Talk:ΔT (timekeeping)

Equation

To basically sum up what Delta T is you just use the equation TT−UT1=Delta T.
— 68.47.224.141 on July 1, 2004.

Delta T is given by 32.184 + (TAI−UTC) + (UT1−UTC) seconds, where (TAI−UTC) is the number of leap seconds. (UT1−UTC) changes continually as the Earth's rotational rate varies; it's actual value must be determined by observation, but is often predicted by formulas. Leap seconds are adjusted to keep abs(UT1−UTC) below 1.0 seconds.

 — Preceding unsigned comment added by 184.58.1.86 (talk) 18:28, 29 May 2020 (UTC) 

Delta minus T

The title of this article was changed by Michael Hardy on December 30, 2004 to Delta minus T and all other instances of it, both here and in other articles, to Δ − T because he misread the hyphen in delta-T as a minus sign. Accordingly, I have changed its title to Delta T, which seems to be more popular than delta-T, to avoid further misunderstandings, and changed all other instances of it into its formal form, ΔT, as part of my rewrite. — Joe Kress 05:04, Jan 16, 2005 (UTC)

Missing unit of measurement?

"This means that at −500 Earth's faster rotation would cause a total solar eclipse to occur 70° to the east of its location calculated using the uniform TT."

I think this sentance needs clarification. What is "−500 Earth's faster rotation"?

Somewhat better wording might be: "Earth's faster rotation at the year −500". Here, astronomical year numbering is being used, were −500 = 501 BC. However, even more clarification seems to be needed, if I could determine a better way to explain the concept. — Joe Kress 06:12, 3 January 2006 (UTC)

Delta T (Psychrometrics)

Just to advise, I work in Air Conditioning and Delta T is commonly used in Psychrometrics (the study of air) to indicate the temperature difference.

A typical formula is $Q=M\times C\times \Delta T$, where: Mass (flow rate) multiplied Constant specific heat capacity (of air) multiplied by the ΔT (difference in temperature) will result in Q (Heat added to air). (Note: This isn't 100% accurate, I'm attempting to find specific information on this which can be used) - ~Xytram~ 15:38, 22 November 2006 (UTC)

I'm adding a disambiguation link at the top of the article, but it would certainly be helpful if those formulas or some other mention of ΔT be in the psychrometrics article. It is standard to indicate such as redirect by placing the target phrase in bold, for example, ΔT ('''ΔT'''), so that the reader's eye is drawn to it when they get to the alternative article. — Joe Kress 23:41, 22 November 2006 (UTC)

Thanks, although I know that dT is used in A/C and I have a little training in psychrometrics, I don't feel qualified enough to edit the main page. I'll post something in the talk page there. ~Xytram~ 20:42, 24 November 2006 (UTC)

n-dot symbol

It appears we have problems with a symbol used for the tidal acceleration of the Moon's mean motion. There used to be a capital Gamma here. However the literature uses a dotted n. Recently I approximated this with an accented n (ń) from the Wiki markup characters, which I found acceptable because primed variables are commonly used to indicate derivatives. Joe Kress inserted a proper unicode char. Unfortunately it does not disply in my Internet Explorer, and I suppose this will bother more readers. Therefore I suggest to write d(n)/dt , for which the dotted n is an abbreviation; I believe it was originally in this article anyway. Tom Peters 09:46, 3 January 2007 (UTC)

I was able to view the n dot character in both Mozilla Firefox and in Internet Explorer 7, but not in IE6 on my old computer (it's a square). Many of the Wiki markup characters are also squares in IE6, but I can view all in either IE7 or Firefox. I'm willing to accept whatever you think is best, ń or d(n)/dt or dn/dt. — Joe Kress 07:42, 4 January 2007 (UTC)

TAI based on TT

The article currently says that TT is based on TAI and inherits its non-uniformity at the 10^-14 level. This is not correct: TAI is actually a realisation of TT, and TT is by definition perfectly uniform. I corrected the article diff, but User:Joe Kress reverted my correction. What's your reasoning, Joe Kress? Your edit summary didn't explain what was wrong with my text. 195.224.75.71 14:28, 7 February 2007 (UTC)

