Greenwich Mean Time
Greenwich Mean Time (GMT) is a time standard based on mean solar time at the Greenwich meridian in Greenwich, London. It developed from astronomical observations made at the Royal Observatory in Greenwich. In ordinary civil use, GMT has the same clock reading as Coordinated Universal Time (UTC) and corresponds to the UTC+0 offset.[1][2]
GMT served as the principal international reference for civil time from 1884 until UTC replaced it in 1972. It remains the standard time of the United Kingdom during the part of the year when daylight saving time is not in effect.
History
[edit | edit source]Mean solar time is calculated by averaging the apparent movement of the Sun across the sky over the course of a year. This removes the daily variation found in apparent solar time and produces a consistent twenty-four-hour day.
The Royal Observatory was established at Greenwich in 1675. Astronomical observations made there allowed Greenwich mean solar time to be calculated and distributed. During the nineteenth century, Greenwich time became increasingly important as railways and telegraph networks required a common time instead of separate local times.
The International Meridian Conference met in Washington, D.C., in 1884. The conference selected the Greenwich meridian as the international prime meridian. GMT subsequently became the main reference for international civil time and the calculation of time differences between locations.
Coordinated Universal Time replaced GMT as the principal international civil time standard in 1972. GMT continued to be used as a named standard time and as a historical timekeeping term.
Relationship with UTC
[edit | edit source]GMT and UTC are technically different standards. GMT was historically determined through astronomical observations of the Earth's rotation and the mean position of the Sun. UTC is maintained through atomic timekeeping and is kept close to rotational time.[1]
Despite this difference, GMT and UTC normally have the same clock reading for civil purposes. A time of 12:00 UTC is therefore also 12:00 GMT. GMT applies no positive or negative offset and corresponds to UTC+00:00.
Civil use
[edit | edit source]The United Kingdom uses GMT as its standard time between the autumn and spring clock changes. When daylight saving time begins, clocks move forward by one hour and the country uses British Summer Time, which corresponds to UTC+1. The clocks return to GMT when daylight saving time ends.[3]
GMT itself does not move forward during the summer. The legal local time changes from GMT to British Summer Time and later returns to GMT.