Talk:Universal Time

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Random Notes

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)

UT0

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

'Technical'

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

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

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

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 68.210.211.188 (talk) 02:02, 7 February 2008 (UTC)

UT1 link in first section

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

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

I suggest that the World Time Zone Map from HMNAO at http://www.hmnao.com/nao/miscellanea/WMTZ/ 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)

Continuation

"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)

Replacement

"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?

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

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 http://maia.usno.navy.mil/ser7/ website. Universal Time Jdlawlis (talk) 02:00, 25 July 2011 (UTC)

Too many solar time articles

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

Pole terminology

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. --88.73.32.237 (talk) 18:51, 27 January 2012 (UTC)

Suggested change to add clarity to the section on Measurement

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

The claim in the lead of this article, introduced by User:156.61.250.250‎, that GMT is defined as UT1, is not true.

As Bernard Guinot writes in the abstract of his paper,[1] "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. 156.61.250.250 (talk) 12:15, 19 April 2015 (UTC)
Answered at Talk:Greenwich Mean Time. Jc3s5h (talk) 13:11, 19 April 2015 (UTC)
Information icon 156.61.250.250 (talk) is one of several London area IP sockpuppets of banned User:Vote (X) for Change

References

  1. ^ 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. 

References

Overcomplication of lead?

In this edit Guy Harris has added to the complexity of the lead. I suggest his edit summary, "Actually, UTC is *not* purely based on Earth's rotation, it's based on Mr. Cesium Atom, with leap seconds added to make it track UT1; only the leap seconds have to do with Earth's rotation", overemphasizes the atomic clock aspect and underemphasizes the leap second aspect. So long as leap seconds are provided, UTC is a perfectly adequate approximation to UT1 for nearly all purposes. Also, the previous version of the lead was not inaccurate; leap seconds are one of the "other adjustments" mentioned. And of course UTC is explained in the "Versions" section. Jc3s5h (talk) 12:39, 3 July 2015 (UTC)

The point is that UT1 and the other variants except for UTC is determined by a process that starts with astronomical observations and possibly smoothing the results, whereas UTC is determined by taking TAI and adjusting it to come close to the astronomical observations; the former process leads to a variable-length second, the latter process leads to 23:59:59 sometimes going to 23:59:60 (or 23:59:58 going to 00:00:00 the next day, or 23:59:59 going to 23:59:60 going to 23:59:61, but those haven't happened yet). I'd say that claiming just that UTC is "based on Earth's rotation relative to distant celestial objects" is an oversimplification. Guy Harris (talk) 16:57, 3 July 2015 (UTC)
The difference between sidereal and atomic standards for time seems highly significant and worth mentioning up front. Ideally the lead would be more clear in distinguishing the different ways of calculating of "Universal Time", along with the reasons for, and the scope of, their present application. An improved lead might possibly devote its first paragraph to explaining Universal Time as a unitary concept, and a second paragraph to describing these different variations. assalaam alaikum, groupuscule (talk) 19:57, 3 July 2015 (UTC)
It seems to me if the lead were written as groupuscule suggests, almost the entire article would be in the lead and hardly anything would be left for the body of the article. It seems to me the chief point to be made in the lead is that Universal Time is a set of similar solar time scales taht are suitable for use throughout the world, for situations where it is inconvenient to use local time. Jc3s5h (talk) 20:09, 3 July 2015 (UTC)
The solar time article says that "Solar time is a reckoning of the passage of time based on the Sun's position in the sky." Universal time is a reckoning of the passage of time based on the Earth's rotation relative to astronomical bodies other than the sun or based on Mr. Cesium Atom with some tweaks in the way the time is represented to keep it from getting too far from the reckoning of the passage of time based on the Earth's rotation.
So there's "solar time" based on the actual passage of the Sun across the sky, and there's "solar time" based on the passage of a bunch of distant objects across the sky, and there's "solar time" based on an atomic clock with tweaks to make sure the HH:MM:SS time based on the atomic clock isn't too far from the HH:MM:SS time based on the passage of the distant objects across the sky.
If the point is to say "these are the replacements for GMT", I wouldn't bother talking about what they're based on in the lede, and just that they try not to diverge too far from a system that says that 00:00:00 is midnight based on the position of the mean sun, and give the details in the "Versions". Just don't say that UTC is based on the movement of anything in the sky; it's based on microwaves from cesium atoms, with the way that the HH:MM:SS display of that clock works tweaked to stay within 0.9 seconds of UT1. Guy Harris (talk) 21:21, 3 July 2015 (UTC)
Guy Harris, would you be willing to suggest a version of the lead comparable to the level of complexity before your edit, but without any wording you feel is misleading? Jc3s5h (talk) 22:23, 3 July 2015 (UTC)

Edit to lead by Groupuscule

I'm afraid there are many serious problems with Groupuscule's attempt to improve the lead:

  • Universal Time (UT) is a time standard based on Earth's rotation which is used around the world to coordinate clocks.
Coordinating clocks is only one of many uses; putting this in the first sentence suggests this is the only, or main, use. But the small number of people directly involved in coordinating clocks (presumably employees of national time laboratories) are vastly outnumbered by the number of people who fly on airplanes, email people in distant parts of the world, amateur radio operators, soldiers, sailors, etc.
True.
  • Midnight at Greenwich is 0:00 UTC. Standard time is calculated for each time zone based on a fixed offset from Greenwich.
Not during the summer.
  • People around the world use this common time system for activities such as transportation and communication, which require coordination at a distance.
See above.
No, ITU only provides the definition of UTC; other entities announce leap seconds and provide values of UT0 and UT1. The remaining versions are calculated by formulas from UT1.
  • Most of these methods, notably UT1, assess Earth's rotation directly based on measurement of distant celestial objects (stars and quasars), with a scaling factor and other adjustments to make them closer to solar time.
True.
I view UTC as a compromise between UT1 and TAI; I don't view it as being based more on one than the other. It's like trying to say if a person is based more on the mother or the father. Jc3s5h (talk) 18:17, 8 July 2015 (UTC)
OK, thanks for these clarifying comments. Perhaps we can work together on a lead which provides more clarity to the lay reader. Here's a revised version of the expanded lead:

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT)—i.e., the mean solar time on the Prime Meridian at Greenwich. Midnight at Greenwich is 0:00 UTC. Standard time is calculated for each time zone based on a fixed offset from Greenwich. Universal Time is used around the world to coordinate clocks, enabling synchronized long-distance activities such as transportation and communication

Universal Time is kept by different methods, which international organizations such as the International Telecommunication Union periodically calibrate.[1] Most of these methods, notably UT1, assess Earth's rotation directly based on measurement of distant celestial objects (stars and quasars), with a scaling factor and other adjustments to make them closer to solar time. Coordinated Universal Time (UTC) counts seconds based on International Atomic Time, which measures the regular activity of cesium atoms. Because these two standards diverge, leap seconds are regularly added to UTC in order to keep it within 0.9 seconds of UT1.

References

  1. ^ Guinot 2011, p. S181.
You could improve this text further, until we have something that works. Regarding coordination of clocks, transportation, & communication, I think it is good to describe these things even if we can't be exhaustive. The phrasing "such as" should make it clear that other activities could be involved. Also: when you write "Not during the summer", are you referring to local shifts to daylight savings time, or to an exception to the UTC = GMT rule? (I'm under the impression that "GMT" as such doesn't change during the summer.) I think our readers will appreciate specific information along these lines. thanks & aloha, groupuscule (talk) 18:50, 8 July 2015 (UTC)
Every few words I see something that is wrong. Lets start with the existing version and you could explain why you think it isn't good enough. Jc3s5h (talk) 19:44, 8 July 2015 (UTC)
The current version is opaque and circuitous, as some comments on this talk page attest. More detail is required to flesh out the precise nature and practical application of "Universal Time". You seem to have some expertise in the matter, which is great, but I suspect that some things which seem obvious to you are not obvious to everyone else. In my opinion the lead should describe precisely the relationship between the terms "Universal Time", "Universal Coordinated Time", and UT1 (if not other forms of UTx), as well as suggest where and how "Universal Time" is applied. Thus some description of the relationship of "Universal Time" to earth's various time zones is warranted. Do you see what I mean? If you think I've gotten the concept totally wrong, try explaining it to me in a way that will correct my misunderstanding—and then maybe we can incorporate your explanation into the lead! groupuscule (talk) 20:02, 8 July 2015 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── OK, lets discuss some of the issues with the version you added at 18:50 UTC. "Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT)—i.e., the mean solar time on the Prime Meridian at Greenwich" is fine.

But when you get to "Midnight at Greenwich" you start to have problems. You have strayed away from terms that have fairly clear definitions. Is "Midnight at Greenwich" the same as "midnight Greenwich Mean Time" or is it midnight as observed by folks walking around the streets of Greenwich? It's different in summer. So try to avoid making up any phrases and stick to reasonably well-defined phrases. Also, it says "Midnight at Greenwich is 0:00". But 24:00 is also midnight at Greenwich. Then "UTC" is tacked on the end. But there is also midnight UT1, plus some obsolete versions of UT.

Next let's look at "Standard time is calculated for each time zone based on a fixed offset from Greenwich." Again, the made-up phrase "fixed offset from Greenwich" is a problem. In practice, both standard time and summer time is calculated for each time zone based on a fixed offset from UTC. But to the best of my knowledge,neither the Supreme Court of the United Kingdom, the Supreme Court of Canada, nor any other counrty that has laws defining time zones in terms of Greenwich Mean Time have taken up the issue of whether Greenwich Mean time is UT1 or UTC; until they do, we can't be completely sure.