Well, I can't speak for Joe, but platonic idealism has nothing to do with time scales, and anonymous edits are suspect anyway. Make a decent account if you want to make serious contributions. In any case, there is no clock that actually runs close to TT, while any cesium clock will show you the time close to TAI. So TT is a theoretical time scale and is derived in practice from TAI. On the other hand TAI does not depend in any way on TT. Tom Peters 15:02, 7 February 2007 (UTC)

I may have been too hasty. I had not seen "platonic" used in relation to time scales, so I assumed it was vandalism, which was reinforced by the separation of several paragraphs at random locations (these minor edits were ignored by Wikipedia) and the change was made by an anonymous editor. Your edits would be given more authority if you sign in to Wikipedia at the upper right hand corner of any page (choose any name or handle). I now see that "platonic" means an ideal definition in contrast to its physical realization. The problem is that Terrestrial Time has both, so we cannot regard TT as ideal and TAI as real. A realization of TT is TT(BIPM06), with an associated error. It would be pedantic to claim that TT without any parenthetical realization can only mean the ideal version. — Joe Kress 01:56, 9 February 2007 (UTC)

I'm reapplying the change. The relation between TT and TAI is already explained in their articles, and Terrestrial Time has included a "platonic" link since I revised it in 2006-04. You must have seen that link, although you don't remember it, because you edited that article yourself for style shortly after my work. Unadorned "TT" does, strictly, refer only to the theoretical time scale. "TT = TAI + 32.184 s" is a common misunderstanding, and to propagate it (and all that it implies) is wrong. I think an encyclopaedia article does need to be strictly accurate, particularly on a matter so closely related to the article's main topic. As for signing in, I prefer not to register. I'm not entirely anonymous: see my user page. Finally, the unwanted line wrapping is a misfeature of Lynx, which I put up with because it doesn't affect how the article renders. 195.224.75.71 09:59, 13 February 2007 (UTC)

Yet I removed it again. "platonic idealism" is a completely inappropriate description of modern theoretical concepts. Plato nor any one of his known followers had anything to say about TT. I see no evidence that astronomers who introduced TT were driven by platonic idealism. As for the anonymity of User:195.224.75.71: you do state the legal name of Andrew Main, and the alias Zefram. Suit yourself, but by continuing contributing under an apparently anonymous IP address you will keep confusing people like you did Joe and me. Tom Peters 21:04, 13 February 2007 (UTC)

I'm reasonably happy with "theoretical" in place of "platonic", which is what you've done. I disagree with your notion that Plato has to be consulted on what qualifies as platonic, but I don't want to argue over the choice of word when I think both options are OK. As you've left the rest of my material about TT and TAI intact, I'm satisfied that the paragraph is now correct. 81.168.80.170 20:13, 14 February 2007 (UTC) (Zefram from a different IP address)

Afterthought: here's a source for "platonic" in the context of time scales: [1] by Steve Allen, a professional astronomer with a paticular interest in time scales. "... TDB has effectively been converted from a Platonic or theoretical time scale into a practical one ..." and "TT is intended as a Platonic ideal, for there is no single realization of it.". It's possible that I picked up "platonic" in this context from that page; I don't recall one way or the other. 81.168.80.170 20:35, 14 February 2007 (UTC)

Future problems

This could pose a problem in the distant future; I don't think fortnights of daylight and darkness would be well tolerated by terrestrial life. Has any thought been given to a plan to grind up the moon or otherwise diminish the threat it poses (admittedly very long term) to public safety? knoodelhed 04:04, 9 February 2007 (UTC)

Delta T indicates the change in time that an event occurs and is the integral of Earth's slowing rotation, so time changes at a much greater rate than Earth's day increases, the latter increasing only 1 ms/day/century over hundreds of millions of years (half the present rate). Earth's oceans will evaporate in 700 million years,[2] virtually halting any further increase in Earth's day at about 26 hours (Earth's day was about 22 hours long 600,000,000 years ago)[3]. In contrast, Delta T (presently 31(years/100)² seconds) will reach 14 days (1,209,600 seconds) in only 20,000 years, at which time Earth's day will have increased by only a third of a second! It has also been shown that the Moon maintains the tilt of Earth's axis at 23.5°±1.3°.[4] If the Moon were to disappear, Earth's axis would wildly fluctuate between 0° and 85°. If it reached 85° it would almost be laying on its side, causing each polar hemisphere to be in night or day for many months, which would kill most life on Earth. Conversely, the Moon allowed life to develop on Earth, so we would not be here if the Moon had not formed. — Joe Kress 08:47, 10 February 2007 (UTC)

