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GMT

Jaeger Le-Coultre Reverso Classic Large Duoface Small Seconds Ref. 3842520

GMT is the acronym for Greenwich Mean Time, the mean solar time at the Royal Observatory located in Greenwich, London. GMT was the international time standard till it was replaced by UTC in 1972.

Mean solar time refers to the time indicated by clocks and watches based on the assumption that the Sun travels at a constant speed and the length of an average day is 24 hours long. 

The Shepherd Gate Clock at the Royal Observatory in Greenwich displays Greenwich Mean Time. Greenwich Mean Time is also a time zone

The GMT time zone for example, is the standard time used in the United Kingdom except during summer. During this period, British Summer Time (BST) is used with time advanced by an hour. Portugal and Iceland are among the countries that use the GMT time zone.

The Royal Observatory became the home of the Prime Meridian in 1884 at longitude 000°00’00”, an imaginary line originating from the Airy Transit Circle Telescope in the Meridian Building that runs to the North and South Poles. While the Equator which lies at the latitude 000°00’00” separates the Northern and Southern hemispheres, the Prime Meridian is what distinguishes longitude east and longitude west. 

The Prime Meridian is also known as the Greenwich Meridian and it is the starting reference point for all the world’s time zones. 

Countries or cities to the east of the Greenwich Meridian have local times ahead of Greenwich Mean Time. Conversely, local times of countries to the west are behind GMT.

Interestingly, the first accurate atomic clock was built that same year in 1955 by Louis Essen and Jack Parry at the National Physical Laboratory in the United Kingdom.

Today, a GMT watch or one described as having the GMT function is commonly taken to mean a timepiece with the second time zone display. The use of GMT is a misnomer and such watches should actually be known as those with second time zone displays or dual-time indications.