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    Leap seconds. was: [6802] Longest year since 1992
    From: George Huxtable
    Date: 2008 Dec 27, 16:59 -0000

    I've altered the threadname, in case this thread develops.
    
    Frank has raised again the question of leap seconds, as copied below, in the
    context of an ill-informed piece in The Times. Frank is right to correct the
    absurdity of those arguments. However, absurd arguments, unless there are no
    others, do not in themselves invalidate the case.
    
    For background reading, I can recommend Steve Allen's-
    
    http://www.ucolick.org/~sla/leapsecs/timescales.html
    
    which seems to me to be a lucid account of a complex subject.
    
    Greg R, in [6805], wrote-
    
    "Besides, the almanacs have been on UT since when - mid 70s? (and thus
    pretty much "disconnected" from "sun time"). ".
    
    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.
    
    ===============================
    
    However the inroduction of leap seconds is, in my view, no more than a
    short-term fudge, to get over the inconstancy of mean Sun time, or
    Time-of-day, compared with atomic time. Short-term, in terms of human
    civilisation, because as the Earth slows more and more, leap seconds will
    become needed more and more often, until a ridiculous state of affairs is
    reached.
    
    In the long-term, of course, we are all dead, but civilisation (we can hope)
    will live on. And we need to bequeath to our descendants that can continue
    logically, without future fudges and upheavals, indefinitely into the
    future.
    
    We have to recognise the fact that there are two measures of time that are
    incompatible, in an unpredictable way, and accept them as different.
    
    One measure, let's call it time-of-day, follows the rotation of the Earth,
    and is gradually getting slower, in a way we can do nothing to change.
    Unless man follows the moles underground, those days will continue to
    regulate human affairs, and the count of those days will give us our dates.
    
    The other measure of time is as uniform as man can possibly make it, and
    will, no doubt, be subject to further refinement over the years. We can call
    it here Atomic Time. It's the timescale  according to which all the laws of
    physics operate, so it's really important to get it right.
    
    Unfortunately, those two measures of time must inevitably drift apart,
    slowly at the start, but the rate of divergence will increase faster and
    faster, as the Earth slows.
    
    If we were to adopt Atomic Time for our human affairs, without the sort of
    fudging that leap-seconds provide, then after 600 years or so, our clocks
    would be an hour ahead of the Sun. The details are unpredictable (and may
    remain so) but the general trend isn't. Two millennia from now, it would be
    midnight at 12 noon by the clock. A thousand years later, our calendar count
    of days would be out by a whole day, compared with the number of times the
    Sun had gone round the sky. And then it would continue, worse and worse, at
    an ever-increasing rate. Scientists would stay happy; they could count on,
    in mega seconds and teraseconds, but humanity would flounder, in confusion.
    
    And that's what the proponents of dropping leap-seconds seem to be
    proposing. When taxed with the resulting absurdities of clock and calendar,
    they blithely suggest- "Oh, we can introduce a leap hour after 600 years or
    so". What a legacy to leave behind us! If the occasional leap second is a
    cause for concern now, what disruptions might an unprecedented shift by an
    hour cause, in a future era? And then, the next leap-hour will be required
    240 years later, and so on. So the principle of imposing a leap-something
    wouldn't be got round, the proposal is no more than a ruse for putting it
    off for a future generation.
    
    ============================================
    
    Is there an alternative? Yes, there is. It just has to be accepted, as a
    fact-of-life that we can do nothing about, that for Earth-dwellers there are
    two natural time-scales; time-of-day. and atomic time. And we can cut them
    loose, to run on quite independently, so one isn't constrained to follow the
    other in any respect.
    
    Indeed, it has already happened. GPS Time was introduced, several years ago.
    Not the UT that the receiver displays, but the basic internal time-scale on
    which the GPS system operates, which ticks on from year to year according to
    atomic time, taking no account of Earth rotations, to synchronise their
    onboard clocks. (The satellites are also informed about Earth rotations, of
    course, and use that to give us our time of day).
    
    Scientists, and indeed anyone else for whom constant time intervals are
    important, could continue to use "GPS seconds" or perhaps some
    near-equivalent, perhaps refined, redefined and ideally renamed. Let's call
    these exact, equal, time-increments "ticks", for now. It would only
    encourage confusion to continue to call them "seconds".
    
    And the rest of us could continue regulating our timepieces, our mantelpiece
    clocks and our wristwatches and chronometers, to run at a rate which
    corresponds to the rotation of the Earth, just as we do now. No personal
    clock that I know of has a prime-mover so inherently accurate that it can
    even detect the Earth's slowing, so that would present no practical problem
    to us as individuals. Providers of standard time broadcasts would need to
    put out two separate signals, one for time-of-day, the other for
    count-of-ticks. The only real difference would be that the interval between
    seconds-of-the-day would have to be made variable, by a marginal amount, to
    keep pace, as precisely as possible, with the Earth's rotation, as it varies
    unpredictably, and slows inevitably.
    
    As for the almanacs, they would continue to predict movements of Sun and
    stars in the sky according to the time-of-day, much as they do now, but
    without the present discrepancy of up to 0.6 seconds. Motion of solar-system
    bodies would require to be pre-calculated by its compilers with reference to
    absolute ticks, as happens now; this affectins the position of the Moon in
    particular.
    
    It's a system that could continue happily for ever.
    
    No doubt there are snags, and no doubt someone will point them out. I hope
    so.
    
    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.
    
    ----- Original Message -----
    From: 
    To: 
    Sent: Monday, December 22, 2008 11:46 PM
    Subject: [NavList 6802] Re: Longest year since 1992
    
    
    
    And once again, some folks are raising silly objections about the possible
    abolition of leap seconds.
    
    See:
    http://www.timesonline.co.uk/tol/news/uk/science/article5361670.ece
    (article written by Mark Henderson, said to be "Science Editor")
    
    The article claims, "Sundials would become even more inaccurate than they
    are already, and it would become almost impossible for sailors to navigate
    by sextant."
    
    As we've discussed previously, accomodating a time standard without leap
    seconds would present no serious problem for celestial navigation. Needless
    to say, the comment about sundials is technically true, but only after
    decades and in a way that no one but an expert would ever notice.
    
    
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