Talk:Greenwich Mean Time

From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Time    (Inactive)
WikiProject icon This article was within the scope of WikiProject Time, a project which is currently considered to be inactive.
WikiProject London (Rated C-class, Mid-importance)
WikiProject icon This article is within the scope of WikiProject London, a collaborative effort to improve the coverage of London on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 Mid  This article has been rated as Mid-importance on the project's importance scale.

Archive of Time Cube discussion

GMT is not UTC

The opening paragraph says that GMT "is arguably the same as Coordinated Universal Time (UTC)." This statement should be removed or changed to "GMT is a time zone subject to daylight saving time rules; UTC is not a time zone. During summer hours, they differ by one hour."

I've noticed a lot of websites saying that GMT and UTC are the same. They differ in every respect, including value (during summer months). There is no argument; they are not the same.

BTW, it was called Greenwich Meridian Time when I was a kid. When did it change to Greenwich Mean Time? Or was I taught the wrong thing way back then.

Warren Gaebel, B.A., B.C.S. (talk) 17:11, 22 November 2012 (UTC)

You were taught incorrectly; it has always been Greenwich Mean Time. This makes the important distinction between mean solar time and true solar time (the time kept by a sundial).

The Geologist — (continues after insertion below)

It always was Greenwich Mean Time and always has been, but a lot of modern teachers have decided that the way things were done was wrong therefore their way is right. Oh and I went to school in the 1950's and 60's. Even my grandfather who died in 1957 always referred to time as Greenwich Mean Time.The Geologist (talk) 15:36, 18 January 2013 (UTC), PhD, FGS.
I would suggest that "Greenwich Mean Time" is undefined, except in certain limited contexts. I defy you to find a definition from an authority over a large part of the world that gives a current definition of "Greenwich Mean Time" that can be unambiguously determined to a small fraction of a second.
I do think that paragraph needs to be cleaned up to explain that GMT is sometimes used as a synonym for UTC (especially outside the British Isles, where people are uninterested in British civil time), but is also used as a synonym for British civil time. Appropriate citations should be given for both meanings. Jc3s5h (talk) 17:57, 22 November 2012 (UTC)
I too remember reading "Greenwich Meridian Time" at times. Google Books clearly supports this.
I'm here because the Coordinated Universal Time, Greenwich Mean Time, Universal Time, and Leap second articles are confusing as to if and how UTC and GMT stay in sync. When leap seconds are added/removed from UTC does this also affect GMT?
Jc3s5h, I agreed with your comment that GMT is is undefined, except in certain limited contexts but we may need to have that stated in a reliable secondary source. The article should also show the various organizations that define GMT and include exactly how they define GMT and where that definition is legally applicable. --Marc Kupper|talk 19:37, 1 December 2012 (UTC)
I think the most appropriate source is the most recent, 3rd, edition of the Explanatory Supplement to the Astronomical Almanac. The contributing authors come from organizations such as the US Naval Observatory, US Geological Survey, Her Majesty's Nautical Almanac Office, National Geodetic Survey, and the Jet Propulsion Laboratory. At least one of these, the US Naval Observatory, is responsible (together with the National Institute of Standards and Technology) for dissemination of legal time in the US. On page 231–2 that source states:

In the United Kingdom, Greenwich Mean Time has been identified with the civil time or Coordinated Universal Time, UTC (§ This connection, however, has never been formalized, so using GMT to refer to UTC should be done with care. For navigation, however, Greenwich Mean Time has meant UT1 (§ 6.8.3). Thus, GMT has two meanings that can differ by as much as 0.9 s, and the term GMT should not be used for precise purposes.

Of course this passage does not directly address to describing British summer time as GMT.
To answer the question about leap seconds and GMT, when GMT is interpreted as UTC, the leap seconds are inserted into UTC and GMT is just another name for this timescale. When GMT is interpreted as UT1, the seconds of UT1, defined by the rotation of the Earth, are slightly different from, and generally longer than, the atomic seconds of UTC. When the accumulated difference approaches 0.9 s a leap second is inserted into UTC to keep it close to UT1. Jc3s5h (talk) 20:06, 1 December 2012 (UTC)
Thank you. Do you know what "§" and "§6.8.3" are referring to? I'm guessing they are sections within Explanatory Supplement to the Astronomical Almanac itself but wanted to confirm that.
I looked at the Universal Time article to better understand UT1. Something that's not clear is if one second in UT1 is exactly one "International second" or if it's 1/86,400 of a mean solar day. --Marc Kupper|talk 04:22, 4 December 2012 (UTC)

────────────────────────────────────────────────────────────────────────────────────────────────────Yes, "§" and "§6.8.3" are indeed references to other sections in the same chapter of the Explanatory Supplement.

