GREENWICH MEAN TIME (GMT) is the mean solar time at the Royal
London . GMT was formerly used as the
international civil time standard, now superseded in that function by
Coordinated Universal Time
Coordinated Universal Time (UTC) . Today GMT is considered equivalent
to UTC for UK civil purposes (but this is not formalised) and for
navigation is considered equivalent to UT1 (the modern form of mean
solar time at 0° longitude); these two meanings can differ by up to
0.9 s. Consequently, the term GMT should not be used for precise
Due to Earth's uneven speed in its elliptical orbit and its axial
tilt , noon (12:00:00) GMT is rarely the exact moment the sun crosses
Greenwich meridian and reaches its highest point in the sky there.
This event may occur up to 16 minutes before or after noon GMT, a
discrepancy calculated by the equation of time .
Noon GMT is the
annual average (i.e. "mean") moment of this event, which accounts for
the word "mean" in "
Greenwich Mean Time".
Originally, astronomers considered a GMT day to start at noon while
for almost everyone else it started at midnight. To avoid confusion,
Universal Time was introduced to denote GMT as counted from
midnight. Astronomers preferred the old convention to simplify their
observational data, so that each night was logged under a single
calendar date. Today
Universal Time usually refers to UTC or UT1.
The term "GMT" is especially used by bodies connected with the United
Kingdom, such as the
BBC World Service
BBC World Service , the
Royal Navy , the Met
Office and others particularly in Arab countries, such as the Middle
East Broadcasting Centre and
OSN . It is a term commonly used in the
United Kingdom and countries of the Commonwealth , including
Australia, New Zealand, South Africa, India, Pakistan and Malaysia;
and in many other countries of the eastern hemisphere. In some
countries (Britain for example)
Time is the legal time
in the winter and the population uses the term. See GMT in legislation
, below, for further explanation.
* 1 History
* 2 Ambiguity in the definition of GMT
* 3 GMT in legislation
* 3.2 Other countries
* 5 Discrepancies between legal GMT and geographical GMT
* 6 See also
* 7 Notes
* 8 References
* 9 External links
History of longitude
Greenwich clock with
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
International Meridian Conference
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 standard time independent of location. Most time zones were based
upon GMT, as an offset of a number of hours (and possibly a half-hour)
"ahead of GMT" or "behind GMT".
Time was adopted across the island of
Great Britain by
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. On 14
May 1880, a letter signed by 'Clerk to Justices' appeared in 'The
Times', stating that '
Greenwich time is now kept almost throughout
England, but it appears that
Greenwich time is not legal time. For
example, our polling booths were opened, say, at 8 13 and closed at 4
13 PM.' This was changed later in 1880, when
legally adopted throughout the island of Great Britain. GMT was
adopted on the
Isle of Man
Isle of Man in 1883,
Jersey in 1898 and
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 redundant
in the process.
The daily rotation of the Earth is irregular (see
ΔT ) and
constantly slows; therefore the atomic clocks constitute a much more
stable timebase. On 1 January 1972, GMT was superseded as the
international civil time standard by Coordinated Universal Time,
maintained by an ensemble of atomic clocks around the world. Universal
Time (UT) , a term introduced in 1928, initially represented mean time
Greenwich determined in the traditional way to accord with the
originally defined universal day ; from 1 January 1956 (as decided by
IAU at Dublin, 1955, at the initiative of
William Markowitz ) this
"raw" form of UT was re-labeled UT0 and effectively superseded by
refined forms UT1 (UT0 equalised for the effects of polar wandering)
and UT2 (UT1 further equalised for annual seasonal variations in earth
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.
— Howse, D. (1997).
Greenwich time and the longitude. London:
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 (see Julian
day ). 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
Time (GMAT) 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.
