Greenwich Mean Time

Greenwich Mean Time (GMT) The mean solar time on the Greenwich Meridian of longitude, which was defined to pass through the Airy Transit Circle at Greenwich in England and was adopted internationally as the zero of longitude at a conference in Washington in 1884. Its acceptance was facilitated by the overwhelming use of the Greenwich Meridian in navigation and the adoption, in the USA and Canada, of time zones based on Greenwich. Originally different towns in Great Britain kept their own local time, varying according to longitude. In the mid-19th century Greenwich time was adopted by railways throughout Britain for the sake of uniformity. However, it was only in 1880 that Greenwich Mean Time became the legal time throughout Great Britain. The international reference time-scale for civil use is now based on atomic clocks but is subject to step adjustments (leap seconds) to keep it close to mean solar time on the Greenwich Meridian. The formal name of the time-scale is UTC (a language-independent abbreviation of coordinated universal time) but it is still widely known as Greenwich Mean Time.