One second of UT1 is effectively one second of mean solar time, because UT1 is the only time scale provided by the scientific community that is intended to be close to mean solar time. However, the expression for UT1 is a linear function of the Earth Rotation Angle, which in turn uses Very-long-baseline interferometry to measure the rotation of the Earth with respect to distant pulsars in other galaxies. The actual position of the Sun is not used in the calculation of UT1. The coefficients in the equation for UT1 were chosen for continuity with the previous definition of sidereal time. If you trace the various definitions back far enough, it turns out that indirectly UT1 is based on the position of the Sun, but there is no direct statement about that in the current definition.

In a paper in a 2011 (v. 48) special issue of Metrologica devoted to time, on page S182, Bernard Guinot wrote

Strictly speaking, the Universal Time, version UT1, is

not a solar time. It is a parameter which, jointly with the coordinates of the moving pole of rotation of the Earth, describes the rotation of the surface of the Earth in space. The knowledge of these parameters is essential for all space techniques with multiple applications, both practical and

scientific, of the utmost importance.

However, all decisions taken until now preserved the role

of UT1, the representation of the mean solar time at the Greenwich meridian as defined by the 1884 Conference, with

a departure which may reach one to two seconds.

It should be noted that since navigational almanacs and software are designed to be used with UT1, the one or two seconds from actual mean solar time described by Guinot will not result in the kinds of small navigation errors that result if navigators don't take the difference between UTC and UT1 into account. Jc3s5h (talk) 09:59, 4 December 2012 (UTC)

"During summer hours, they differ by one hour" by is wrong. GMT is always approximately equal to UTC, never differs by one hour. Because GMT is the time only used in winter in Britain, while BST is the time used in summer in Britain. GMT is approximately equal to UTC, while BST is approximately equal to UTC+1. (P.S. the term "approximately" here is only for "leap seconds") -- Yejianfei (talk) 16:46, 30 January 2015 (UTC)
With all due respect, you are just a Wikipedia editor. There is no particular reason for anyone to pay any attention to your pronouncements. Please look at Greenwich Mean Time#Summer time where reliable sources are cited which make contrary claims; one says GMT is always approximately UT, the other says sometimes the summer time observed in the UK is called GMT. Can you provide a really convincing reliable source that proves one of the cited sources is spouting nonsense? And I'm not talking official definitions, I'm talking about all strata of people in the UK from the National Physical Laboratory down to the sign giving the operating hours of a fishmonger. Jc3s5h (talk) 17:51, 30 January 2015 (UTC)
"Greenwich Meridian Time" is a canard same as "After Death" for the abbreviation "AD". Language is conservative. Thus we still "hang up" the telephone even though that action has not been performed since before the war. So because we used Greenwich mean time before 1972 we still call our winter time GMT although it's something completely different.
I can tell you from here in the UK that the population does not and never has referred to British Summer Time as "Greenwich mean time". I'm going to check these articles and if I find any such claims out they will go. (talk) 09:52, 31 January 2015 (UTC)

User Jc3s5h wrote: "I do think that paragraph needs to be cleaned up to explain that GMT is sometimes used as a synonym for UTC (especially outside the British Isles, where people are uninterested in British civil time), but is also used as a synonym for British civil time." User Marc Kupper wrote: "I'm here because the Coordinated Universal Time, Greenwich Mean Time, Universal Time, and Leap second articles are confusing as to if and how UTC and GMT stay in sync. When leap seconds are added/removed from UTC does this also affect GMT?"

I just edited the introduction section thoroughly because I found it confusing and a bit misleading. I added a note that GMT's function as the international standard civil time has been superseded by UTC; that UTC is kept within 0.9 of UT1 by adding/subtracting leap seconds; and that "GMT" is still misused as a synonym for UTC in Britain and some other places. Where the time zones used in Britain are mentioned, I explain that "GMT" and "BST" actually mean UTC+0 and UTC+1.

I hope this is satisfactory. Teemu Leisti (talk) 23:38, 25 April 2015 (UTC)

Thank you for your help in sorting out the confusing usages. Dbfirs 06:42, 26 April 2015 (UTC)
No worries. Though what I saw as clarification -- that "GMT" is generally misused as a synonym for UTC+0 -- has now been edited out. I really don't have the time and energy at the moment to defend my opinion on what should appear here. Teemu Leisti (talk) 11:07, 27 April 2015 (UTC)


Paris does not, as stated in the article lie East of 7 Degrees 30 Minutes, it's Longitude is in fact 2°20′14.025″ East. This means that local noon occurs just after 11:49 GMT or UCT. The reason why France does not keep GMT and this was recorded at the nference in 1884 which fixed the Greenwich Meridian as the Prime Meridan, is because France / Paris was not selected by the majority of delegates. Ever since then France has refused to accept GMT / UTC as the time reference.The Geologist (talk) 15:31, 18 January 2013 (UTC)