GMT IN LEGISLATION
Legally, the civil time used in Britain is called still "Greenwich
mean time" (without capitalisation), according to the Interpretation
Act 1978 , with an exception made for those periods when the Summer
Time Act 1972 orders an hour's shift for daylight saving. The
Interpretation Act 1978, section 9, provides that whenever an
expression of time occurs in an Act, the time referred to shall
(unless otherwise specifically stated) be held to be
time. Under subsection 23(3), the same rule applies to deeds and other
During the experiment of 1968-1971, when the British Isles did not
Time during the winter, the all-year British
Time was called British Standard
In Britain, UTC+0 is disseminated to the general public in winter and
UTC+1 in summer.
BBC radio stations broadcast the "six pips" of the
Signal . It is named from its original generation at the Royal
Greenwich Observatory , and is aligned to either
Greenwich Mean Time
British Summer Time
British Summer Time as appropriate for the time of year.
A coded radio signal is broadcast in the UK by
Time from NPL .
Several countries define their local time by reference to Greenwich
Mean Time. Some examples are:
* Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead
* Ireland: Standard
Time (Amendment) Act, 1971, section 1, and
Interpretation Act 2005, part iv, section 18(i).
* Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1).
This refers to 'standard time' for the several provinces, defining
each in relation to '
Greenwich time', but does not use the expression
Greenwich mean time'. Several provinces, such as Nova Scotia (Time
Definition Act. R.S., c. 469, s. 1), have their own legislation which
specifically mentions either "
Greenwich Mean Time" or "
Those countries marked in dark blue on the map above use BST/Western
Time and advance their clock one hour in summer. In
the United Kingdom, this is
British Summer Time
British Summer Time (BST); in the Republic
of Ireland it is called Irish Standard
Time (IST) —officially
changing to GMT in winter. Those countries marked in light blue keep
their clocks on UTC/GMT/WET year round.
DISCREPANCIES BETWEEN LEGAL GMT AND GEOGRAPHICAL GMT
LEGAL TIME VS LOCAL MEAN TIME
1 h ± 30 m behind
0 h ± 30 m
1 h ± 30 m ahead
2 h ± 30 m ahead
3 h ± 30 m ahead
European winter European summer
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 (
UTC+1 in particular);
conversely, there are European areas that use UTC, even though their
'physical' time zone is UTC−1 (e.g., most of
Portugal ), or UTC−2
(the westernmost part of
Iceland ). Because the UTC time zone in
Europe is 'shifted' to the west,
Lowestoft in the
United Kingdom at
only 1°45'E is the easternmost settlement in Europe in which UTC is
applied. Following is a list of the 'incongruencies': Countries (or
parts thereof) west of 22°30'W ("physical" UTC−2) that use UTC
* The westernmost part of Iceland, including the northwest peninsula
and its main town of
Ísafjörður , which is west of 22°30'W, uses
Iceland is the westernmost point in which UTC is
Countries (or parts thereof) west of 7°30'W ("physical" UTC−1)
that use UTC
Canary Islands (
* Most of Portugal, including
Braga , Aveiro , and
Coimbra . (Only the easternmost part, including cities such as
Bragança and Guarda , lies east of 7°30'W.) Since the Treaty of
Windsor in 1386 (the world's oldest diplomatic alliance),
maintained close ties to Britain, which possibly explains its choice
Madeira , even further to the west, also employs UTC. A more
likely explanation is that during the mid-1970s, when
Portugal was on
Central European Time
Central European Time all year round, it did not begin to get light in
Lisbon in winter until 08:30.
* Western part of Ireland , including the cities of Cork , Limerick
* Westernmost tip of
Northern Ireland , including the county town of
County Fermanagh ,
* Extreme westerly portion of the
Outer Hebrides , west of Scotland
; for instance,
Vatersay , an inhabited island and the westernmost
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 (autonomous region of the
Danish Kingdom ), Mykines
Iceland , including
* Northeastern part of
Greenland , including
This arch that stretches over a highway indicates the prime
Spain . Countries (mostly) between meridians 7°30'W
and 7°30'E ("physical" UTC) that use
Spain (except for the Canary Islands, which use UTC). Parts of
Galicia lie west of 7°30'W ('physical' UTC−1), whereas there is no
Spanish territory east of 7°30'E ('physical' UTC+1). Spain's time is
the direct result of Franco 's Presidential Order (published in
Boletín Oficial del Estado of 8 March 1940) abandoning Greenwich
Time and advancing clocks one hour 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 (most likely
enacted to be in synchrony with
Italy , with which the
Franco regime was unofficially allied) 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
* Most of
France , including the cities of
Lyon . Only small parts of
Alsace , Lorraine and
Provence are east of
7°30'E ("physical" UTC+1).