Contradiction and nonsense

The first paragraph of the article states: "[the extensive ice age icecaps melted], allowing the land under them to begin to rebound upward in the polar regions, which has been continuing and will continue until isostatic equilibrium is reached." Fair enough: I've hear this theory before, and have no problem with it. However, the article then goes on to say: "This "glacial rebound" brings mass closer to the rotation axis of the Earth, which makes the Earth spin faster (law of conservation of angular momentum)..." (my emphasis),

That doesn't make any sense. Surely the ground is moving upwards as stated earlier in the paragraph, due to rebounding after the ice age glaciers melted. Just because this rebounding is "in the polar regions", that doesn't make the direction of rebounding towards the centre of gravity; if it was, the land would be going down, not up. No, the net movement is outwards, away from the centre of gravity and not towards it.

Someone please explain if I have missed something here, or if this is the problem I think it is.

WikiReaderer 01:14, 23 October 2007 (UTC)

The quote says that mass is closer to the rotation axis, not the center of gravity. The glacial uplift represents extra mass in the polar regions, which are close to the axis, when measured perpendicular to that axis. — Joe Kress 03:46, 24 October 2007 (UTC)

It doesn't make sense to me, either. The South Pole is covered by land, and the North Pole by ocean. The effects at each pole must be different. Also, the melted water must travel away from the poles, moving the mass farther from the axis, not closer. Can the correct explanation be made clearer? Is the Earth made less oblate somehow? Unfree (talk) 01:28, 7 November 2009 (UTC)

Yes. The volume of the Earth remains constant during the polar uplift, so while the polar regions are displaced outward along the axis, equatorial regions must come closer towards the axis. While the displacement of the polar regions along the axis doesn't change their contribution to the Earth's moment of inertia (apart maybe from some second-order effects such as redistribution of ocean water), the smaller distance of the equatorial bulge from the axis reduces the total moment of inertia.-- Tosch (talk) 09:40, 9 May 2012 (UTC)

Unnecessary Complexity

"Terrestrial Time (TT) is a theoretical uniform time scale, defined to provide continuity with the former Ephemeris Time (ET)." This sounds unnecessarily close to "theoretically uniform". If it means what I think it means, Terrestrial Time is an invariant time scale, and there's nothing "theoretical" about it, except that it's artificial, in the sense that it's defined independently of astronomical observations. "Continuity with" is meaningless to me. Perhaps that's supposed to indicate that TT is close to ET. Beats me. How can there be continuity between the two concepts? "Former Ephemeris Time" seems to me to imply that there are at least two definitions of Ephemeris Time, one "former" and another other than "former", which is misleading. If that means what I think it means, "former" ought to be deleted entirely, or replaced by something like "formerly used (for something)". Unfree (talk) 01:16, 7 November 2009 (UTC)

I would think it would not only be independent of astronomical observations, but also independent of atomic clocks. After all, any real collection of atomic clocks is slightly imperfect. As far as "continuity with" is concerned, I think that just means the starting value of TT is set equal to the value of ET at that time, as best as can be determined. --Jc3s5h (talk) 01:42, 7 November 2009 (UTC)

The present trend in Delta-T values

In the very long term, it is indeed true to state (in the main article): "[Delta-T] will continue to increase at an ever faster (quadratic) rate in the future".

At the same time, it is interesting to note the diagram on p.33 in the [UK publication] New Scientist of 30th January 1999, illustrating Stephenson and Morrison's suggestion that there might be an oscillation effect - so, (if that oscillation is real), during the next few centuries Delta-T will not increase quite as much as envisaged. But perhaps this is still too speculative to qualify for a mention on the main page, is it? DLMcN (talk) 20:50, 18 March 2011 (UTC) DLMcN (talk) 20:52, 18 March 2011 (UTC)

The Bibliography near the bottom of our main page does actually include Stephenson and Morrison's paper "Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990", and the [additional] abstract given by JSTOR ends with the remark: "Moreover, it is shown that besides this accelerative component, there is a fluctuation in the l.o.d. with a semi-amplitude of ~4 ms and a period of ~1500 yr". DLMcN (talk) 21:13, 18 March 2011 (UTC)

Initial rating

From Unknown, initially rating this article as C-Class on the project's quality scale and Low-importance on the project's importance scale. Senator2029║talk 06:47, 10 December 2011 (UTC)

2A01:E34:EC0A:CB40:CED:BCDA:7E58:C7C5 (talk) 05:39, 7 September 2022 (UTC) Earth rotation fictitious time gap (ERFTG) would be a suitable and pertinent name.

