Greenwich Mean Time or GMT is mean (average) solar time at the Greenwich Meridian or Prime Meridian, 0 degrees longitude. The Prime Meridian is the reference point for every time zone in the world.
The time displayed by the Shepherd Gate Clock at the Royal Observatory in Greenwich, London, is always GMT. When the sun is at its highest point exactly above the Prime Meridian at the Royal Observatory, it is 12:00 noon at Greenwich.
Until 1972, when it was replaced by UTC, GMT was also the basis of international civil time standard. It is still used as civil time in UK, when Daylight Saving Time is not in use.
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.
Did you know?
- The name of the clock that shows Greenwich Mean Time at the Royal Observatory in Greenwich, London, UK, is Shepherd Gate Clock
- Apart from UK, the offset GMT+0 (0 hours time difference from GMT) is used in several other countries
- Offsets or time differences are generally written as UTC/GMT plus or minus a number of hours
- In the UTC standard, there is a commitment to keep within 0.9 seconds of GMT.
- In the age of the internet, the adjustments are distributed via NTP
Check out the GMT timestamp to see how accurate is your computer time.
View here real-time clocks showing GMT/UTC offsets for 24 time zones across the world.
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 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 Astronomer Royal and 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.
GMT was replaced as the international time standard in 1972 by UTC (Universal Coordinated Time). UTC is based on atomic measurements.
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