475028°W) of the Greenwich observatory. It was long marked by a brass strip in the courtyard, now upgraded to stainless steel, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.
The Greenwich meridian passes through the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana, and Queen Maud Land in Antarctica.