Talk:Universal Time

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Random Notes[edit]

Merged with the Universal time article which was on the identical subject. A capital 'T' is correct. It is a proper noun. Bluelion 07:01 Mar 27, 2003 (UTC)

See Wikipedia:Naming conventions (capitalization). -- looxix 21:08 Mar 27, 2003 (UTC)

Yea, I've read that quite a few times. It says: Unless the term you wish to create a page for is a proper noun, do not capitalize second and subsequent words. This article is about Universal Time (UT0, UT1, UTC, etc.) It is not about universal time. It's no more valid to title this article Universal time than it would be to title an article 'David letterman' or 'Emily post'. I've noticed articles on other time scales are misnamed, too, although some (like Terrestrial Time) are correct. Someone (not I) has correctly made the Terrestrial time article a redirect to the properly titled Terrestrial Time article. (I am curious about miscapitalization of article titles where the first letter should be lowercase, like pH which is an article entitled PH. What 'PH' is I'm sure I don't know. It's good for a chuckle, though.)

No problem for me (I prefer it like this), but I'm not sure what the "offical" way is. -- looxix 23:22 Mar 27, 2003 (UTC)


I'm not sure what to make of this paragraph.

  • "the rotational time of a particular place of observation" would suggests a sun-time, different for each longitude, but "However, ... different observatories will find a different value for UT0 at the same moment" suggest that theoretically there should be only one.
  • Yet, that lat sentence suggests it's OK to get variation in the values, whereas "because of polar motion, the geographic position of any place on Earth varies" suggests variations would mean the observer failed to correctly take this polar motion into account when calculating UT0.
  • One would expect "a simple subtraction yields UT0" to be useful to determine which interpretation is correct, but unfortunately the actual substraction is not given, somewhat reducing the amount of information in that sentence. Aliter 18:35, 24 Mar 2005 (UTC)
I agree that the wording should be improved. But that will require some explanation of polar motion within the paragraph. The problem is how best to do that without excessively increasing the length of the paragraph. — Joe Kress 07:33, Mar 25, 2005 (UTC)

Subscript text


I had found the Versions section to be extremely difficult to read and understand. It needs to be revised for flow and content, but without sacrificing important information a more knowledgeable reader would find useful. -- RedPoptarts 22:38, 8 March 2007 (UTC)

IERS Conventions[edit]

This article needs revision in the light of the IERS Convensions (2003) [1] and USNO Circular 179 [2]. Unfortunately, this is diffucult to do and still remain "not too technical". ExtonGuy 02:25, 17 March 2007 (UTC)

Let's walk through the steps[edit]

Suppose I'm an observatory and I want to know the current time according to UTC. I guess I locate an extragalactic radio source with a known position in the sky, and use that to infer the angle of the Sun relative to the Greenwich meridian, which gives me UT0. Then I apply various corrections to get UT1. Then I add the requisite number of leap seconds to get UTC. Do I have that all right? --Doradus (talk) 02:20, 26 November 2007 (UTC)

That is not correct. You would add leap seconds to TAI to produce UTC. If you know UT1, then UTC will always be less than a second away from it (in fact, less than 0.9 seconds). The differential for any particular day can be found in bulletins issued by the IERS. For example, the 2008-07-31 issue of IERS Bulletin-A gives the prediction that today (2008-08-07), UT1-UTC is -0.45635 seconds. --Mathew5000 (talk) 12:16, 7 August 2008 (UTC)

Not a single mention of conversion[edit]

Or even of the differences between it and traditional time zones. How could you pinheads miss the most obvious thing this article needs? —Preceding unsigned comment added by (talk) 02:02, 7 February 2008 (UTC)

UT1 link in first section[edit]

click on it and it redirects back to the the 'Universal Time' main page. :D

ie it presntly goes nowhere.

perhaps it should link down to "versions"? Jellyboots (talk) 10:45, 23 April 2008 (UTC)

Digressing history[edit]