Although your history may seem reasonable given Anglo-French competitiveness, it is wrong. The French delgates to the International Meridian Conference never proposed that the Paris meridian should be Earth's Prime Meridian. Instead they proposed a neutral meridian that would not cut Europe or America, suggesting either the Azores or the Bering Strait. (Proceedings of the International Meridian Conference, pp. 36-84, vote pp. 84-85) The only other initial meridian proposed was Greenwich. [pp.87-99] Because most delegates did not adopt the French proposal, France abstained from voting on all proposals that mentioned or implied Greenwich. accepted resolutions and votes, pp. 199-203
Nevertheless, France kept Greenwich Mean Time for most of the 20th century, but without using the word "Greenwich" in the law. France had already adopted Paris Mean Time for the entire country on 14 March 1891 (except that all French railway times were five minutes slow). In March 1897 a bill was introduced into the Chamber of Deputies that stated "The legal time in France and Algeria is the mean time of Paris, retarded by 9 minutes, 21 seconds." This bill languished in the Senate for 12 and a half years. Beginning midnight March 10/11, 1911, France's legal time was defined as "Paris Mean Time, retarded by nine minutes twenty-one seconds", which happens to be Greenwich Mean Time. The bill also made French railway times conform. Despite the lack of "Greenwich" in the law, newspaper articles stated that this was really l'heure anglaise de Greenwich. Time signals transmitted from the Eiffel Tower changed to Greenwich mean time on 1 July 1911. French hydrographic charts adopted the Greenwich meridian as of 1 January 1914 as a direct result of the sinking of the Titanic on 15 April 1912. On 25 March 1917, zone time based on Greenwich was adopted by French ships at sea, which had been using apparent time. A detailed discussion is provided by Ian R. Bartky, One time fits all (2007) pp. 127-134, 138-153. This remained French legal time until 1978 when Coordinated Univrsal Time was adopted, which also does not use the word "Greenwich". [1][2][3] The equivalent astronomic difference in longitude (including vertical deflection) between Paris and Greenwich corresponding to 9 minutes 21 seconds was 2°20'15". The longitude of the Paris Observatory (Cassini's meridian) was given in the Astronomical Almanac through 1980 as -9m20.91s (-2°20'13.65"), and from 1981 to the present (2013) as -2°20.2' (-2°20'12" or -9m20.8s). — Joe Kress (talk) 00:58, 12 February 2013 (UTC)
At the time the vast majority of the World's shipping was British-registered and as the Royal Navy under such people as George Vancouver, James Cook, William Bligh, et-al, had made accurate Admiralty charts of places that no-one else had ever surveyed (hence if the French, Russians or anyone else wanted safe travel for their ships in these areas they had to use the only available accurate navigational charts, i.e, Admiralty ones) the logical choice for any international primary meridian was Greenwich and therefore GMT, as that was what the RN had used in all its charts, and it was their charts - or illicit copies of them - that were used by almost the entire world shipping community, both civilian and naval.
That's why Greenwich was chosen rather than Paris. — Preceding unsigned comment added by (talk) 17:22, 6 February 2015 (UTC)

House of Lords debate

In the UK the law says the time (in winter) is GMT, but the National Physical Laboratory time transmissions are UTC. A bill was introduced in the House of Lords to settle the confusion by declaring declaring that GMT is UTC, but the law was never passed by the House of Lords. The debate may be found here.

This is why we can't make statements like "GMT is the same as UTC" or "GMT is different from UTC". Some authorities think they're the same, some think they're different, and the House of Lords refuses to answer the question. Jc3s5h (talk) 22:10, 13 August 2014 (UTC)

Dispute accuracy of History section

The claim in the "History" section, introduced by User:‎, that GMT is defined as UT1, is not true.

As Bernard Guinot writes in the abstract of his paper, "The International Conference held in 1884 at Washington defined a universal time as the mean solar time at the Greenwich meridian (GMT). Now, the Universal Time, version UT1, is strictly defined as proportional to the angle of rotation of the Earth in space. In this evolution, the departure of UT1 from GMT does not exceed one or two seconds." Clearly there cannot be a departure between two time scales if they are defined to be the the same. Jc3s5h (talk) 20:07, 18 April 2015 (UTC)

You're citing the abstract of the paper, not the paper itself? Unless you remedy that I'll remove the tag. The only circumstance in which Guinot would be relevant would be if he had experimentally timed an event by both GMT and UTC1 and found a discrepancy. (talk) 12:16, 19 April 2015 (UTC)

I read the paper. Guinot has been involved in many of the critical papers involved in time and earth orientation; anonymous Wikipedia editors have no standing to question his qualifications to make a statement (although other high-quality sources could be used to show other points of view or other terminology preferences).
Once such difference in terminology preference is with the online glossary of the Astronomical Almanac which says "In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC); in this volume, UT always refers to UT1." Guinot does not limit the current meaning so narrowly:

