History of Greenwich Mean Time: The Story of the World's Time Standard
Greenwich Mean Time (GMT) has its origins in the Royal Observatory in Greenwich, London, founded in 1675 by King Charles II to solve the problem of finding longitude at sea. Over the following centuries, GMT evolved from a local astronomical time to the world's primary time standard, driven by the needs of railways, the telegraph, and international commerce. The 1884 International Meridian Conference formalised Greenwich as the Prime Meridian (0 degrees longitude), making GMT the basis for global time zones. Today, while UTC has replaced GMT as the official standard, GMT remains deeply embedded in British culture and is still used as civil time in the UK during winter months. For the current time in the UK, visit Time.now.
The Founding of the Royal Observatory and Early Timekeeping
The Royal Observatory was established in 1675 with the primary goal of improving navigation by creating accurate star charts. Astronomers Royal, such as John Flamsteed and Edmond Halley, used the observatory's telescopes to measure the positions of stars and planets, which were essential for determining longitude at sea. Timekeeping was crucial: by comparing the local time of a ship's location with the time at a known reference point (such as Greenwich), sailors could calculate their longitude. The observatory's clocks were among the most accurate of their day, and by the late 18th century, Greenwich Mean Time was widely used by British mariners.
The Railway Revolution and the Standardisation of Time
Before the 19th century, towns across Britain kept their own local time, based on the sun's position. For example, Oxford time was 5 minutes behind London, and Bristol was 10 minutes behind. This caused chaos when railways began to connect cities. In 1840, the Great Western Railway adopted London time (GMT) for its timetables, and by 1847, most railway companies had followed suit. The Railway Clearing House officially adopted GMT in 1847, and it became known as 'Railway Time'. The telegraph accelerated this standardisation: the Royal Observatory began transmitting time signals via telegraph in 1852, allowing clocks across the country to be synchronised. By 1855, 98% of public clocks in Britain were set to GMT, and it was legally adopted as the standard time for the entire country in 1880.
The 1884 International Meridian Conference: Greenwich Becomes the Prime Meridian
As global travel and communication expanded, the need for a single prime meridian became urgent. In October 1884, delegates from 25 nations met in Washington, D.C., for the International Meridian Conference. After much debate, they voted to adopt the meridian passing through the Royal Observatory in Greenwich as the Prime Meridian (0 degrees longitude). This decision was influenced by the fact that two-thirds of the world's shipping already used Greenwich-based charts. The conference also established the 24-hour time zone system, with GMT as the reference point. This landmark event cemented Greenwich's place at the centre of global timekeeping. For a detailed comparison of GMT and its modern successor, see GMT vs UTC.
GMT and the Development of Time Zones
Following the 1884 conference, nations gradually adopted time zones based on GMT. The world was divided into 24 zones, each 15 degrees of longitude wide, with GMT as the starting point. Britain itself used GMT as its standard time, though the introduction of British Summer Time (BST) in 1916 added a seasonal shift. Today, many countries still reference GMT for their time zones, although Coordinated Universal Time (UTC) has technically replaced GMT as the scientific standard since 1972. However, GMT remains in common use, especially in the UK, where it is the official civil time during winter. For more on the difference between GMT and BST, visit GMT vs BST.
Key Dates in the History of GMT
| Year | Event |
|---|---|
| 1675 | Royal Observatory founded by King Charles II |
| 1830s-1840s | Railways adopt London time; 'Railway Time' spreads |
| 1852 | Time signals first transmitted by telegraph from Greenwich |
| 1880 | GMT legally adopted as standard time in Britain |
| 1884 | International Meridian Conference selects Greenwich as Prime Meridian |
| 1916 | British Summer Time introduced (first daylight saving period) |
| 1972 | UTC introduced as successor to GMT; GMT remains in common use |
GMT Today: Legacy and Usage
Although UTC is now the international standard for timekeeping, GMT remains widely used in the United Kingdom, Ireland, and several other countries. In the UK, GMT is observed from the last Sunday in October until the last Sunday in March, when clocks are set back one hour from British Summer Time. The Royal Observatory continues to be a popular tourist attraction, and the Prime Meridian line is marked at the site. For those needing to convert between London and New York times, London To New York Time provides a handy tool. For a broader view of world times, TimeRight.Now offers global city times, and CurrentTime.now provides a multi-city world clock. European and US time information can be found at European time and date and US time and date.
Frequently Asked Questions
What is the difference between GMT and UTC?
GMT is a time zone based on the mean solar time at the Royal Observatory in Greenwich. UTC (Coordinated Universal Time) is a more precise atomic time standard that replaced GMT as the basis for civil time in 1972. In practice, they are often used interchangeably, but UTC does not observe daylight saving time. For more details, see GMT vs UTC.
Why is Greenwich the Prime Meridian?
Greenwich was chosen as the Prime Meridian at the 1884 International Meridian Conference because the Royal Observatory had a long history of accurate timekeeping, and the majority of the world's shipping already used Greenwich-based charts. The decision was a compromise that established a single reference line for longitude and time.
When did the UK start using GMT as standard time?
GMT was adopted as the standard time for all of Great Britain in 1880, following decades of use by railways and telegraph systems. Before that, towns kept their own local mean time.
Does the UK use GMT all year round?
No. The UK uses GMT during the winter months (from the last Sunday in October to the last Sunday in March) and switches to British Summer Time (BST), which is GMT+1, for the summer. For the 2026 dates, see British Summer Time 2026.
What is the difference between GMT and BST?
GMT is the standard time at the Prime Meridian, while BST is the daylight saving time observed in the UK during summer, which is one hour ahead of GMT. BST runs from late March to late October. Learn more at GMT vs BST.
Is GMT still used in astronomy?
Historically, GMT was used in astronomy, but today astronomers use Universal Time (UT1) or Coordinated Universal Time (UTC) for precision. However, the term GMT is still sometimes used informally in astronomical contexts.