Digital clock showing GMT
- The name of the clock that shows Greenwich Mean Time at theRoyal Observatory in Greenwich, London, UK, is Shepherd GateClock
- GMT is used as Standard Time in the UK and several othercountries and is also atime zone
- Offsets or time differences are generally written asUTC/GMT plus or minus anumber of hours.
The definition
Greenwich Mean Time or GMT is clock time at the Royal Observatory inGreenwich, London. It is the same all year round and is not affected bySummer Time (Daylight Saving Time) clock changes.
When the sun is at its highest point exactly above thePrime Meridian, it is 1200noon at Greenwich.
Check out the GMT timestamp tosee how accurate your computer time is.
GMT is also a time zone, used by the United Kingdom of Great Britain andNorthern Ireland (UK) when Daylight Saving Time isnot in use, from October to March.
The Greenwich Meridian (Prime Meridian or Longitude Zerodegrees) marks the starting point of every Time Zone ofthe time zone map.
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 locationon earth if you know how many degrees it is east or west of Greenwich.
Where is Greenwich,England ?
- Longitude 0° 0' 0"
- Latitude 51° 28' 38"N (North of the Equator)
Viewhere real-time clocksshowing GMT/UTC offsets for 24 time zones across the world.
GMT is still widely used as the standard time against which all the othertime zones in the world are referenced.
Historical timeline
GMT was originally set up to aid naval navigation when travel around theglobe started to open up with the discovery of the “New World” (America) inthe fifteenth century.
Read more here about the connectionbetween accurate time-keeping, GMT and sea voyages.
Greenwich was a royal park and palace on a hill to the south of the RiverThames east of London.
In 1675 the great race to create accurate maps for navigators had begun andCharles II offered the land to The Royal Society for Britain's firstnational observatory. Christopher Wren was commissioned to design the domedbuilding.
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John Flamsteed was appointed the Astronomer Royal. British mapmakers beganto set Longitude from Greenwich and in 1884 it was adopted as the PrimeMeridian.
Railways and National Time
With the introduction of the railways (railroads) in the mid-nineteenthcentury, Britain needed a national time system to replace the local timeadopted by major towns and cities.
As Greenwich, due to the presence of the Royal Observatory, was the nationalcentre for time and had been since 1675, the choice was obvious.Nevertheless, time as shown by the clocks at the Royal Observatory was notadopted officially by Parliament until 2 August 1880 .
GMT was then adopted by the United States (USA) on 18 November 1883. Thechosen moment was at noon, when the telegraph lines transmitted time signalsto all major cities. Prior to that there were over 300 local times in theUSA.
On 1 November 1884, GMT was adopted universally at the InternationalMeridian Conference in Washington, DC, USA. As a result, the InternationalDate Line was drawn up and 24 time zones were created.
Today, GMT is used as the UK’s civil time. GMT has been referred to as“UT1", which directly corresponds to the rotation of the Earth, and issubject to that rotation’s slight irregularities. It is the differencebetween UT1 and UTC (Universal Coordinated Time) that is kept below 0.9s bythe application of leap seconds.
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