Greenwich Mean Time Explained
Greenwich Mean Time began as the time kept at the Royal Observatory in Greenwich, London, and became a world standard in the 19th century.
The Royal Observatory
The Royal Observatory was founded in 1675 to help sailors find their longitude at sea. Time at Greenwich became the standard for British charts and navigation.
Railways and a national time
In the 1840s the railways adopted Greenwich time across Britain so that timetables made sense. Before that, a city like Bristol kept its own local time about ten minutes behind London.
1884: the world agrees
At the International Meridian Conference in Washington in 1884, delegates chose Greenwich as the prime meridian, the zero line for longitude and the basis of world time zones.
Frequently asked questions
Where is the prime meridian?
It runs through the Royal Observatory in Greenwich, south-east London, at zero degrees longitude.
Clock change guides
- What Is Daylight Saving Time (DST)? A Plain-English GuideWhat daylight saving time is, how it works in the UK, Europe and the US, why it exists, its history, who uses it and the argument over scrapping it.
- Why Do the Clocks Go Back in October? The Real ReasonThe clocks go back in autumn to return the UK to GMT, so winter mornings are lighter. The history, the rule, what time it happens and what changes for you.
- When Do the Clocks Go Forward Again After Going Back?When the UK clocks go forward again after the autumn change, how long the UK stays on GMT, when evenings get lighter and how to remember to reset your clocks.