In this article, you will learn what GMT is London. Greenwich Mean Time is the yearly average of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich. Essentially, mean time is clock time rather than solar time. Solar time varies throughout the year, as the time interval between the Sun crossing a set meridian line changes.
What GMT is London?
GMT stands for Greenwich Mean Time, the local clock time at Greenwich. From 1884 until 1972. GMT was the international standard of civil time. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service. Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round.
London uses Greenwich Mean Time (GMT) during standard time and British Summer Time (BST) during Daylight Saving Time (DST), or summer time.
How did Greenwich Mean Time become the international standard?
In 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World.
There were two main reasons for this. The first was that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.
The recommendation was based on the argument that naming Greenwich as Longitude 0º would be of advantage to the largest number of people.
As the reference for GMT, the Prime Meridian at Greenwich therefore became the center of world time and the basis for the global system of time zones.
The Airy Transit Circle (telescope) became the telescope that would define the Prime Meridian of the World. Astronomer Royal George Biddell Airy designed it, and it is located at the Royal Observatory Greenwich.
It was recommended that the meridian line would indicate 0° longitude. Therefore this also became the start of the Universal Day. The meridian line is marked by the cross-hairs in the Airy Transit Circle eyepiece.
What is the Difference between GMT and UTC?
In practice, GMT and UTC share the same time on a clock, which can cause them to be interchanged or confused. However, there is an essential difference between the two:
UTC is a time standard that forms the base of time zones worldwide and is not used as a time zone in any country. Time zones are defined by their offsets from UTC. Since GMT has a UTC offset of zero, there is no time difference between the two.
GMT is a time zone used in the UK and other European and African countries.
A source of confusion is that GMT used to be the world's time standard before UTC was introduced. Until 1972. time zones were defined by their offset from GMT based on mean solar time at the prime meridian in Greenwich near London. This article is about what GMT is London.
