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    Re: Leap seconds
    From: George Huxtable
    Date: 2008 Dec 30, 11:10 -0000

    Greg R wrote-
    "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...). "
    Yes, I am being pedantic. On a subject such as this, so easy to
    misunderstand, we have to be precise about our words. Greg R and I are
    completely failing to communicate properly. If we can't agree about this
    initial matter, there's no point in further discussion, so let's have
    another go. Geoffrey Kolbe's intervention has been particularly helpful in
    clarifying matters.
    The words in question came from a posting [6805] by Greg R, in which he
    wrote- "Besides, the almanacs have been on UT since when - mid 70s? (and
    thus pretty much "disconnected" from "sun time")."
    Greg R has not since repeated that statement, but continues to defend it.
    He continues, in [6849]-
    "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?"
    No, we can't agree on that. Greg R's phraseing is woolly. What does he mean
    by "based on"? Yes, the STARTING POINT is the counting of seconds, of Atomic
    Time, unrelated to the rotation of the Earth. But then (and this is the
    crucial bit) that count of seconds is SERIOUSLY MODIFIED, by the insertion
    of an unpredictable leap-second here and there, to create UT which IS then
    closely in step with the rotation of the Earth, within less than a second.
    That's the time-scale that almanacs work to. It allows almanacs to be
    printed some years in advance of their predictions, without needing to know
    whether, or when, leap-seconds will need to be introduced in the interim.
    --- On Mon, 12/29/08, George Huxtable  wrote:
    > From: George Huxtable 
    > Subject: [NavList 6848] Re: [NavList /] Re: Leap seconds
    > To: NavList@fer3.com
    > Date: Monday, December 29, 2008, 11:00 AM
    > Greg R wrote-
    > ----- Original Message -----
    > From: "Greg R." 
    > To: 
    > Sent: Monday, December 29, 2008 12:09 AM
    > Subject: [NavList /] Re: Leap seconds. was: [6802] Longest
    > year since 1992
    > 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?
    > =========================
    > What I questioned was Greg R's statement in [6805], as
    > follows-
    > "Besides, the almanacs have been on UT since when -
    > mid 70s? (and thus
    > pretty much "disconnected" from "sun
    > time"). "
    > And I pointed out that the almanacs have remained closely
    > "connected" with
    > "sun time", precisely because of the insertions
    > of those leap seconds, by
    > which UT is made to correspond closely with the rotation of
    > the Earth..
    > I wonder what it was that happened, in the mid 70s,
    > according to Greg R, to
    > bring that about the "disconnection" he refers
    > to? In one way or another,
    > the Nautical Almanac has followed mean Sun time at
    > Greenwich, within a
    > fraction of a second, since 1834, and continues to do so.
    > Greg R  has complained that "I think you
    > misinterpreted what I was trying to
    > say", and now "You're still missing my
    > point".
    > But he hasn't withdrawn those words that I quoted, and
    > questioned, above,
    > about the almanacs being "disconnected" from
    > "sun time". And I don't know
    > how to interpret those words any differently than the way I
    > did.
    > If we can find a way to agree about this minor matter,
    > perhaps we can go on
    > to discuss leap seconds more fruitfully.
    > George.
    > contact George Huxtable, at  george@hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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