Ruth Belville - the
Time Lady, daughter of John Henry
Belville and personal distribution of
Time via a watch.
24-hour watch —24-hour wristwatch
Marine chronometer —synchronised with GMT, and used by ships to
calculate their longitude
Time in the
Swatch Internet Time
Swatch Internet Time —alternative, decimal measure of time
Western European Summer Time
* ^ Hilton and McCarthy 2013, p. 231–2)
* ^ A B McCarthy & Seidelmann 2009, p. 17.
* ^ Astronomical Almanac Online 2015, Glossary s.v. "Universal
* ^ Howse 1997, p. 114
* ^ CLERK TO JUSTICES. "Time, Actual And Legal". Times, London,
England, 14 May 1880: 10. The Times Digital Archive. Web. 18 Aug.
* ^ Bartky, Ian R. (2007). One
Time Fits All: The Campaigns for
Global Uniformity. Stanford University Press. p. 134. ISBN 0804756422
. Retrieved 18 August 2015.
* ^ A B Myers (2007).
* ^ UT1 as explained on IERS page
* ^ Astronomical Supplement to the Astronomical Almanac. University
Science Books. 1992. p. 76. ISBN 0-935702-68-7 .
* ^ Howse 1997, p. 157.
* ^ A B Dumortier, Hannelore, & Loncke (n.d.)
* ^ Seago & Seidelmann (c. 2001)
* ^ Standard
Time Act, 1968.
* ^ "BOE Orden sobre adelanto de la hora legal en 60 minutos".
Retrieved 2 December 2008.
* ^ "B.O.E. #68 03/08/1940 p.1675". Retrieved 2 December 2008.
* ^ A B "B.O.E. #68 03/08/1940 p.1676". Retrieved 2 December 2008.
* ^ "Hábitos y horarios españoles". Retrieved 27 November 2008.
* Astronomical Almanac Online. (2015). United States Naval
Observatory and Her Majesty's Nautical Almanac Office.
* Dumortier, J, Hannelore, D, & Loncke, M. (n.d.). "Legal Aspects of
Time services in Europe". AMANO. Retrieved 8 July 2009.
* Guinot, Bernard (August 2011). "Solar time, legal time, time in
use". Metrologica 48 (4): S181–185.
Bibcode : 2011Metro..48S.181G.
doi :10.1088/0026-1394/48/4/S08 .
* Hilton, James L and McCarthy, Dennis D. . (2013). "Precession,
Nutation, Polar Motion, and Earth Rotation." In Sean Urban and P.
Kenneth Seidelmann (Eds.), Explanatory Supplement to the Astronomical
Almanac 3rd ed. Mill Valley CA: University Science Books.
* Howse, D. (1997).
Greenwich time and the longitude. London: Philip
* Interpretation Act, R.S.C. 1985, c. I-21. (2005).
* Interpretation Act 1978. UK Law Statute Database. (UK statute)
* Interpretation Act 2005. British and Irish Legal Information
Institute . (Irish statute)
* McCarthy, D., and Seidelmann, P. K. (2009). TIME—From Earth
Rotation to Atomic Physics. Weinheim: Wiley-VCH.
* Myers, J. (2007). History of legal time in Britain. Retrieved 4
* Seago, J.H., & Seidelmann, P. K. (c. 2001). 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.
Time Act, 1968.
Irish Statute Book . Office of the
Attorney General. (Irish statute)
Time (Amendment) Act, 1971. British and Irish Legal
Information Institute . (Irish statute)
Greenwich Mean Time