Greenwich Mean Time originally referred to the mean solar time at the Royal Observatory
in Greenwich, London, which later became adopted as a global time standard. It is for the most
part the same as Coordinated Universal Time, and when this is viewed as a time zone, the
name Greenwich Mean Time is especially used by bodies connected with the United Kingdom,
such as the BBC World Service, the Royal Navy, the Met Office and others particularly in
Arab countries, such as the Middle East Broadcasting Center and OSN. It is a term commonly used
in the United Kingdom and countries of the Commonwealth, including Australia, South Africa,
India, Pakistan and Malaysia, and many other countries in the Eastern Hemisphere.
Before the introduction of UTC on 1 January 1972, Greenwich Mean Time was the same as
Universal Time, a standard astronomical concept used in many technical fields.
In the United Kingdom, GMT is the official time during winter; during summer British
Summer Time is used. GMT is the same as Western European Time.
Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich
meridian and reaches its highest point in the sky there, because of Earth's uneven speed
in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from
noon GMT, a discrepancy calculated by the equation of time. Noon is the annual average
time of this event, prompting the inclusion of "mean" in "Greenwich Mean Time".
Historically the term GMT has been used with two different conventions, sometimes numbering
hours starting at midnight and sometimes starting at noon. The more specific terms UT and UTC
do not share this ambiguity, always referring to midnight as zero hours. Astronomers preferred
the latter GMT convention to simplify their observational data so that each night was
logged under a single calendar date.
History
As the United Kingdom grew into an advanced maritime nation, British mariners kept at
least one chronometer on GMT to calculate their longitude from the Greenwich meridian,
which was by convention considered to have longitude zero degrees, internationally adopted
in the International Meridian Conference of 1884. Synchronisation of the chronometer on
GMT did not affect shipboard time, which was still solar time. But this practice, combined
with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances
based on observations at Greenwich, led to GMT being used worldwide as a reference time
independent of location. Most time zones were based upon this reference as a number of hours
and half-hours "ahead of GMT" or "behind GMT". Greenwich Mean Time was adopted across the
island of Great Britain by the Railway Clearing House in 1847, and by almost all railway companies
by the following year, from which the term "railway time" is derived. It was gradually
adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official
time. This changed in 1880, when Greenwich Mean Time was legally adopted throughout the
island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and
Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time. Hourly time
signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the
time ball at the observatory obsolete in the process.
The daily rotation of the Earth is somewhat irregular and is slowing down slightly; atomic
clocks constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the
international time reference by Coordinated Universal Time, maintained by an ensemble
of atomic clocks around the world. Universal Time, a term introduced in 1928, initially
represented mean time at Greenwich determined in the traditional way to accord with the
originally defined universal day; from 1 January 1956 this "raw" form of UT was re-labeled
UT0 and effectively superseded by refined forms UT1 and UT2. Leap seconds are nowadays
added to or subtracted from UTC to keep it within 0.9 seconds of UT1.
Indeed, even the Greenwich meridian itself is not quite what it used to be—defined
by "the centre of the transit instrument at the Observatory at Greenwich". Although that
instrument still survives in working order, it is no longer in use and now the meridian
of origin of the world's longitude and time is not strictly defined in material form but
from a statistical solution resulting from observations of all time-determination stations
which the BIPM takes into account when co-ordinating the world's time signals. Nevertheless, the
line in the old observatory's courtyard today differs no more than a few metres from that
imaginary line which is now the Prime Meridian of the world.
Ambiguity in the definition of GMT Historically GMT has been used with two different
conventions for numbering hours. The long-standing astronomical convention dating from the work
of Ptolemy, was to refer to noon as zero hours. This contrasted with the civil convention
of referring to midnight as zero hours dating from the Romans. The latter convention was
adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of
12 hours, or half a day earlier. The instant that was designated December 31, 5 GMT in
1924 almanacs became January 1, 0 GMT in 1925 almanacs. The term Greenwich Mean Astronomical
Time was introduced to unambiguously refer to the previous noon-based astronomical convention
for GMT. The more specific terms UT and UTC do not share this ambiguity, always referring
to midnight as zero hours. Greenwich Mean Time in legislation
Several countries define their local time by reference to Greenwich Mean Time. Some
examples are: United Kingdom: The Interpretation Act 1978,
section 9 provides that whenever an expression of time occurs in an Act, the time referred
to shall be held to be Greenwich mean time. Under subsection 23(3), the same rule applies
to deeds and other instruments. Belgium: Decrees of 1946 and 1947 set legal
time as one hour ahead of GMT. Republic of Ireland: Standard Time Act, 1971,
section 1, and Interpretation Act 2005, section 18(i).
Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1).
Time zone Those countries marked in dark blue on the
map above use Western European Summer Time and advance their clock one hour in summer.