Bad, bad name

The article sorely needs to be renamed: "ΔT" is not a name but merely a symbol that is meaningful only to specialists of this particular discipline -- i.e., an "algebraic jargon". Wikipedia articles must be written -- and named -- for the benefit of non-specialist readers. Thus, for example, the article about temperature must be titled "temperature" and not "T".
So please find (or invent) an appropriate name for this quantity, using English words, and rename the article with it. --Jorge Stolfi (talk) 03:24, 24 May 2012 (UTC)

• Well, I have looked around in the web and -- surprise -- there seems to be no proper name for this quantity; everybody calls it just "Delta T". Which is bad, because other specialties (psychrometry, theormodynamics, statistical mechanics, simulated annealing, ...) also have their "Delta T"s. --Jorge Stolfi (talk) 18:03, 24 May 2012 (UTC)

How about naming it "ΔT (astronomy)" or "ΔT (timekeeping)" AstroLynx (talk) 06:51, 25 May 2012 (UTC)

A typical approach in Wikipedia when a term applies to several fields is that the first usage to have an article written about it gets the term as the title. As other articles come along, there is something in parentheses to distinguish from the first article. Also, a disambiguation page would be created and each article would have an {{about}} template at the top pointing to the disambiguation page. So far it seems that this is the only quantity where the name and the symbol are the same; other fields have an English name for the quantity. So so far there is no need to distinguish this page from similarly named articles. Jc3s5h (talk) 11:01, 25 May 2012 (UTC)

Or how about "Time differential" or "Solar/Astronomical time differential". Martinvl (talk) 11:53, 25 May 2012 (UTC)

A simple time differential is indicated as Δt (small t) (such as commonly found in math class or science class) 70.24.251.208 (talk) 04:36, 4 June 2012 (UTC)

How about renaming United States Constitution to "A founding document"?

"ΔT" has a much more specific meaning than "Time differential" or "Solar/Astronomical time differential". It is the difference between, in effect, atomic time at sea level on the surface of the earth and UT1, which is a specific approximation to mean solar time. Jc3s5h (talk) 12:03, 25 May 2012 (UTC)

Sounds like its time for an ambitious editor to put together a disambiguation page for other uses of ΔT. SteveMcCluskey (talk) 16:11, 25 May 2012 (UTC)

I just checked NIST's publication (p. 11, §1.3) on the SI System of units and found that the abbreviation for the dimension time or duration is T, while the abbreviation for the dimension thermodynamic temperature is θ. This pretty well confirms that ΔT refers properly to time, not to temperature. SteveMcCluskey (talk) 18:18, 5 June 2012 (UTC)

You might want to read that again. Page 11, gives "T" is the symbol for thermodynamic temperature, and "t" for time duration. Using a symbol as the name for this article is incredibly confusing since it is used for al sorts of things. Most commonly, for the temperature difference in thermodynamic processes.TR 13:23, 7 June 2012 (UTC)

We have to pay attention to details. The column for the fundamental dimensions lists Roman T as the symbol for the dimension time or duration and θ as the symbol for the dimension thermodynamic temperature. The column for quantities denotes italic, lower case t as the symbol for a quantity of time and italic, upper case T as the symbol for a quantity of temperature.