The name of a time and of a time scale either indicates its goal (such as Universal Time) or the mode of construction (such as Ephemeris Time), or both (such as International Atomic Time). Universal Time is a nice name, although stamped with the grandiloquence of the 19th century, because it indicates the goal of its definition, not the technique to build the time scale. But the need to distinguish versions of UT adorned its acronym with suffixes: Universal Time has versions UT0, UT1, UT2, UTR, UTC. As shown by Sadler [9] this situation is far from being satisfactory. [p. S185]

The paper does not define Greenwich Mean Time in so many words, but begins with the methods stated or implied by the 1884 Washington Conference that selected the Airy Transit Circle at the Greenwich Observatory as 0° longitude, measurement of actual transits of the Sun rather than measuring star transits, and using the vertical indicated by spirit levels at Greenwich rather than the normal to a reference ellipsoid. He then describes various changes in techniques and definitions, and estimates how much departure each change would contribute to the difference between the methods implied by the 1884 conference and UT1. I understand the structure of the paper, combined with the abstract, to mean Guinot considered Greenwich Mean Time to be the time measured in the way indicated in the 1884 conference.
Guinot (p. 182) also mentions "In 1955, the International Astronomical Union (IAU) officially decided to select a fixed point on the moving equator as the origin of longitude and to call UT1 (Universal Time 1) the universal time referred to this point." McCarty and Seidelmann (p. 17) devote most of a page explaining why "the use of the term 'Greenwich Mean Time' or its abbreviation 'GMT' remains a source of confusion today." They point out several times when respected authorities have stopped using GMT or recommended that GMT should no longer be used: the IAU in 1928, the 1939 American Ephemeris, astronomical almanacs after 1960, and another recommendation from the IAU in 1976. Since the same organization, the IAU, recommended the discontinuance of GMT in 1928 and introduced UT1 in 1955, it seems implausible they would define GMT as UT1; I would want to see a citation to an IAU publication saying they did such a strange thing before I would believe it. Jc3s5h (talk) 13:06, 19 April 2015 (UTC)
Never mind what the IAU recommended - they are not a judicial body. The British Parliament legislated Greenwich Mean Time back in the 1880s. It has not repealed that legislation, so people like yourself who suggest that leap seconds apply to Greenwich Mean Time are wrong. You have not answered the question I posed - does Guinot have experimental evidence that Greenwich Mean Time has drifted from UT1 - yes or no? Are you suggesting that in all those years from the introduction of Universal Time in 1925 astronomers had no idea of the relationship between it and Greenwich Mean Time?
In Talk:Coordinated Universal Time I explained what the Royal Greenwich Observatory has to say on the matter:

UT universal time; counted from 0h at midnight; unit is mean solar day

prior to 1925 [Greenwich Mean Time] was reckoned, for astronomical purposes, from Greenwich mean noon.

86,400 seconds of Greenwich Mean Time is one mean solar day and it's counted from 0h at midnight: thus it is (and always has been) in frequency and phase exactly the same as Universal Time. The Royal Greenwich Observatory has decided that, unless otherwise stated, "Universal Time" means UT1. Therefore, Greenwich Mean Time is UT1.

The idea that our legislators cannot tell the time is only in your head. The 1968 Standard Time Act provided that the legal time throughout the year should be Greenwich Mean Time plus one hour. If they did not know what Greenwich Mean Time was they would not have said that. They also said that "Nothing in this statute shall affect the use of Greenwich Mean Time in astronomy and navigation". Astronomers and navigators know exactly what Greenwich Mean Time is - if they didn't there would be an awful lot of shipwrecks which hasn't happened. (talk) 15:04, 19 April 2015 (UTC)

If has questions about Guinot's paper he or she should read it. As for the British legislation on time, this is discussed in a paper by Seago, Seidelmann, and Allen. UTC is the defacto legal time throughout the world, and the UK government funds the National Physical Laboratory, which disseminates UTC to the British public, and who then conduct their affairs according to that time scale, while usually describing it as GMT. As for what the Royal Greenwich Observatory says, there are several organizations that have existed with variations of that name, so I can't comment on what they say without seeing the exact text. Jc3s5h (talk) 16:32, 19 April 2015 (UTC)
Concerning the comment above that "If they [legislators] did not know what Greenwich Mean Time was they would not have said that" one might find it entertaining to read relevant debates in the House of Lords:
Jc3s5h (talk) 19:13, 19 April 2015 (UTC)
Lord Fraser of Carmyllie is (or was) a politician, not a scientist. However, he points out that Coordinated Universal Time and Greenwich Mean Time are in agreement to less than one second. Coordinated Universal Time is also in agreement with UT1 to within a second, so that throws Guinot's two second difference out of the window. Since his core argument has been disproved, why bother to read his paper?
You may recall that Lord Chesterfield, who piloted the Gregorian calendar through the Lords in 1750, admitted that he knew very little about the subject, but just made some superficially learned argument to stop the noble lords getting bored and throwing out the measure. Same here.