In the United Kingdom, this is known as British Summer Time; in the Republic of Ireland it
is called Irish Standard Time—officially changing to GMT in winter. Those countries
marked in light blue keep their clocks on UTCWET year round.
Discrepancies between legal GMT and geographical GMT
Since legal, political, social and economic criteria in addition to physical or geographical
criteria are used in the drawing of time zones, actual time zones do not precisely adhere
to meridian lines. The 'GMT' time zone, were it drawn by purely geographical terms, would
consist of the area between meridians 7°30'W and 7°30'E. As a result, there are European
locales that despite lying in an area with a 'physical' UTC time use another time zone;
conversely, there are European areas that use UTC, even though their 'physical' time
zone is UTC−1, or UTC−2. Because the UTC time zone in Europe is 'shifted' to the west,
Lowestoft in Suffolk, East Anglia, England at only 1°45'E is the easternmost settlement
in Europe in which UTC is applied. Following is a list of the 'incongruencies':
Countries west of 22°30'W that use UTC The westernmost part of Iceland, incl. the
northwest peninsula and its main town of Ísafjörður, which is west of 22°30'W, uses UTC. Bjargtangar,
Iceland is the westernmost point in which UTC is applied.
Countries west of 7°30'W that use UTC Canary Islands
Most of Portugal, incl. Lisbon, Porto, Braga, Aveiro, and Coimbra. Since the Treaty of Windsor
in 1386, Portugal has maintained close ties to Britain, which possibly explains its choice
of UTC. The Madeira Islands, even further to the west, also employ UTC. A more likely
explanation is that during the mid-1970s, when Portugal was on Central European Time
all year round, it did not begin to get light in Lisbon in winter until 08:30.
Western part of Ireland, incl. the cities of Cork, Limerick, and Galway.
Westernmost tip of Northern Ireland, incl. the county town of County Fermanagh, Enniskillen
Extreme westerly portion of the Outer Hebrides, west of Scotland; for instance, Vatersay,
an inhabited island and the westernmost settlement in Great Britain, lies at 7°54'W. If uninhabited
islands or rocks are taken into account St Kilda, west of the Outer Hebrides, at 8°58'W,
and Rockall, at 13°41'W, should be included. Westernmost island of the Faroe Islands, Mykines
Iceland, including Reykjavík
Countries between meridians 7°30'W and 7°30'E that use UTC+1
Spain. Parts of Galicia lie west of 7°30'W, whereas there is no Spanish territory east
of 7°30'E. Spain's time is the direct result of Franco's Presidential Order abandoning
Greenwich UTC time in favour of UTC+1 effective 23:00 16 March 1940. This is an excellent
example of political criteria used in the drawing of time zones: the time change was
passed "in consideration of the convenience from the national time marching in step according
to that of other European countries". The Presidential Order included in its 5th article
a provision for its future phase out, which never took place. Due to this political decision
Spain is two hours ahead of its local mean time during the summer, one hour ahead in
winter, which possibly explains the notoriously late schedule for which the country is known.
In Portugal, which is a mere one hour behind Spain, the timetable is quite different.
Most of France, including the cities of Paris, Marseilles and Lyon. Only small parts of Alsace,
Lorraine and Provence are east of 7°30'E. Monaco
Andorra Belgium
Netherlands Luxembourg
See also 24-hour watch—24-hour wristwatch
Radio clock Marine chronometer—synchronised with GMT,
and used by ships to calculate their longitude Sandringham Time
Swatch Internet Time—alternative, decimal measure of time
Western European Summer Time Notes
References Dumortier, J, Hannelore, D, & Loncke, M..
"Legal Aspects of Trusted Time services in Europe". AMANO. Retrieved 8 July 2009.
Howse, D.. Greenwich time and the longitude. London: Philip Wilson.
Interpretation Act, R.S.C. 1985, c. I-21.. CanLII.
Interpretation Act 1978. UK Law Statute Database. Interpretation Act 2005. British and Irish
Legal Information Institute. Myers, J.. History of legal time in Britain.
Retrieved 4 January 2008. Seago, J.H., & Seidelmann, P. K.. National
Legal Requirements for Coordinating with Universal Time. Steve Allen of University of California
Observatories. Retrieved 8 July 2009. "Six pip salute". BBC News. Retrieved 9 July
2009. Standard Time Act, 1968. Irish Statute Book.
Office of the Attorney General. Standard Time Act, 1971. British and Irish
Legal Information Institute. External links
Greenwich Mean Time World Time Zone Map with current time
International Earth Rotation and Reference Systems Service
Royal Observatory, Greenwich The original BBC World Service GMT time signal
in mp3 format Rodgers, Lucy. "At the centre of time". BBC
News. Retrieved 20 October 2009.