What is Greenwich Mean Time?

The history of GMT: from sailing to railways, longitude 0 and the Prime Meridian.

GMT and UK time on digital clocks

Greenwich Mean Time, or GMT, is the mean (average) solar time at the Greenwich Meridian or Prime Meridian, 0 degrees longitude.

The time displayed by the Shepherd Gate Clock at the Royal Observatory in Greenwich, London, is 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, GMT was the basis of the international civil time standard, when it was replaced by UTC (Coordinated Universal Time). It is still used as civil time in the UK when Daylight Saving Time is not in use.

GMT as a Time Zone

Apart from the UK, GMT is used as a time zone in several other countries.

Time Zones and Longitude 0

The Prime Meridian, longitude 0, is the reference point for every time zone in the world. Offsets or time differences are generally written as UTC/GMT plus or minus a number of hours, e.g. UTC+7 / GMT+7.

Every 15° longitude represents one hour's difference in time: (24 × 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.

View digital clocks for all 24 time zones.

Precise Time

The modern astronomical equivalent of GMT is UT1, which directly corresponds to the rotation of the Earth and is subject to that rotation's slight irregularities. UTC (Coordinated Universal Time) is kept within 0.9 s of UT1 by the application of leap seconds. In the age of the internet, these adjustments are distributed via NTP, which is how we are able to find out the exact time to the second.

Where it all began

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 (the Americas) 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 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 (Coordinated Universal Time). UTC is based on atomic measurements.

Do you like clocks and calendars?

Read about the Bristol clock with two minute hands, as well as many other topics in the time-related articles section of the site.