Greenwich Mean Time, which is not Greenwich Mean Time any more because it is not calculated on a mean - at noon.

is obvious nonsense.

This is an official statement by the British Government confirming that Greenwich Mean Time is not Coordinated Universal Time. Proof of Lord Fraser's ignorance is his conflating delta T and the equation of time. Be that at it may, as the official representative of Her Majesty's Government he confirms that Greenwich Mean Time is and will remain the legal time in these islands. You have said you are not legally trained. Your reference to a "de facto" standard being a legal standard confirms this. The two concepts are opposites.

Further down Lord Fraser categorically confirms that Greenwich Mean Time will always be within 0.9 seconds of Coordinated Universal Time. Since UT1 is kept within the same range GMT=UT1. The statement that the fictitious mean sun was abandoned in 1925 is wrong. It is still used to derive the analemma by which the relationship of sundial time to mean time is established.

I see from Lord Tanlaw's speech that ITU Recommendation TF460 of 1975 was that GMT should continue to be equivalent to Universal Time. Why didn't you mention that? What is the serial number and content of the 1976 recommendation which you do mention? The debate proceeded on the basis that GMT = UT1. The Bill was rejected and that is the end of the matter.

As for your claim that the British public use UTC and call it GMT that is rubbish. I used to work in Westminster, and every fifteen minutes we would hear the bongs of Big Ben. Sound travels at 1100 feet per second, so some people hearing those bongs might set their watches to GMT, others to UTC, others to UT0, others to UT2, others to UTR and so on. The precision of Big Ben is maintained by placing pennies on the mechanism to ensure the correct weight, so the claim that it can distinguish to sub - second levels of accuracy is risible. We are not as stupid as you think. A time display on a GPS unit will be treated as a GPS time, a UTC display will be treated as UTC, a computer display as that of the relevant operating system and so on. Nobody checking a digital watch is going to argue about whether it displays GMT or UTC. There is only one observatory at Greenwich, although the observing moved to Herstmonceux, but the civil service personnel involved are the same.

To wrap this, the RGO publishes times of astronomical phenomena, calculated according to UT1. These are reproduced in almanacs, calendars, diaries and newspapers which inform readers that these are GMT times or, (as the case may be), British Summer times, where the one - hour offset is applied. Apart from that, there is no discrepancy whatsoever between these times, so GMT = UT1.

There is one caveat - before 1925 publishers were sometimes caught out by the 12 - hour difference in starting the day, so sometimes their times are twelve hours out. (talk) 11:20, 20 April 2015 (UTC)