I'm not a big fan of "Universal Time is used around the world to coordinate clocks, enabling synchronized long-distance activities such as transportation and communication". Defining time zones is one issue. Setting clocks to a fixed offset, that is coordinating clocks, is a combination of examining a source of time and applying a defined time zone offset. Engaging in aviation and international communication is still another activity; the people doing this may have a UTC clock on their wall or the corner of their computer screen, or they may be making a mental adjustment from local time to UTC.

"Universal Time is kept by different methods, which international organizations such as the International Telecommunication Union periodically calibrate" isn't really right either. Yes, there are international organizations, mainly the International Earth Rotation and Reference Systems Service, which provide the difference between UTC and UT1. UT0 is not provided by any international organization; I'm perfectly free to go out in my back yard, measure some stars with my theodolite and a stopwatch, and call it UT0. Nobody in the official or quasi-official timekeeping community provides UT0 anymore. Which brings up the point that no one is in charge of the term "UT" or "universal time". It's a loose term that applies to any attempt to keep the mean solar time at Greenwich, even if it's my cuckoo clock that I inherited from my great-grandfather.

"Most of these methods, notably UT1, assess Earth's rotation directly based on measurement of distant celestial objects (stars and quasars), with a scaling factor and other adjustments to make them closer to solar time." Well, they all are intended to be close to mean solar time, which is different than apparent solar time; nowadays UT1 is usually used as the timescale closest to mean solar time, but before 1972 UT2 was the timescale that UTC tracked.

"Coordinated Universal Time (UTC) counts seconds based on International Atomic Time, which measures the regular activity of cesium atoms. Because these two standards diverge, leap seconds are regularly added to UTC in order to keep it within 0.9 seconds of UT1." Better replace "these two standards" with "UT1 and TAI". Otherwise you have a circular argument. Jc3s5h (talk) 22:41, 8 July 2015 (UTC)

Thanks for your thoughtful comments. Here's another attempt:

Universal Time (UT) is a set of closely related time standards based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT)—i.e., the mean solar time on the Prime Meridian at Greenwich. Midnight in Universal Time equals midnight in Greenwich Mean Time. Universal Time is used as a worldwide standard, enabling synchronized long-distance activities such as transportation and communication

Universal Time is kept by different methods, some of which are periodically calibrated by international organizations.[1] Most of these methods (UT0, UT1, UT2) assess Earth's rotation directly based on measurement of distant celestial objects (stars and quasars), with a scaling factor and other adjustments to make them closer to mean solar time. Coordinated Universal Time (UTC) counts seconds based on International Atomic Time (TAI), which measures the regular activity of cesium atoms. Because TAI, an atomic constant, diverges from UT1, based on the Earth's rotation, leap seconds are regularly added to UTC in order to keep it within 0.9 seconds of UT1.

References

  1. ^ Guinot 2011, p. S181.


Does this look any better? The last two sentences in the first paragraph describe (1) How a lay person could figure out approximately what time it is in UT, and (2) Why UT has any significance whatsoever. I think these are important questions to answer in the lead. I have changed the statements to better reflect some of your concerns. Again, you might consider tinkering with the language yourself. shalom, groupuscule (talk) 21:25, 10 July 2015 (UTC)
I'd change "Because TAI, an atomic constant, ..." to "Because TAI, an atomic time scale, ...". Guy Harris (talk) 21:55, 10 July 2015 (UTC)
OK, changing this in the text. groupuscule (talk) 22:08, 10 July 2015 (UTC)
That's not bad, with three exceptions. I agree with Guy Harris's comment at 21:55. Also, I would remove the sentence "Midnight in Universal Time equals midnight in Greenwich Mean Time" because it doesn't really add anything, and also because different people have different ideas of what GMT is, so it's better not to say it is "equal" to anything; just try to ignore GMT as much as possible. Finally, we should add something about UT never being advanced or retarded due to summer time (also known as daylight saving time). Jc3s5h (talk) 22:10, 10 July 2015 (UTC)

A further rewrite:

Universal Time (UT) is a set of closely related time standards based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT)—i.e., the mean solar time on the Prime Meridian at Greenwich. Midnight in Universal Time corresponds approximately to midnight on the Prime Meridian but never changes for daylight savings time. Universal Time is used as a worldwide standard, enabling synchronized long-distance activities such as transportation and communication

Universal Time is kept by different methods, some of which are periodically calibrated by international organizations.[1] Most of these methods (UT0, UT1, UT2) assess Earth's rotation directly based on measurement of distant celestial objects (stars and quasars), with a scaling factor and other adjustments to make them closer to mean solar time. Coordinated Universal Time (UTC) counts seconds based on International Atomic Time (TAI), which measures the regular activity of cesium atoms. Because TAI (an atomic time scale) diverges from UT1 (based on the Earth's rotation), leap seconds are regularly added to UTC in order to keep it within 0.9 seconds of UT1.