A check of Google Scholar, however, makes it clear that there are different uses of this term. As I suggested above, there is a need for a disambiguation page. SteveMcCluskey (talk) 19:40, 7 June 2012 (UTC)

This article should simply be named "Difference between Universal Time and Terrestial Time". (In accordance with the policy that article names should not be abbreviations).TR 13:31, 7 June 2012 (UTC)

The Wikipedia Manual of Style addresses the use of abbreviations as titles in two places:

• The section on article titles says: "Avoid abbreviations: Abbreviations and acronyms are generally avoided unless the subject is almost exclusively known by its abbreviation (e.g. NATO and Laser).... For more details, see Acronyms and initialisms in page titles."
• That section places a slightly different emphasis on the matter: "An acronym or initialism should be used in a page name if the subject is almost exclusively known by its acronym or is widely known and used in that form (e.g. NASA and radar)."

Depending on your source, abbreviations or acronyms are either allowed or preferred when the subject is almost exclusively (or widely) known by that abbreviation. Since ΔT is almost exclusively known by that abbreviation, that seems to be the preferred title. SteveMcCluskey (talk) 20:01, 7 June 2012 (UTC)

Let's look at a reliable source, the glossary of the Astronomical Almanac. There is an entry for ΔT and it is defined as "the difference between Terrestrial Time (TT) and Universal Time (UT): ΔT=TT-UT1". There are no entries beginning with the word "difference". There is no entry named "Time differential". There are no entries named "Solar/Astronomical time differential" and no similar terms that begin with "solar" or "astronomical".

McCarthy and Seidelmann (p. 124, see article for full bibliographic information) use "ΔT and Ephemeris Time Revised" as a section title.

So I believe we should follow the reliable sources and keep the article where it is. It would be appropriate to add a redirect from "Difference between Terrestrial Time (TT) and Universal Time (UT1)". Of course I would change my mind if someone can find a reliable source that uses some multi-word term for ΔT as a title, glossary entry, or dictionary entry. Jc3s5h (talk) 14:00, 7 June 2012 (UTC)

I have created the redirect Difference between Terrestrial Time (TT) and Universal Time (UT1). Jc3s5h (talk) 14:07, 7 June 2012 (UTC)

I have created a disambiguation page ΔT (disambiguation). I added some pages I could find that use the term ΔT (or Δt) for other measures of time or temperature. Please add others as seems appropriate. SteveMcCluskey (talk) 20:44, 7 June 2012 (UTC)

The name of this article is clearly inappropriate and should be changed to something more descriptive. "T" is used for too many things in different contexts. TimothyRiass suggestion "Difference between Universal Time and Terrestial Time" sounds fine for me. Using sources that deal specifically with astronomy and time to support the current name is totally irrelevant for the name of an article in a general encyclopaedia. Liiiii (talk) 15:58, 27 August 2015 (UTC)

As there's already ΔT (disambiguation), just rename the present article to ΔT (astrometry), with an automatic redirect. fgnievinski (talk) 23:13, 27 August 2015 (UTC)

Leap Second links here as an expansion of a section "Slowing rotation of the Earth". And there is another article Day length fluctuations. So this article should be titled something like Day length increase. Explain the phenomenon then introduce Delta T as effectively a measure of the cumulative increase since about 1900 (although formally it's the measure of theach increase since 1970 plus an offset). Humphrey Jungle (talk) 23:19, 18 October 2018 (UTC)

Based on what appears to be consensus in the discussion above, I've now moved this article to ΔT (timekeeping) and fixed all the links and redirects to reflect this. I hope this now resolves the problem. -- The Anome (talk) 11:18, 24 October 2020 (UTC)

WikiProject Physics Rating

I recently rated this article for Wikiproject Physics and was asked to give some pointers on how it could be made up to a B Class (criteria here). Looking through the assessment criteria:

1. The article is suitably referenced, with inline citations where necessary (the first paragraph might need an additional one or two but it's enough for me)
2. The article reasonably covers the topic, and does not contain obvious omissions or inaccuracies (only just though)
3. The article has a defined structure
4. The article is reasonably well-written
5. The article contains supporting materials where appropriate
6. The article presents its content in an appropriately understandable way

I'll address each point in order;

• The article doesn't really address why ΔT is important or the impact of changes to it. Additionally, it glosses over why 1820 is such an important year for calibrating time measurements. You could also consider including the history/adoption of ΔT, though this is possibly a little less important.
• Currently there isn't really any structure at all. The article needs a lead section and you could consider putting the paragraphs about Universal Time and Terrestrial Time into their own sections with links to their (respective) main articles.
• I have added a clarify tag to the sentence(s) about glacial rebound. They are correct, however as already evidenced further up this talk page could perhaps be worded slightly more clearly. It would perhaps be worth mentioning that the effects of glacial rebound of the land mass outweigh the effects on the moment of inertia of the release of liquid water to the equator, hence bringing mass closer to the rotation axis, (etc.).