There is no official conclusion from these debates, because the bill they were debating never became law. The general sense of the debate I get is the speakers thought there was a distinction between GMT and UTC, but they also thought that the time being disseminated to the general public, often under the name GMT (in winter), was actually UTC. There was clearly no consensus among the speakers that GMT was UT1; it could be UT0, UT1, UT2, or a hypothetical UT measured with the Airy transit circle using 19th century methods (which of course is not available since the transit circle is no longer in operation). Jc3s5h (talk) 14:11, 20 April 2015 (UTC)
There is a saying Lex non curat de minimis. That maxim has been applied here. The official conclusion from these debates is that there is nothing to worry about - time signals do indeed indicate UTC but as the time you receive depends on how far you are from your radio set and the time you adjust your watch to depends on how quickly you move nothing needs to be done. I endeavoured to explain that the legislators are aware that GMT matches UTC to within 0.9 seconds which makes it UT1. I think it is time you dropped the stick and moved on. (talk) 14:30, 20 April 2015 (UTC)
I'm within a few feet of my radio set when I check the time, and my eyes can move very quickly to my watch. I don't use digital radio (in fact there is no digital signal where I live). I've always believed that the GMT (or BST) time signal was accurate UTC to within a few milliseconds. What's all this about 0.9 seconds? I know of no time signal that uses UT1. I would claim that "mean solar time on the Prime Meridian at Greenwich" (UT1) is NOT Greenwich Mean Time, though they might once have shared a definition. Dbfirs 18:52, 21 April 2015 (UTC)
I agree with the first part of's comment at 14:30, 20 April 2015 (UTC): "The official conclusion from these debates is that there is nothing to worry about - time signals do indeed indicate UTC but as the time you receive depends on how far you are from your radio set and the time you adjust your watch to depends on how quickly you move nothing needs to be done." The Parliament as a whole did not feel a compelling need to do anything about a difference on the order of a second. I don't agree that the Lords in the debate thought GMT was UT1. There grasp of the matter was rather shaky, so when Lord Tanlaw said in June 1997 "The question will no doubt arise as to why there is any necessity to change the law from GMT to UTC when in practice the maximum difference between the two time-scales (UTC and UT) can never exceed 0.9 of a second" we really can't take that as a statement by Tanlaw that GMT is UT1, because Tanlaw also said "The reason why this cannot be done is that UT--that is old GMT since 1975--is a general designation of all time-scales based on the Earth's rotation and therefore becomes ambiguous in that the term represents more than one time-scale." Also in December 1996 he said "Does he further agree that GMT can also be Mean Solar Time—GMST—on the Greenwich meridian? Is he aware that after 150 years Greenwich Mean Time has been removed from the list of international timescales in the current Astronomical Almanac for the reason given on sheet B5 which is, and I quote: 'Greenwich Mean Time is ambiguous'?" Clearly Tanlaw thought there were multiple possible definitions of GMT. Jc3s5h (talk) 19:57, 21 April 2015 (UTC)
The Astronomical Almanac (on page B4) says "The name Greenwich Mean Time (GMT) is not used in this Almanac since it is ambiguous and is now used, although not in astronomy, in the sense of UTC in addition to the earlier sense of UT; prior to 1925 it was reckoned, for astronomical purposes, from Greenwich mean noon (12h UT)." Who writes this rubbish? Somebody in the United States Naval Observatory, no doubt. — Preceding unsigned comment added by (talk)
To Britons, the "Mean" in Greenwich Mean Time is the giveaway. We understand it to be the mean solar time at Greenwich, which is an observatory situated in a pleasant park south east of London. It is the mean version of the time which our sundial would show if it were transported from wherever it happens to be to the Prime Meridian. Sundials don't do leap seconds, so we are well aware that Greenwich Mean Time is not Coordinated Universal Time. — Preceding unsigned comment added by (talk) 12:40, 22 April 2015 (UTC)
Seago, Seidelmann and Allen confirm that "UT1 ... is a precise astronomical measure of the rotation of the Earth on its axis, synonymous with mean solar time at the meridian of Greenwich, sometimes known simply as Greenwich mean time (GMT)". I note that the paper says "UTC is the legal basis for time - keeping for most countries in the world, and de - facto is the time scale used in most others". That's the difference between de jure and de facto - a live - in girlfriend may be a common law wife, but the man is free to marry someone else if he chooses.
The paper confirms "The acronym GMT survives as a common navigational synonym for UT1 despite admonitions from as far back as 1928 that astronomers are advised not to use the letters GMT in any sense for the present". And again "Such imprecise descriptions are sometimes coupled with factually incorrect statements: one example that has now multiplied into the definitions of some dictionaries and technical glossaries is the unusual claim that 'GMT was replaced by UTC in 1986' ".
In summary, "Mean solar time at Greenwich (also known as Greenwich mean time, GMT, or Universal Time, UT1) is an astronomical measure of Earth rotation referenced to the international reference meridian." (talk) 12:57, 22 April 2015 (UTC)
I suppose this all goes to emphasise that "GMT" is not as clearly defined as I thought, but the British Government (or at least their Nautical Almanac Office) seems to think that GMT has been the same at UTC since 1972. Dbfirs 17:10, 22 April 2015 (UTC) (GMT)
Thanks for the link. I've made some changes. (talk) 11:19, 24 April 2015 (UTC)
Your claim that GMT is often considered to be identical to Coordinated Universal Time in the UK is an urban myth, like "Greenwich Meridian Time". The best way to deal with urban myths is to deny them the oxygen of publicity. I'm guessing that you live in the far north of England, but northeners are canny folk. We understand that we cannot avoid coming into contact with metres, kilogrammes, litres, degrees Centigrade and the like but we don't confuse them with yards, feet, inches, pounds, ounces, gallons and degrees Fahrenheit. We know what Greenwich Mean Time is and we know that Coordinated Universal Time is an intruder like these other units. Why, even the name is different. (talk) 17:47, 24 April 2015 (UTC)
It's hard to determine whether most of the British population considers GMT identical to UTC, but it's obvious that they must observe UTC (in the winter) because that is the only time scales disseminated from official sources. And everyone knows they very often call the time they observe in the winter "GMT". And by "observe", I mean the time people set their timekeeping devices to, to the best of their ability. The fact that nearly all of these devices will add a few seconds per day of uncertainty, and that some timekeeping devices are hard to set any closer than a minute, does not change the time scale to which these devices are being steered by periodically resetting them. Jc3s5h (talk) 18:24, 24 April 2015 (UTC)
I'm not sure what all that was about,, or why time or units should be different in London, but did you read the text of the link that you acknowledged? "The time-scale used for general purposes in the United Kingdom is Greenwich Mean Time (GMT)" and "Since 1972 January 1 the Greenwich time signals have been based on an internationally-adopted time-scale known as “Coordinated Universal Time” (UTC)". What do you conclude from these two official statements? Dbfirs 18:17, 24 April 2015 (UTC)
What everybody else (but you, apparently) concludes - Greenwich Mean Time is based on the motion of the sun and Coordinated Universal Time is based on the movement of atoms. (talk) 18:24, 24 April 2015 (UTC)'s assertion at 18:24, 24 April 2015 (UTC), "What everybody else (but you, apparently) concludes - Greenwich Mean Time is based on the motion of the sun and Coordinated Universal Time is based on the movement of atoms", just isn't so. All the astronomical authorities agree GMT is obsolete and, therfore, has no current precise definition. Coordinated Universal Time is based on both the atomic transitions and the motion of the Sun; the length of the seconds is based on atomic transitions while the insertion of leap seconds is based (through more than a century's worth of derivations) on the motion of the Sun. Jc3s5h (talk) 18:35, 24 April 2015 (UTC)
So the Astromomical Almanac, infoplease, Whitakers' Almanac and Adam Hart - Davis are all lying, are they? — Preceding unsigned comment added by (talk) 18:42, 24 April 2015 (UTC)
In real life, it is possible to disagree on definitions without a lie being intended. Dbfirs 19:03, 24 April 2015 (UTC)
GMT is the gold standard. It's the timescale by which Coordinated Universal Time is regulated. Jc3s5h's comment that "All the astronomical authorities agree GMT is obsolete and, therfore [sic], has no current precise definition" shows how out of touch with reality he is. Dbfirs is also out of touch with reality. We are now in politically - correct 2015. The unsourced claim that Britons regard Coordinated Universal Time as identical to Greenwich Mean Time is added solely to make British people look stupid. They simply do not care about the difference between the two timescales. Dbfirs, on the other hand, will no doubt, come 1 AM on the morning of July 1, be rushing around his house moving the display on all his timepieces back by one second. This is the sort of trivia we do not insert into articles. It may well be that there are people who believe that the moon's phases are caused by the passage of the Earth's shadow across its surface. It may well be that some of those people are British. We do not record that. Still less do we write "Some British people believe that the moon's phases are caused by the passage of the Earth's shadow across its surface". (talk) 09:26, 25 April 2015 (UTC)
Your sarcasm and ad hominem arguments are not welcome here. This page is for improvements to the article. In fact many of my clocks are corrected to "GMT" by the 60kHz transmission from Anthorn Radio Station, so I will have no need to rush around on July 1st. I have, in the past, observed the extra second being inserted to bring my clocks in line with UTC which the official link that I provided seems to regard as the same as GMT. I do agree with you that the odd second difference is not really significant for most purposes. Dbfirs 09:51, 25 April 2015 (UTC)