References

  1. ^ Guinot 2011, p. S181.

I think this is in keeping with your comments, Jc, but still gives the reader some anchor for understanding how UT corresponds to times of day. Of course it is very tricky to find the right wording, since our natural language and syntax do not prepare us well to discuss even the concept of "universal time". I also changed the commas in the last sentence to parentheses so readers can more easily discern the two main clauses. shalom, groupuscule (talk) 21:26, 11 July 2015 (UTC)

I can see you've been making a lot of efforts to get the lead technically correct, but is the plain sense being forgotten? "A set of closely related time standards" mightn't be meaningful to the naive reader of Wikipedia. Imo, the first two sentences of the current lead are fine. It's when it comes to the third sentence the whole thing falls apart. That's the time to refer to the different standards.
The discussion of midnight is a distraction: midnight has a precise astronomical definition and stays with UT throughout the year. Obviously people continue to refer to 12am as midnight during summer time so you need to qualify it, but saying "approximately" is wrong. This suggests it varies as with solar times which it doesn't. Chris55 (talk) 23:10, 11 July 2015 (UTC)
So what time is it, at this instant, in Universal Time? If there's more than one answer for a given instant, UT isn't a standard, it's a set of standards. If the naive reader is reading this article, one part of their naiveté that needs to be dispelled is the notion that there's only one flavor of UT, so I think the first sentence should make it clear that there isn't a single time called "Universal Time".
And we need to somehow indicate that UT doesn't pay attention to daylight savings time/summer time rules. If discussing midnight (a term that I suspect most naive users probably don't know has a precise astronomical definition, separate from "when my clock says 12:00 AM/00:00/0h0m/whatever") isn't the way to do it, what would you suggest? Guy Harris (talk) 23:53, 11 July 2015 (UTC)
There's plenty of space to discuss these things in the article. We're talking about the lead. And one needs a sense of proportion. Most readers will not know that solar time varies up to 16 minutes from the mean during the year, nor that midnight is only exact if you are on the appropriate meridian. In contrast they probably are aware that if you change your clock on one day in the spring, the earth doesn't instantly change in sync. (In UK we're currently running on BST not GMT.) Since all solar times are actually computed from sidereal times the precise definition is clearly going to be difficult, particularly as scientific and engineering needs get continually more exacting. Chris55 (talk) 10:39, 12 July 2015 (UTC)
One thing that is noticeably lacking from the article is that UT uses a 24 hour clock. I say this because although it was Fleming's original motivation, many Americans still haven't caught on. A possible lead sentence taken from the Time standard article is: "Universal Time (UT) is a time scale based on the mean solar day, defined to be as uniform as possible despite variations in Earth's rotation." This avoids getting into the standards issue too soon and allows you to discuss the different implementations of the idea. Chris55 (talk) 12:58, 12 July 2015 (UTC)
No, universal time is about what the time is, not how it is represented. It is perfectly correct for me to write that the date and time as I write this is Julian day 2457216.089236 UT1.
I agree that the 24 hour clock was not written in to the 1884 agreement, only 24 time zones (more the pity). However the Julian day is counted from midday not midnight, and it was this ambiguity which led to the introduction of the term "Universal Time" in the first place.
Furthermore, that sentence from Time standard is wrong. Mean solar time is meant to smooth out variations due to non-uniformity in the Earth's revolution about the Sun in it's orbit and the tilt of the equator with respect to the plane of the orbit. Variations in the Earth's rotation on its axis are much smaller than these other variations, and were not verified until the 1930s, thousands of years after the invention of mean solar time. Universal time is just mean solar time at 0° longitude (with all the caveats expressed earlier in the thread). Jc3s5h (talk) 14:16, 12 July 2015 (UTC)
You're getting confused between mean solar time and UT. UT was only created in 1884 (and named in 1935). Yes, it's "just" mean solar time at the Greenwich Meridian, but that's the whole point. It's an international agreement not a fundamental constant of nature. I do not interpret "variations in Earth's rotation" as anything to do with its ellipsoidal orbit or the tilt of its orbit. Chris55 (talk) 20:12, 12 July 2015 (UTC)
In solar system astronomy, "rotation" only refers to the spinning motion of a planet on its axis. The planet "revolves" about the Sun in an orbit. See, for example, http://spaceplace.nasa.gov/review/dr-marc-earth/earth-rotation.html. The Earth's rate of rotation on its axis is so nearly constant that it couldn't be confirmed until the invention of quartz-crystal electronic oscillators in the 1930s. Jc3s5h (talk) 23:41, 12 July 2015 (UTC)