Greenwich has spent a third of a millennium cracking the conundrum of time. Since King Charles II established Flamsteed as the first Astronomer Royal in 1675. The opening of the new world and the search for accurate time-keeping to aid navigation gave the world GMT and the Prime Meridian.
Your questions answered:
We have had many requests about linking our GMT page to other pages which feature GMT. Well now your chance use the IBM aqui to set up these links and get the world linked on time!
What is Greenwich Mean Time (GMT)?
Why is the Navy so involved in time-keeping?
What is Co-ordinated Universal Time (UTC)?
What time does the time-ball drop?
When was British Summer Time (BST) introduced?
When will British Summer Time (BST) be in force this year (1997)?
What is the definition of a second?
What is the International Atomic Time Scale?
What is the SI unit of time?
Do you have any more time questions you would like answered?
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What is Greenwich Mean Time (GMT)?
Originally set-up to aid naval navigation when the globe started to open up with the discovery of the "New World" (America) in the fifteenth Century.
It was not forced on to "land-lubbers" until the introduction of the railways (railroads) in the mid nineteenth century. In the 1840 's with the introduction of the railways (railroads) there was a need, in Britain, for a national time system to replace the local time adopted by major towns and cities. Greenwich was the national centre for time and had been since 1675. It was not adopted officially by Parliament until 2 August 1880.
Greenwich Mean Time (GMT) was adopted by the United States (USA) at noon on 18 November 1883 when the telegraph lines transmitted time signals to all major cities. Prior to that there were over 300 local times in the USA.
Greenwich Mean Time (GMT) was adopted universally on the 1 November 1884 when the International Meridian Conference in Washington, DC, USA met. From then the International Date Line was drawn up and 24 time zones created.
Today, GMT is used as the UK's civil time, or UTC. But to navigators, GMT has referred to "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 that is kept below 0.9s by the application of leap seconds.
Why is the Navy so involved in time-keeping?
The earliest users of accurate time were the navy. It was vitally important to aid navigation so an accurate position of ships could be determined. Historic Maritime Greenwich is the centre of time because King Charles II established the first Royal Observatory in the Royal Park at Greenwich in 1675. It was set-up to improve naval navigation. Britain at that time was a strong sea power.
The US Naval Observatory (USNO) was established in Washington, DC, in 1830 to co-operate with the Royal Greenwich Observatory to develop accurate time-keeping for the navy. It still is today and the main clock on this site is sourced from the US Naval Observatory in Washington DC.
What is Co-ordinated Universal Time (UTC)?
Replaced Greenwich Mean Time (GMT) as the World standard for time in 1986. It is based on atomic measurements rather than the earth's rotation. Greenwich Mean Time (GMT) is still the standard time zone for the Prime Meridian (Zero Longitude).
What time does the time-ball drop at Greenwich?
At Greenwich the bright orange time-ball on the observatory roof drops at 1pm every day. It has done this every day since 1833. 1pm was chosen to enable the astronomers to undertake telescopic observations at noon.
In Washington DC, USA the time-ball was installed in 1845 and dropped every day at noon but alas is no more!
When was British Summer Time (BST) introduced?
British Summer Time was introduced in the UK in 1916.
British Summer Time was permanently in force during the Second World War from February 1940 until October 1945 and again from February 1968 until October 1971.
Double summer time was in force from 1941-1947 except for 1946.
When will British Summer Time (BST) be in force this year (1997)?
The Summer Time Act 1972 defined the period of British Summer Time to start at 2 am (GMT) on the morning of the day after the third Saturday in March or, if that was Easter Day, the day after the second Saturday. It was to end at 2 am (GMT) on the day after the fourth Saturday in October.
The duration of British Summer Time (BST) can be varied by Order of Council and in recent years has been changed so as to bring the date of the start of Summer Time into line with that used in Europe.
The rule for 1981-1994 defined the start of Summer Time as the last Sunday in March and the end as the day following the fourth Saturday in October. The time of change was altered to 1 am (GMT).
There is no rule for the dates of Summer Time (BST) for the years 1995, 1996 and 1997.
The agreed dates are:
For 1995 BST started on March 26 and ended on October 22
For 1996 BST started on March 31 and ended on October 27
For 1997 BST starts on March 30 and ends on October 26
all changes to be at 1 am GMT. It is unclear what will happen after 1997.
In 1996 all clocks in Europe were changed the same date for the first time.
Source:
What is the definition of a second?
Originally based on the Earth's rotation this has been found to be irregular and has in more recent times been replaced by atomic time.
The overhead sun at noon can arrived 16 minutes 18 seconds early or up to 14 minutes 28 seconds late. In fact only 4 days per year have 24 hours in them. These days occur on, or about 25 December, 15 April, 14 June and 31 August. The rest are longer or shorter as measured from the overhead midday sun.
A more precise measure of time was required. So there have been many definitions of the second throughout time:
1884 - International Meridian Conference (Washington DC): based on a mean solar day at Greenwich, England.
1956 - Ephemeris Time: Based on lunar observations
1958 - Atomic Time: Based on clock data from numerous countries
1967 - Atomic Time: based on radiation patterns of the caesium element.
1972 - Co-ordinated Universal Time (UTC) introduced.
1984 - Dynamical time - based on motion of celestial bodies
1986 - UTC replaced GMT.Despite time being measured highly accurately by atomic means the earth time stills rules. Should earth time and atomic time get out of step scientists adjust time by subtracting or adding "leap seconds" on the last day of June or December.
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