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    Re: Leap seconds
    From: Geoffrey Kolbe
    Date: 2008 Dec 30, 09:25 +0000

    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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