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    Re: Leap seconds. was: [6802] Longest year since 1992
    From: Greg R_
    Date: 2008 Dec 28, 10:52 -0800
    George Huxtable wrote:
     
    > 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.
     
    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.
     
    --
    GregR
     
     
     
    ----- Original Message -----
    From: "George Huxtable" <george@hux.me.uk>
    Sent: Saturday, December 27, 2008 8:59 AM
    Subject: [NavList 6836] Leap seconds. was: [6802] Longest year since 1992

    >
    > 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: <
    frankreed@HistoricalAtlas.net>
    > To: <
    NavList@fer3.com>
    > 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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