Greenwich Meridian Line

The Greenwich meridian is a prime meridian, a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. From 1884 to 1974, the Greenwich meridian was the international standard prime meridian, used worldwide for timekeeping and navigation. The modern standard, the IERS Reference Meridian, is based on the Greenwich meridian, but differs slightly from it. 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 (51°28′40.1″N 0°0′5.3″W) of the Greenwich observatory. The prime meridian was therefore long symbolised by a brass strip in the courtyard, now replaced by stainless steel, and since 16 December 1999, it has been marked by a powerful green laser shining north across the London night sky. The Global Positioning System (GPS) receivers show that the marking strip for the prime meridian at Greenwich is not exactly at zero longitude (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. In the past, this offset has been attributed to the establishment of reference meridians for space-based location systems such as WGS-84 (which the GPS relies on) or to the fact that errors gradually crept into the International Time Bureau timekeeping process. The actual reason for the discrepancy is that the difference between geodetic coordinates and astronomically determined coordinates everywhere remains a localized gravity effect due to vertical deflection; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.