British astronomers have long used the Royal Observatory as a basis for measurement . Four separate meridians have passed through the buildings , defined by successive instruments . The basis of longitude , the meridian that passes through the Airy transit circle , first used in 1851 , was adopted as the world 's Prime Meridian at the International Meridian Conference on 22 October 1884 ( voting took place on 13 October ) . Subsequently , nations across the world used it as their standard for mapping and timekeeping . The Prime Meridian was marked by a brass ( later replaced by stainless steel ) strip in the Observatory 's courtyard once the buildings became a museum in 1960 , and , since 16 December 1999 , has been marked by a powerful green laser shining north across the London night sky
International Meridian Conference
International Meridian Conference
Royal Observatory, Greenwich