( also the Royal Greenwich Observatory )
an observatory (= a building from which to study the stars, weather, etc.) at Greenwich , London, England. Its original purpose was to study the stars in order to help sailors find their position at sea, and its work led to the widespread use of Greenwich Mean Time and the Greenwich Meridian, an imaginary line running through Greenwich from north to south around the earth. International time zones are still measured from the Greenwich Meridian. The building is now part of the National Maritime Museum .
See also Astronomer Royal .