Lastly it's worth noting that the article should generally be compliant with the Manual of Style.

I hope this helps! I'd be happy to reassess if any editors feel these points have been addressed. — Preceding unsigned comment added by Newty23125 (talk • contribs) 14:49, 10 June 2012 (UTC)

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Latest Work of L. V. Morrison and F. R. Stephenson

I updated the values of Delta T based on the latest work of Morrison and Stephenson, and added a reference to support the update. Acorann left my updates but deleted the reference, saying that there were already references to the work of L. V. Morrison and F. R. Stephenson. The existing references do not include all of the material I consulted. I could add the additional references, but combining works together, even from the same authors, is Original Research. To avoid that, I did the combining in a paper, which I referenced. Should I just add the additional references, or should I reference the paper that combines their results? John Sauter (talk) 21:36, 19 May 2016 (UTC)

Do not cite your original research which was published in an unreliable source, wikimedia.org. Read and follow the relevant venerability verifiability policy and guideline on identifying reliable sources. Jc3s5h (talk) 11:56, 20 May 2016 (UTC)

The works of Morrison and Stephenson are primary sources. I wrote a secondary source on Delta T, and the closely related topic of leap seconds, which draws almost entirely on the works of Morrison and Stephenson. To be a suitable source for Wikipedia, such a paper should be published in a respected, academic, peer-reviewed journal. The world of respected, academic, peer-reviewed journals is open only to authors supported by prestigious academic institutions. One journal that I investigated wants $3000 to consider a paper which would not be, upon publication, locked behind a paywall. I regard this as a racket: the publishers do not pay authors or reviewers for their work, but profit from their readers at the expense of the general public, who is denied access to the papers, or from authors, who must pay to be published.

I understand that Wikipedia needs reliable sources to have a useful encyclopedia. However, I do not understand why reliable sources can only come through the respected journals. I therefore ask the Wikipedia editors who are interested in this topic to read and review my paper. I will update the paper based on the feedback I get. At some point I expect the paper will be good enough, in spite of the fact that I have not paid $3000 to have it printed in a respected journal, that it will be referenced as a source in this Wikipedia article.

The article is available here: John Sauter (4 May 2016). "Extending Coordinated Universal Time to Dates Before 1972" (PDF). John Sauter (talk) 00:50, 21 May 2016 (UTC)

Dear John Sauter: Wikipedia is not the place for experts to review original research. You can publish it without paying fees in, e.g., Journal of Geodesy, although it'll be behind a paywall, which is not a problem for Wikipedia. If you insist on open access, there's the Journal of Geodetic Sciences, which you might consider asking for a fee waiver. If you further insist on so-called platinum open access, try Boletim de Ciências Geodésicas, which has a an official impact factor. Thanks for your understanding. fgnievinski (talk) 10:35, 21 May 2016 (UTC)

Please. The world of respected, academic, peer-reviewed journals is not open only to authors supported by prestigious academic institutions. All papers are considered on merit. Quite frankly, I don't see the merit in John's paper. We have enough problems with people converting dates prior to 1582 into Gregorian without people trying to convert times (which are all local mean solar time) into something else as well. 92.19.252.220 (talk) 09:18, 24 May 2016 (UTC)

Problems arising from new section headings

The recent addition of section headings sharpened a tension in this article between the idealistic version of a simple quadratic expression for ΔT, now in a separate section of Values prior to 1955, and the non-quadratic form of non-tidal terms discussed in the rest of the article. It leaves the reader with the impression that the complexities discussed earlier in the article can be ignored prior to 1955, when in fact, they really become significant when considering such long-term calculations.

