Talk:ΔT

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To basically sum up what Delta T is you just use the equasion TT-UT=Delta T.
— 68.47.224.141 on July 1, 2004.

Contents 1 Delta minus T 2 Missing unit of measurement? 3 Delta T (Psychrometrics) 4 n-dot symbol 5 TAI based on TT 6 Future problems

[edit] 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)

[edit] 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)

[edit] 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 , 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)

[edit] 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)

[edit] 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)

[edit] 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)