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., USA, 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.
The Greenwich Meridian passes through the Airy transit circle 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.