Greenwich Mean Time

Greenwich Mean Time, abbreviated GMT, is the mean solar time associated with longitude 0 degrees at the Royal Observatory in Greenwich, England. Mean time is clock time based on the average length of a solar day; it smooths out the small seasonal variations in apparent solar time. The Prime Meridian passes through Greenwich and provides a geographical reference for measuring longitude. GMT is often used casually as a time zone name, but historically it refers to a time standard tied to Greenwich rather than a complete modern system for coordinating atomic clocks worldwide.

Astronomers at the Royal Observatory used local mean time to organize observations. In the nineteenth century, railways and telegraph networks made a common time increasingly useful: when towns kept separate local times, railway timetables could be difficult to coordinate. British railway companies adopted Greenwich-based time across their networks, and the Railway Clearing House adopted it in 1847. Greenwich Mean Time became Britain's legal standard time in 1880. The Royal Observatory's time signals, distributed by telegraph, also helped make accurate common time available beyond the observatory.

At the 1884 International Meridian Conference, the Greenwich meridian was recommended as the prime meridian of the world, giving longitude zero a common international reference. GMT consequently became a widely used basis for time zones and international timekeeping. In the twentieth century, astronomers adopted Universal Time, a related mean-solar-time system, and Coordinated Universal Time (UTC) became the modern worldwide time standard, combining atomic timekeeping with adjustments that keep it close to Earth's rotation. GMT is still used in everyday contexts and is the legal standard time in the United Kingdom during winter; British Summer Time is used in summer.

GMT also had practical importance at sea. Navigators could compare local time with the time at Greenwich to help determine longitude. In the eighteenth century, the Nautical Almanac published lunar-distance data calculated using Greenwich observations, while accurate marine timekeepers offered another solution to the longitude problem. These methods helped establish Greenwich time as a useful reference well before it became an international civil standard.