I'm not certain how best to go about resolving this but it seems to me that the simple quadratic expression should be presented as a simplified model, while the more complex analyses based on the historical observations collected by Stephenson and Morrison more adequately should be presented as more complete expressions of the Earth's changing rotation over the long term. --SteveMcCluskey (talk) 20:52, 6 December 2016 (UTC)

Important recent article

This article of last December is probably the most comprehensive and up-to-date on this subject, so I suggest it be cited. Zero^talk 11:14, 31 October 2017 (UTC)

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Calculating delta T

The given equations do not calculate delta T. They also introduce a symbol T meaning the Earth's rotation period, but in the expressionext delta T the symbol T means the time since the epoch. Humphrey Jungle (talk) 23:24, 18 October 2018 (UTC)

You're right, the section "Calculating ΔT" including the two sub-sections, provide some information related to calculating ΔT, but don't actually calculate it. It's also confusing that the ΔT is a term of art, and does not mean the difference between two values of the same variable T. We could fix the first problem by rewriting the section, but the second problem is not something we can fix, because that's the way the scientific community defines the term. Jc3s5h (talk) 14:43, 19 October 2018 (UTC)

I've replaced T in the equations with P. Humphrey Jungle (talk) 08:57, 7 November 2018 (UTC)

Diagram showing present and recent values of Delta-T

The UK Hydrographic Office in Taunton, Somerset, has issued a diagram which gives a really excellent illustration of the final statement in the opening paragraph of this Wikipedia article - [namely: "Since 2017, the length of the day has happened to be very close to the conventional value, and ΔT has remained within a second of 69 seconds"].

This diagram is temporarily viewable in my private website at http://www.dlmcn.com/deltaT_March_2023_Short.pdf ... It could presumably be uploaded here if we get permission from the UK Hydrographic Office? --DLMcN (talk) 20:13, 6 March 2023 (UTC)

That^ UK office is presently servicing its website - so the diagram will probably be published officially quite soon. --DLMcN (talk) 21:28, 6 March 2023 (UTC)

Permission to use the diagram was received in this e-mail:

Ref:  CS-271713-G8T5  Your enquiry has been ... logged under this unique Reference Number, which should be quoted in any further correspondence with us.

Good day,

Thank you for your enquiry, this has been shared with our HMNAO team and they have provided the following;

“Thank you for your kind words, we are happy for you to use our diagrams for the Delta-T Wikipedia article. Unfortunately due to our site’s essential maintenance we do not have a permanent link to these images. To complicate matters further we also update these diagrams automatically every month and so placing them in another location in a static fashion would quickly become obsolete.

For now we are happy to send you the new files each month for updating, and when our website returns we will be sure to let you know of their new location.”

Kind regards Shaun Cockram <customerservices@ukho.gov.uk>, Customer Services, UK Hydrographic Office, Admiralty Way, TAUNTON, Somerset, UK, TA1 2DN, Tel: +44 (0)1823 484444 www.Admiralty.co.uk

--DLMcN (talk) 19:28, 14 March 2023 (UTC)

@DLMcN: No permission is required to just link to an internet resource. On the other hand, if we wish to display the image ourselves, permission to publish on Wikipedia is not enough. A free license is required, which is defined at WP:Image use policy. Images without such a license are always removed before long. Images hosted on Commons have a similar restriction, see here. Cheers, Zero^talk 01:22, 15 March 2023 (UTC)

Thank you, Zero, for your comment and advice. I have been trying to explore and make sense of the various Wikipedia links on this topic, but confess that it is not at all clear exactly what I need to do in this particular context, to get the necessary licence and permission. If you could please point me in the right direction, that would be much appreciated. Thanks in anticipation, --DLMcN (talk) 08:06, 15 March 2023 (UTC)

ΔT is NOT parabolic

It's an error made in the past, in the 50's, to assume ΔT is parabolic. If it was, ΔT in 1870 equals that in 1900 and ΔT in 1620 would equal that in 2024. It's obviously wrong.

Furthermore, parabolic ΔT has a minimum point in 1893, which has no physical meaning. 2A01:E0A:9EE:BD20:E8B4:7D59:4CEC:B377 (talk) 10:47, 2 July 2024 (UTC)

This topic is already being discussed at Talk:Ephemeris time#ΔT is NOT parabolic. This page probably would have been a better place for the discussion, but since it's there, it should probably stay there. I also feel that starting this discussion without mentioning the other one amounts to forum shopping. Jc3s5h (talk) 12:46, 2 July 2024 (UTC)