I added the time zone map as I figure that many people coming here will want to know the difference between UT and their own local time zone. I recommend deleting the last two paragraphs in the "Universal Time and standard time" section(starting with "Charles F. Dowd"), as they are basically duplications of info from the Time zone article and have no real bearing on the topic of UT. I will delete them if no one objects.--Dwane E Anderson (talk) 01:32, 13 February 2009 (UTC)

World Map of Time Zones[edit]

I suggest that the World Time Zone Map from HMNAO at is superior to the one in the article. It is in PDF (if you click on the image to get higher resolution) and needs to be put into a format that can be used in Wikipedia articles, PNG perhaps. If someone knows how to do that and feels so inclined then please feel free to do it.

Alexselkirk1704 (talk) 21:59, 28 July 2009 (UTC)

The licensing terms of the map mentioned by Alexselkirk1704 are not suitable for Wikipedia. I'm no copyright expert, but I don't think converting it to a different format would solve the copyright problem. --Jc3s5h (talk) 00:11, 29 July 2009 (UTC)

Pesky Lawyers

Alexselkirk1704 (talk) 19:21, 29 July 2009 (UTC)

Another problem with Her Majesty's Nautical Almanac Office map is that it ignores the 45 minute offsets, including Nepal and Chatham Islands. Both the HMNAO and CIA maps split Kiribati with the International Date Line. — Joe Kress (talk) 03:57, 30 July 2009 (UTC)


"It is a modern continuation of Greenwich Mean Time (GMT)..." This makes no sense at all to me. Unfree (talk) 01:40, 7 November 2009 (UTC)

Perhaps it could be phrased better. Universal Time is an umbrella term for several related time scales: UT1, UTC, and UT2 (the latter is essential obsolete). Collectively, they serve the same purposes that GMT used to serve. --Jc3s5h (talk) 02:14, 7 November 2009 (UTC)


"As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT)." This makes no sense to me. It could mean that one term has replaced another, but I doubt it. Unfree (talk) 01:48, 7 November 2009 (UTC)

UT is a measure of the rotation of the Earth. Atomic clocks and other techniques have revealed that the rotation of the Earth is slightly irregular. Astronomers need several kinds of time scale, including a time scale that is as uniform as possible. Ephemeris Time was the first effort at a uniform time scale; it was the independent variable in the equations of motion of the bodies in the solar system. Later it was replaced by TT. For practical purposes, TT is the same as International Atomic Time (TAI), except there is an offset so the value of TT matches the value of ET on 0 h 0 m 0 s January 1, 1977 TAI (see Resolution A4, Recommendation III in the XXIst General Assembly of the International Astronomical Union (1991). --Jc3s5h (talk) 02:31, 7 November 2009 (UTC)

Where does time start?[edit]

At the big bang of course! But the calendar as we use it in the West didn't start at the year 1 or at the big bang, it started on 1 January 46. So presumably all the different definitions of time have a starting 'reference moment'? What are these moments and how were they agreed?The Yowser (talk) 16:00, 27 August 2010 (UTC)

Except for UTC, all the time scales are aimed at providing a form of mean solar time at Greenwich. The exact method of calculation for the various methods has changed over time without a name change. When one method is changed to another, it is generally done so that the new method flows smoothly from the previous method without any time steps. Since the rotation of the Earth repeats everyday, there is no need to think of a starting point.
UTC is a stepped time that ticks at the same rate as International Atomic time, but with the insertion of leap seconds. Since it is necessary to keep track of the total number of leap seconds that have been inserted (and some other sub-second steps and frequency deviations that were used in the early days of UTC) it is useful to think of a starting time.
The international group that, by treaty, regulates the International System of Units is the General Conference on Weights and Measures. In between meetings of that group, the work of the conference is carried on by the International Committee for Weights and Measures. That Committee, in the 1950s, created the Comité Consultatif pour la Définiton de la Seconde (CCDS) to coordinate the work of scientists working on atomic time and ephemeris time. "In 1967, the CCDS standardized atomic time scales by establishing the origin of International Atomic Time (TAI) to be in approximate agreement with 0h UT2 on 1 January 1958." (McCarthy and Seidelmann 2009, p. 83, 87) Jc3s5h (talk) 16:51, 27 August 2010 (UTC)
What happened 1 January 46? Jc3s5h (talk) 16:52, 27 August 2010 (UTC)
I suspect The Yowser is refering to the beginning of the Julian calendar on 1 January 45 BC. If we separate time from the calendar, then time is cyclical, passing through zero whenever one date changes to the next date. For the Western clock, zero hours is midnight because the ancient Romans changed the date at midnight. Every time zone has its own midnight, chosen by each country. — Joe Kress (talk) 07:50, 29 August 2010 (UTC)