I have restored the version of the article that recognizes the ambiguity in GMT and have replaced the 39 year old citation to a general-interest source with a citation to the Explanatory Supplement to the Astronomical Almanac 3rd ed. (2013), a very widely cited authoritative astronomy text prepared by authors from the US Geological Survey, US Naval Observatory, Her Magesty's Nautical Almanac Office, Observatorie De Paris, National Geodeetic Survey, Jet Propulsion Laboritory, and several observatories and universities. I included a quote in the citation which specifically states that GMT can be regarded as either Universal Time (no variety specified). Jc3s5h (talk) 11:09, 25 April 2015 (UTC)

I wonder if we could compromise on saying that GMT is Universal Time without being dogmatic about which variety? Dbfirs 12:53, 25 April 2015 (UTC)
But it's not. The astronomers say it's UT1. When someone can make 40 million pounds trading in seconds on the stock exchange that's important. (talk) 12:59, 25 April 2015 (UTC)
I agree that, historically, it was the precursor of UT1, and that astronomers continue this definition, even though they average the UT1 observations over observatories round the world, not just at Greenwich. The problem is that traders use UTC and refer to it as GMT (wrongly in your opinion). The fraction of a second difference is indeed important in automated trading. Dbfirs 13:25, 25 April 2015 (UTC)
In every system I've come across the timestamps are UTC. If you've seen different please tell me which systems were involved. (talk) 13:38, 25 April 2015 (UTC)
I think you are correct that the normal international timestamp is "UTC" where exact timing is critical. Nevertheless it is common to associate this with GMT. (For example: [4], [5], [6], [7]). Dbfirs 14:50, 25 April 2015 (UTC)
Everybody's human. Everybody makes mistakes. I don't think it's Wikipedia's job to rub it in. It's common for people to say that we use the "Georgian calendar", but that's not so notable that we mention it in the article. Many people think we use a calender - so much so that Joe Kress made a redirect Gregorian calender>Gregorian calendar. Again, when people come to Wikipedia they want scientific information, not a discourse on common spelling mistakes. (talk) 15:06, 25 April 2015 (UTC)
I appreciate that you think these are all mistakes, but the "error" is so common, even in official government documents (see HM Nautical Almanac Office above), that we have an obligation to report how the abbreviation "GMT" is actually used. Dbfirs 16:46, 25 April 2015 (UTC)

In a recent paper by Rots et al. (2015) it is clearly stated (several times) that GMT is continuous with UTC.

