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    Re: Leap seconds
    From: Greg R_
    Date: 2008 Dec 30, 09:18 -0800

    --- On Tue, 12/30/08, Geoffrey Kolbe  wrote:
    
    > So, this is the situation today. UTC is not tightly coupled to "solar 
    > time"
    
    Agreed (why George is arguing over his finely-parsed meaning of the word "disconnected" is beyond me).
    
    > but at present it is forced to track UT1 to within 0.9 seconds by the 
    > injection of a leap-second as required.
    
    Exactly. Which brings us back to my original assertion that whatever time 
    scale we use as navigators, as long as it matches the time that is published 
    in the almanacs it will work just fine for solving celnav problems. Whether 
    or not we use leap-seconds on that time scale is totally irrelevant.
    
    --
    GregR
    
    
    
    --- On Tue, 12/30/08, Geoffrey Kolbe  wrote:
    
    > From: Geoffrey Kolbe 
    > Subject: [NavList 6850] Re: [NavList /] Re: Leap seconds
    > To: NavList@fer3.com
    > Date: Tuesday, December 30, 2008, 1:25 AM
    > At 03:27 30/12/2008, you wrote:
    > >
    > >
    > >George:
    > >
    > >I don't know if you're being pedantic here, or
    > if we have an honest 
    > >failure to communicate (or maybe a little of both...).
    > >
    > >But I think we can both agree that the current
    > timescale that most of us 
    > >navigators use (i.e. UT) is based on an atomic
    > standard, right? Which has 
    > >no connection at all to what is commonly called
    > "solar time" (hence my use 
    > >of "disconnected" from it).
    > >
    > >And if you don't understand my example showing that
    > it really doesn't 
    > >matter what timescale we use to solve navigation
    > problems (as long as our 
    > >timepieces and the almanacs are using the same scale),
    > then I really don't 
    > >know any other way to try to word that so it makes
    > sense to you.
    > >
    > >--
    > >GregR
    > 
    > Greg
    > 
    > I have to share George's disquiet at your proposition
    > that UT "has no 
    > connection at all" with "solar time" by
    > virtue of the fact that is is 
    > "based on an atomic standard". I assume that by
    > UT, you actually mean UTC, 
    > "Coordinated Universal Time", or more commonly,
    > "Broadcast Time" as 
    > transmitted by various terrestrial radio transmitters
    > around the world and 
    > on which our civil time is based.
    > 
    > The fact is that the atomic standard on which the second is
    > based was fixed 
    > during the 1960s as being 9,192,631,770 periods of the
    > radiation 
    > corresponding to the transition between the two hyperfine
    > levels of the 
    > ground state of the cesium 133 atom, because this
    > corresponded to the 
    > fraction 1/31,556,925.9747 of the tropical year for 1900
    > January 0 at12 
    > hours ephemeris time. A tropical year is the period between
    > two vernal 
    > equinoxes, so this definition tied the length of an atomic
    > clock second 
    > into being the same as the length of a "solar
    > time" second as it was at the 
    > start of the year 1900. It was done this way in recognition
    > of the fact 
    > that the "solar time" second is a constantly
    > varying entity, as measured by 
    > an accurate clock.
    > 
    > Now, there are a multitude of time systems based on the
    > atomic second, many 
    > of which - like GPS time - do not recognize the fact that
    > the earth's 
    > rotational period is gradually slowing down. However, UTC
    > is not one of 
    > those time systems. When atomic clocks were first built in
    > the 1950s, they 
    > were constantly adjusted so that their output corresponded
    > to "solar time" 
    > or GMT as it was in those days. This was nothing new as the
    > time shown by 
    > chronometers or observatory clocks had always been adjusted
    > to correspond 
    > with "solar time" in one guise or another. The
    > fact that these were now 
    > atomic clocks was neither here nor there. At first,
    > milli-seconds were 
    > interspersed on an ad-hoc basis to keep the atomic clocks
    > tightly coupled 
    > to GMT. But finally this evolved so that Broadcast Time
    > (UTC) would be kept 
    > within 0.9 seconds of UT1 (the modern version of GMT) by
    > the agreed 
    > addition (or subtraction) of leap-seconds either at the
    > start or the middle 
    > of the year.
    > 
    > So, this is the situation today. UTC is not tightly coupled
    > to "solar 
    > time", but at present it is forced to track UT1 to
    > within 0.9 seconds by 
    > the injection of a leap-second as required. It is not true
    > to say, then, 
    > that UTC "has no connection at all" with
    > "solar time". It does, both by the 
    > length of an atomic clock second and by the absolute UTC
    > time kept by 
    > atomic clocks. What the Working Group 7a of the
    > International 
    > Telecommunications Union are recommending is that we do
    > away with the leap 
    > second so that UTC no longer tracks UT1 or "solar
    > time".
    > 
    > The British representatives to Working Group 7a  would
    > appear to be against 
    > the proposition to do away with the leap-second. I still
    > have not found 
    > anything which gives the reasoning behind their position.
    > Newspapers talk 
    > about GMT and the Prime Meridian running through Greenwich
    > Observatory as 
    > being standards which would cease to exist if the
    > leap-second was done away 
    > with, and allude that this is why the UK is against the
    > move. The fact is 
    > that GMT as a time standard no longer exists and the Prime
    > Meridian - as 
    > defined by the WGS84 system which the UK has now
    > effectively adopted for 
    > its maps - now puts the Prime Meridian through a dustbin on
    > the footpath 
    > some tens of metres to the East of the Greenwich
    > Observatory! In the 
    > absence of further details, I can only conclude that the UK
    > and China are 
    > seeking concessions within the ITU on other matters and are
    > using this as a 
    > bargaining counter.
    > 
    > Geoffrey Kolbe
    > 
    > 
    
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