Graph of TAI-UT1 and TAI-UTC from 1962 to the present[edit]

I don't know if this graph belongs on this page or not, but I have created a graph showing the number of seconds that have accumulated between the UTC/UT1 clocks and the TAI atomic clock standard, established in 1959. It is interesting to note that the international agencies had to issue corrections several times during a year before deciding on leap seconds in 1972. The data comes from the website. Universal Time Jdlawlis (talk) 02:00, 25 July 2011 (UTC)

Too many solar time articles[edit]

Please see Wikipedia talk:WikiProject Time#Too many solar time articles Jc3s5h (talk) 17:31, 2 August 2011 (UTC)

Pole terminology[edit]

UT0 [] is uncorrected for the displacement of Earth's geographic pole from its rotational pole. But in the Geographical pole article, "geographic[al] pole" is defined as the exact intersection of the rotational axis with the surface, and the CIO (year 1900) pole is called "cartographic pole". I don't know what the standard terminology is, but if there is one, Wikipedia should use it throughout and try not to make it more complicated than it already is. -- (talk) 18:51, 27 January 2012 (UTC)

Suggested change to add clarity to the section on Measurement[edit]

The section on Measurement can perhaps be made more explanatory. Here's an attempt. Exactly where to put it, I am not sure. The first two sentences are already there. If no one seriously disagrees with my proposed explanation, I'll put it after those sentences, in place of some or all of the rest of the paragraph. Suggestions would be welcome.

The rotation of the Earth is somewhat irregular, and is very gradually slowing due to tidal acceleration. Furthermore, the length of the second was determined from observations of the Moon between 1750 and 1890.

Because such a second based on astronomical observations is irregular and changing, the second for the last few decades has, instead, been based on an extremely stable frequency associated with atoms. Although our clocks now give us days that are precisely 86,400 SI (atomic) seconds long, the mean solar day (astronomically determined and the basis of UT) is actually a few milliseconds longer than that. Typically over a few months these milliseconds add up to about a second. Therefore, since they need to reflect UT (in other words, need to be synchronous with night and day), our atomic master clocks need to be adjusted every few months by pausing them for one second to bring them back in line with UT. Each decision to make the adjustment is made by an international committee, and when such a "leap second" is added, it is always added to the last minute of 30 June or 31 December, making that minute exactly 61 seconds in duration. Wikifan2744 (talk) 05:51, 31 August 2014 (UTC)

The passage might be confusing because it seems to treat the atomic second, and its predecessor, the ephemeris second, as the "real" second, and the second defined as 1/86,400 of a mean solar day as some kind of second-class approximation to the real second. Also, the leap seconds are only applied to UTC, but the passage makes it seem like they are applied to all varieties of UT. Finally "a few months" is misleading; the leap seconds are applied about every 18 months. Jc3s5h (talk) 06:47, 31 August 2014 (UTC)

Dispute accuracy of article[edit]

The claim in the lead of this article, 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 same. Jc3s5h (talk) 19:59, 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) 12:15, 19 April 2015 (UTC)

Answered at Talk:Greenwich Mean Time. Jc3s5h (talk) 13:11, 19 April 2015 (UTC)