Talk:Terrestrial Time

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level of clarity[edit]

i find this article incredibly difficult to understand. should it be rewritten in a less technical fashion? —Preceding unsigned comment added by Richar4034 (talkcontribs) 23:53, 21 January 2007

It's a very technical subject. The detail and precise terminology does have to be there. I oppose a wholesale rewrite, but if a less technical explanation (which would inevitably be less complete) can be added then that would be a good thing. I'm not sure how to go about that though: I find the article very clear. What do you suggest? Perhaps if you ask about particular points of confusion here then this would point to ways to explain it better. 20:23, 14 February 2007 (UTC)

Yes, but I'm inclined to agree that there is room for improvement. The initial paragraph really only says that Terrestial Time takes relativistic effects into account. Most people will have some concept of what SR and GR are all about, even if terms like proper time, and perhaps even time dilation will be unfamiliar. Another issue is that many readers will be familiar with special relativity, as it really does not require advanced mathematics, but general relativity will be more mysterious. Or will it? Greg Woodhouse 21:07, 19 March 2007 (UTC)

TT is a coordinate time, not a proper time[edit]

IAU Resolution B1.9 (2000) as seen at reiterates the content of IAU Resolution A4 (1991) wherein TT is defined as a coordinate time, not a proper time. This distinction was further explicated by Gerard Petit at IAU Joint Discussion 16 (2006, Prague) as cataloged at (the actual presentation is at ). To wit, TT is defined as a linear transformation of TCG. TCG is a coordinate time, therefore TT is a coordinate time. At the sub-nanosecond level this distinction is important. Steven L Allen 19:18, 1 July 2007 (UTC)

Need to update TT − UTC[edit]

Even though the date of the last leap second was updated, the value of TT-UTC was not; should be updated to 66.184 s. User:Arnold Rots —Preceding unsigned comment added by (talk) 03:05, 5 October 2009 (UTC)

Are you confusing this article with another? This article does not mention leap seconds or TT−UTC, although maybe it should. It does mention TT−UT1, which is not the same thing. — Joe Kress (talk) 00:48, 6 October 2009 (UTC)


It could be useful to reference Unix/Epoch time - 1453329945 (talk) 22:45, 20 January 2016 (UTC)

How so? They are not really closely related: they are connected by the chain TT-TAI-UTC-Unix. In particular, Unix time does not keep step with TT, neither at the seconds level (due to its treatment of leap seconds) nor at the nanoseconds level (due to TAI imperfectly realising TT). I don't see a reason to mention Unix time in the TT article. 2001:8B0:80D:0:0:0:0:1 (talk) 21:25, 6 March 2016 (UTC)

Relativistic relationship[edit]

A lot has been written about the General Relativistic effect of altitude here, but nothing on the different rotation speeds at different latitudes.

I infer from the article that TT is defined for someone who is at sea level and who is not moving along with the rotating surface of the earth. Is that inference correct? Whether it's correct or not, I think the article should spell it out. Rwflammang (talk) 21:52, 29 November 2012 (UTC)

There is quite a lot wrong with the 'current definition' section (I'll propose to edit it soon if not already improved). At present, where the text gives the formula that defines TT wrt TCG, the text says they are both linear counts of SI seconds. That's misleading in this context: the essential point underlying the formula is that TCG and TT are counts of SI seconds but only in relation to different reference frames. As observed at the surface of the earth, TCG does not appear to be going at SI-second rate, it looks faster from here (by about 0.5 sec/yr) than a local clock here keeping the SI seconds rate. It's this terrestrially-observed TCG value, steadily accruing at the faster rate and not at SI-seconds rate (so far as it looks to us here), that's the TCG value that enters into the formula, which naturally has to compare observations made at the same place.

The explanation is in terms of (gravitational) time dilation, but the wikipedia articles on that subject are also not as clear as they might be. Firstly, the link between TCG and SI seconds is highly theoretical: it is a coordinate time scale that could theoretically be matched in rate by the 'proper time' of a clock going at SI-seconds rate, but only if that clock were to be situated and locally observed entirely outside the earth's gravity-well and also at rest with respect to the geocenter. The apparent excess rate of such a hypothetical TCG clock (if it were to be observed here at the earth's surface) arises because we are some way down in the Earth's gravity well. For an observer outside the gravity well and at rest relative to the geocenter (and equipped with a SI-seconds clock that's local to him), our SI clocks down here consequently would look a little sluggish (about 0.5 sec/year), even though, for us observing here, our local SI clocks seem to us to keep SI seconds just fine. Vice versa, the clocks up there look, or would look, to us (observing them from down here) as if they are going a little too fast. This is nowadays a real observable effect, as the clocks even a little way up, in the GPS satellites, have had to be slightly de-rated (slower than SI seconds by some number of microseconds per day from the point of view of somebody riding with them in the satellite) to allow for this effect and make sure they don't look too fast when 'seen' by us from down here. Terry0051 27 October 2015