The appendix to this paper further specifies "A.8. Greenwich Mean Time (GMT) is an ill-defined timescale that nevertheless continues to persist in popular parlance as well as scientific papers. Its use is to be discouraged, but if encountered it should be interpreted as UTC, with the caveat that it is rather loosely defined as such and any assertions as to the precision of the time stamps should be regarded with caution."

The question whether GMT in UK civil timekeeping is interpreted as UTC or UT1 can easily be settled by looking how leap seconds are dealt with. If leap seconds are inserted GMT = UTC, if not, GMT = UT1. AstroLynx (talk) 14:29, 27 April 2015 (UTC)

I'm happy to confirm that GMT does not have leap seconds. The authors of the paper you cite say that GMT should be interpreted as UTC, implying leap seconds but they adduce no evidence showing that anybody ever has added leap seconds to it. They appear to know very little about their subject. They say "most terrestrial time stamps prior to 1972 should be expressed as UT (see Sect. A.9) and we recommend specifically that GMT be interpreted as UT for such dates." UTC came in long before 1972, so being logical they should recommend that GMT be treated as UT as much after 1972 as before.
In their definitions section they say as much:

"In time scale UTC the integer part of the seconds field runs from 00 to 60 (in order to accommodate leap seconds); in all other time scales the range is 00 to 59." (talk) 16:22, 27 April 2015 (UTC)

Information icon (talk) is one of several London area IP sockpuppets of banned User:Vote (X) for Change
One of the authors in the cited paper is a UK astronomer - surely he should know something about his trade. You should also look into the paper by Hohenkerk & Hilton (2011), online here, from which I cite (p. S196)
"In many civil applications UTC is often called Greenwich Mean Time (GMT). There is no known legal connection, at least in either the UK or the US, between GMT and UTC. In the UK the name GMT does mean UTC. However, for navigation GMT has meant UT1. Thus, GMT has two meanings that can differ by as much as 0.9 s. For this reason GMT should not be used for precise purposes".
Let us know how your clock behaves on 1 July (1h BST). AstroLynx (talk) 16:48, 27 April 2015 (UTC)
@AstroLynx:, I don't think that's a valid way to show that GMT in UK civil timekeeping is UTC or UT1. There aren't all that many readily available time sources that are accurate to a tenth of a second or so. For the few that are, they could be compared to an accurate source of UTC during a time when DUT1 is large in magnitude (approaching a magnitude of 0.9 s). If the observed difference is close to 0, the source is keeping UTC; if the observed difference is close to DUT1, it's keeping UT1.
For less accurate systems, it would be necessary to observe the process used to reset the source when the inaccuracy becomes large enough to motivate the source owner to reset it. If the source owner resets to a source of UTC; the keeps UTC. Since the scientific community does not directly disseminate UT1, a source owner who wanted to reset the source to UT1 would have to find a time signal that provides some other time scale (UTC, GPS, GLONASS, etc.), look up the difference between the time signal and UT1, and reset the source taking into account the looked-up difference. Of course, most time sources are reset in private and the world does not know for certain how each owner does it, but I would be extremely surprised if any significant number of time source owners in the UK look up the difference between the time signal they are using and UT1.
The fact that a particular time source does not emit 23:59:60 hours 30 June 2015 does not prove it keeps UT1; it may just be keeping a loose approximation to UTC and the one second error will be corrected the next time the time source is reset. Jc3s5h (talk) 16:44, 27 April 2015 (UTC)
I don't live in the UK but one could check whether the coming leap second on 1 July is announced by UK media. Where I live leap seconds are inserted when appropriate (as everywhere else in the world - am I to believe that the UK is the only exception to this rule?). AstroLynx (talk) 16:53, 27 April 2015 (UTC)
If this leap second follows the pattern of previous ones, it will be widely announced in the press, radio and TV, and will be indicated by an extra "pip" in the Greenwich Time Signal. Dbfirs 20:01, 27 April 2015 (UTC)


is 'GMT' EXACTLY the same C.U.T zero-hour?

CorvetteZ51 (talk) 11:21, 30 June 2015 (UTC)

There is no precise definition for GMT because the scientific community, which would provide an exact definition, no longer uses the term. Thus it isn't EXACTLY the same as anything. Also, the correct abbreviation for Coordinated Universal Time is "UTC" in all languages. It isn't clear what the original poster means by "zero-hour". Jc3s5h (talk) 18:21, 30 June 2015 (UTC)