Talk:Greenwich Mean Time

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Archive of Time Cube discussion

GMT is 'atomic time', or 'earth angle time' ?[edit]

[Note: above heading was added at 13:08, 2 October 2011 by User: CorvetteZ51.

There is no answer for this. The scientific community has retired the term "GMT". Some countries, like the UK, have laws that set civil time as GMT, or an offset from GMT, but the UK parliament has declined to make any law clarifying the current meaning of GMT in the law. As far as I know, no other country with such laws has clarified the meaning of GMT since the introduction of UTC. Jc3s5h (talk) 14:27, 2 October 2011 (UTC)

OK, I give up. UTC is what I call, 'cogged - international atomic time', which is kept close to the mean-sun with 'leap seconds'. UTC is always adjusted in units of a whole second. OTOH, UT-zero is continuously adjustable and represents the angle of the earth. which of those two us closer to GMT? CorvetteZ51 (talk) 09:47, 6 October 2011 (UTC)
The only country I know of that officially bases its time scale on "Greenwich Mean Time" is the UK. Who knows if their law is even valid, it might be overridden by some European Community law. Of course, there may be other countries that base their time on GMT, I just don't know about them.
The UK House of Lords debated a bill that would have changed to UTC. One sentence from the debate is "The use of Greenwich Mean Time for scientific purposes duly died out, although the term lives on in everyday parlance. One reason the scientific community no longer uses the term GMT--as the noble Lord, Lord Tanlaw, explained to us--is that we are no longer really sure what GMT is." The bill did not pass. So my interpretation is that the UK has been asked to clarify what GMT is, or else stop using it, and they refused to act. Jc3s5h (talk) 12:00, 6 October 2011 (UTC)
Of course we refused. Greenwich is in England, and we invented the concept of standard time. --Redrose64 (talk) 12:57, 6 October 2011 (UTC)

GMT is not UTC[edit]

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)


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)


I have added some more wiki links on "time zone", trying not to overdo it.

This help request has been answered. If you need more help, place a new {{help me}} request on this page followed by your questions, contact the responding user(s) directly on their user talk page, or consider visiting the Teahouse.

Is there a policy on multiple links to terms used frequently in an article?

My personal preference would be one & no more than one per para, or perhaps 1 per section; one per article is not enough IMHO, especially in a long article, where the reader may only be interested in one section. D A Patriarche (talk) 05:23, 3 March 2014 (UTC)

  • From the MoS: "Generally, a link should appear only once in an article, but if helpful for readers, links may be repeated in infoboxes, tables, image captions, footnotes, and at the first occurrence after the lead." Bjelleklang - talk 08:00, 3 March 2014 (UTC)

House of Lords debate[edit]

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)

See the third thread above. No question was asked. An MP introduced a bill to change our legal time from Greenwich mean time to Coordinated Universal Time and the House rejected the idea. Very sensible. Joining Coordinated Universal Time would be like joining the European Monetary Union - a total disaster. UTC is set to start drifting away from God's time this November. (talk) 09:52, 31 January 2015 (UTC)

Dispute accuracy of History section[edit]

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)