The definition
Greenwich Mean Time or GMT is the clock time at the Royal Observatory in Greenwich, London. It is the same all year round and is not affected by clock changes. When the sun is at its highest point exactly above the Prime Meridian, it is 12:00 noon at Greenwich.
The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) is the reference point for every Time Zone in the world.
Every 15° longitude represents one hour's difference in time: (24 x 15 = 360, the degrees of a circle). You can work out the time at every location on earth if you know how many degrees it is east or west of Greenwich.
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Check out the GMT timestamp to see how accurate your computer time is.
View here real-time clocks showing GMT/UTC offsets for 24 time zones across the world.
GMT was replaced as the international time standard in 1972 by UTC (Universal Coordinated Time). UTC is based on atomic measurements.
GMT has been referred to as “UT1", which directly corresponds to the rotation of the Earth, and is subject to that rotation’s slight irregularities. It is the difference between UT1 and UTC (Universal Coordinated Time) that is kept below 0.9s by the application of leap seconds.
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Historical timeline
GMT was originally set up to aid naval navigation when travel around the globe started to open up, with the discovery of the “New World” (America) in the fifteenth century.
Read more here about the connection between accurate time-keeping, GMT and sea voyages.
Greenwich was a royal park and palace on a hill to the south of the River Thames east of London.
In 1675 the great race to create accurate maps for navigators had begun and Charles II offered the land to The Royal Society for Britain's first national observatory.
Christopher Wren was commissioned to design the domed building. John Flamsteed was appointed the Astronomer Royal. British mapmakers began to set longitude lines from Greenwich.
Railways and National Time
With the introduction of the railways (railroads) in the mid-nineteenth century, Britain needed a national time system to replace the local time adopted by major towns and cities.
As Greenwich, due to the presence of the Royal Observatory, was the national centre for time and had been since 1675, the choice was obvious. Nevertheless, time as shown by the clocks at the Royal Observatory was not approved officially by Parliament until 2 August 1880 .
GMT was then adopted by the United States (USA) on 18 November 1883. The chosen moment was at noon, when the telegraph lines transmitted time signals to all major cities. Prior to that there were over 300 local times in the USA.
On 1 November 1884, the Greenwich Meridian was adopted universally at the International Meridian Conference in Washington DC, USA. As a result, Greenwich Mean Time became the time standard and the 24 time zones were created.