NavList:

   

Reply

You're still missing my point, which was (perhaps obliquely) related to 
whether we need leap seconds inserted into the UT scale or not. 

What I'm trying to say (and I really don't know how to re-word this to make it 
any clearer) is that whatever time scale the navigation community agrees on 
(either with or without "leap seconds"), as long as the time that we use in 
the field (from a chronometer, radio time signal, "atomic" watch, 
whatever...) matches the time used in the various almanacs it's *totally 
irrelevant* whether or not that time matches the actual "sun time" that's 
been used for centuries. 

In other words, if the almanac shows a time of 16:00:00 for noon at Greenwich 
on a given day (granted that's a huge difference from UT, but it works for an 
example), as long as my local timepiece also matches that time scale I 
shouldn't have any problem working a sun sight based on that (any more that I 
would with a timepiece set to UT as is the current practice).

Granted it would be nice if our navigation timescale were reasonably close to 
"sun time", but as a practical matter whether it does or not is really 
irrelevant to solving the navigation problem.

Make more sense now?

--
GregR



--- On Sun, 12/28/08, George Huxtable  wrote:

> From: George Huxtable 
> Subject: [NavList 6845] Re: Leap seconds. was: [6802] Longest year since 1992
> To: NavList@fer3.com
> Date: Sunday, December 28, 2008, 1:33 PM
> > Greg R, in [6805], wrote-
> >
> > "Besides, the almanacs have been on UT since when
> - mid 70s? (and thus
> > pretty much "disconnected" from "sun
> time"). ".
> 
> to which I commented-
> 
> > On the contrary, though, the leap seconds are inserted
> just so that UT,
> > incrementing at a constant rate compatible with atomic
> time, keeps in step
> > with the Mean Sun, to within 0.6 seconds, as do the
> almanacs. They remain
> > closely connected with Sun time.
> 
> And Greg has replied-
> 
> "I think you misinterpreted what I was trying to say -
> i.e. that our time 
> reference (based on a multiple of the period of the Cesium
> atom) is 
> something that's not related to the motion of the earth
> (and by reference, 
> also the sun) any longer (which is exactly why there's
> a need for leap 
> seconds periodically, in an effort to keep the two roughly
> "in sync").
> 
> My point was that since we're now on this atomic
> timescale (that really 
> isn't in sync with "sun time"), as long as
> the almanacs and our timekeepers 
> were on the same time scale (so that we could accurately
> mark the date/time 
> of a celestial event), does it really matter what actual
> "time" the event 
> occurred? It could be some random number not even related
> to "sun time", but 
> as long as we could use it as a reference point for
> starting our 
> calculations (i.e. GHA/Dec) the actual value is actually
> irrelevant."
> 
> Next from George-
> 
> Well, all I had to go on was Greg R's words, in [6805],
> which I quoted. It 
> was those words that I was questioning.
> 
> But really , it's more complicated than Greg makes out.
> There really ARE two 
> time-scales, that run at irreconcilable rates, with the
> differences 
> unpredictable in advance. It's a fact of life. And they
> have different 
> effects. Take the implications for astronomers. The way
> they point their 
> telescopes towards a star depends on the rotation of the
> Earth. So that 
> depends on a time-scale that's gradually slowing, and
> unpredictably. But now 
> think about the motions of , say, Mars, in the sky,
> relative to the Sun and 
> stars. That is completely unaffected by the revolutions of
> the Earth. If the 
> Earth stopped spinning on its axis, Mars' motion round
> the Sun would 
> continue unaffected. It's governed by the physical laws
> of gravity and 
> relativity, for which, unchanging atomic time is relevant.
> 
> Yes, Greg R can use some sort of formula to convert one
> time-scale to the 
> other, knowing the history of the way Earth rotation speed
> has varied in the 
> past. But he CAN'T do that into the future, because of
> the unpredictability 
> of the slowing. And that's really the basic reason why
> efforts to constrain 
> two such time-scales to run in step are, in the long-term,
> doomed to fail.
> 
> George.
> 
> 
> 
> 
> 
> 

--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply