Greenwich Meridian

This sheet marks the 125th Anniversary of the Greenwich Meridian being adopted worldwide as the prime meridian.

Britain has a long tradition of leadership in navigation and timekeeping. Before Cook s voyages of discovery in the eighteenth century there was no practical means of measuring longitude at sea. The Royal Observatory at Greenwich was established to solve the problem of measuring longitude, and played a vital part in the discovery and development of navigational methods.

Before the days of worldwide communications and travel there was little need for universal time. Each country, and often different parts of each country, used their own local time. In Britain the railways ran to a national time scale, which became known as railway time, but it was not until 1880 that Greenwich Mean Time (GMT) became the legal standard. There were similar standards in other countries. As with time, so with longitude - each country had its own system based on its national observatory. In 1884 delegates from 25 countries met in Washington DC to decide on a single worldwide system of time and longitude.

Britain had two special claims for the site of the prime meridian and with it the base for universal time. First, the Greeenwich Observatory had, for over a century, been the source of the British Nautical Almanac, which was almost universally used for navigation at sea. Second, the Greenwich meridian had already been adopted by the USA and Canada as the origin for the division into standard time zones used by the railroads. From 1884 onwards all countries adopted the Greenwich meridian as the zero of longitude.