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What is Greenwich Mean Time (GMT)?

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Greenwich Mean Time or GMT is the clock time at the Royal Observatory in Greenwich, London. When the sun is at its highest point exactly above the Prime Meridian, it is 1200 noon at Greenwich.

GMT is still widely used as the standard time against which all the other time zones in the world are referenced. It is the same all year round and is not affected by Summer Time or Daylight Saving Time.

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.

GMT was not forced on to "land-lubbers" until the introduction of the railways (railroads) in the mid-nineteenth century. The developing railway network meant that 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, GMT was not adopted officially by Parliament until 2 August 1880 .



Greenwich Mean Time (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, GMT was adopted universally at the International Meridian Conference in Washington, DC, USA. As a result, the International Date Line was drawn up and 24 time zones were created.

Today, GMT is used as the UK's civil time, or UTC. 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 that is kept below 0.9s by the application of leap seconds.

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