What is "prime meridian"

Meridian \Me*rid"i*an\, n. [F. m['e]ridien. See Meridian, a.]

1. Midday; noon.

2. Hence: The highest point, as of success, prosperity, or the like; culmination.

   I have touched the highest point of all my greatness, And from that full meridian of my glory I haste now to my setting.
   --Shak.

3. (Astron.) A great circle of the sphere passing through the poles of the heavens and the zenith of a given place. It is crossed by the sun at midday.

4. (Geog.) A great circle on the surface of the earth, passing through the poles and any given place; also, the half of such a circle included between the poles.

   Note: The planes of the geographical and astronomical meridians coincide. Meridians, on a map or globe, are lines drawn at certain intervals due north and south, or in the direction of the poles.

   Calculated for the meridian of, or fitted to the meridian of, or adapted to the meridian of, suited to the local circumstances, capabilities, or special requirements of.

   All other knowledge merely serves the concerns of this life, and is fitted to the meridian thereof.
   --Sir M. Hale.

   First meridian or prime meridian, the meridian from which longitudes are reckoned. The meridian of Greenwich is the one commonly employed in calculations of longitude by geographers, and in actual practice, although in various countries other and different meridians, chiefly those which pass through the capitals of the countries, are occasionally used; as, in France, the meridian of Paris; in the United States, the meridian of Washington, etc.

   Guide meridian (Public Land Survey), a line, marked by monuments, running North and South through a section of country between other more carefully established meridians called principal meridians, used for reference in surveying. [U.S.]

   Magnetic meridian, a great circle, passing through the zenith and coinciding in direction with the magnetic needle, or a line on the earth's surface having the same direction.

   Meridian circle (Astron.), an instrument consisting of a telescope attached to a large graduated circle and so mounted that the telescope revolves like the transit instrument in a meridian plane. By it the right ascension and the declination of a star may be measured in a single observation.

   Meridian instrument (Astron.), any astronomical instrument having a telescope that rotates in a meridian plane.

   Meridian of a globe, or Brass meridian, a graduated circular ring of brass, in which the artificial globe is suspended and revolves.

n. The reference line at 0° longitude, passing through Greenwich, England, from which longitude east and west is measured.

n. meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England)

A prime meridian is a meridian (a line of longitude) in a geographical coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its antimeridian (the 180th meridian in a 360°-system) 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.

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., United States, 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 alleged marking strip for the 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.