<P> Coordinates: 51 ° 28 ′ 40.12" N 0 ° 00 ′ 05.31" W ﻿ / ﻿ 51.4778111 ° N 0.0014750 ° W ﻿ / 51.4778111; - 0.0014750 </P> <P> 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 US 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 . </P> <P> The plane of the prime meridian is parallel to the local gravity vector at the Airy transit circle (51 ° 28 ′ 40.1" N 0 ° 0 ′ 5.3" W ﻿ / ﻿ 51.477806 ° N 0.001472 ° W ﻿ / 51.477806; - 0.001472 ﻿ (Airy Transit)) 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 . </P>

Why was greenwich chosen as the location for the prime meridian