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A couple of relevant items:


-- Richard Langley

On Sat, 19 Apr 2008, George Huxtable wrote:

>
>
>Geoffrey Kolbe has written-
>
>"Well, my Long Term Almanac is nominally good up to the year 2050 and will
>actually work OK up until 2060. So this is a very real question for me. I am
>assuming that if leap seconds are abolished, anyone using my tables - which
>assume GMT, or UT1 as it is called these days - will know the difference
>between broadcast time and GMT and use this as a correction to broadcast
>time."
>
>And following a question from Lu Abel, elaborated-
>
>"Actually, the theory used to generate my tables includes an estimation for
>delta T, the difference between ephemeris time and Universal Time. How delta
>T will vary in the future is not that well understood, of course, which
>means that by 2050 my tables will probably be out by a second or so. "
>
>=========================
>
>Response from George-
>
>I wonder how confident Geoffrey really is about that projection into the
>future, to within a second "or so".
>
>Let me propose a simple test. If we take the date on which his tables were
>based, and arrive at the number of years, to 2050, that the prediction of
>delta-t was to be valid for (so, 42 years if the tables were issued in 2008,
>for example). And then take whatever projection Geoffrey uses to predict
>delta-T to 2050, and instead apply it backwards, for the corresponding
>number of years, to "predict" changes in delta-T in the past. Then, even
>though we are now enabled to use a certain amount of hindsight in making
>that projection, I ask him what's the maximum difference, over those years,
>between delta-t, "predicted" that way, and delta-t as it actually happened.
>
>It's an interesting question, what should be done in our measurement of
>time, as the rotation of the Earth slows more and more into the distant
>future. A second defined as a fraction of a day (which I will call an
>Earth-second) will diverge more and more from the "scientific", constant,
>second, to which physical laws conform. That is a fact of life, quite
>inevitable, and we can do nothing about it except adapt to it as best we
>can. To me, the answer isn't clear-cut.
>
>It's a problem that can only get worse, faster, and faster, into the future,
>and we need to bequeath to posterity a system that is practical and
>applicable into the far distance, not one that will call for some major
>upheaval at some future date long after we're all dead and gone.
>
>The second was defined to correspond to the rotation of the Earth around
>1900, and delta-t was set roughly to zero about then, and it's been growing
>ever since, at a steadily increasing rate.
>
>There are two main causes at work here. One is rather well understood, now,
>and can be readily predicted. It's due to slowing of the Earth's rotation,
>due to the action of the tides, caused mainly by the gravity-gradient of the
>Moon (and, to a much lesser extent, the Sun). This component can be
>measured, rather well, because the same forces have a corresponding effect
>on the Moon, driving it out to a larger radius from Earth, which can be
>measured by radar ranging. Over the long term, that is the major effect,
>that will cause delta-t to grow from its present 38 seconds or so to about
>an hour, in a thousand years time. By then, leap seconds would have to be
>inserted, not at intervals of a couple of years as at present, but every
>couple of months. Another thousand years, and it will have reached four
>hours, because it changes according to a square law. 5,000 years from now,
>the times will have diverged to put the two dates one whole day different,
>and by then there will be a leap second needed at fortnightly intervals! So,
>in the far future, will leap-seconds be a viable proposition? I only ask.
>
>The other main cause of the variation in the Earth's rotation is due to
>fluctuating motion in the fluid core. This is combined with smaller faster
>changes that result from winds and ocean currents, and much slower changes
>due to continental drift, which we can ignore for now. But its those effects
>of the fluid motion that are unpredictable, and mask, to a large extent, the
>predictable changes in rotation rate over time-scales of decades at a time.
>It's these fluctuations that I predict will cause Geoffrey Kolbe a bit of
>grief.
>
>So what would be the practical effect of arresting  change in delta-t,
>presumanly at its current value, so that Earth-time is forced to follow the
>constant seconds that scientists use? For navigators, not a lot, I predict.
>Almanac-makers, including Geoffrey Kolbe, would be able to produce their
>wares for dates far into the future, instead of them having to anticipate
>changes in the value of delta-t, for a few years ahead. Instead of using a
>special time-scale (ephemeris time, now called terrestrial dynamical time)
>to compute their dynamics, differing unpredictably from the UT (same as GMT)
>that ordinary mortals use, then everyone would be forced to use the same
>rational time as astronomers do.
>
>The snag comes when converting sky-positions to a geographical position with
>respect to the Earth's surface, in calculating Greenwich Hour Angles (GHA).
>In doing that, the increasing  and unpredictable discrepancy corresponding
>to delta-t will have to be allowed for. If the almanac is to remain valid
>for many years ahead, that can't be done within its pages, but only by the
>navigator, knowing what the correction happens to be at his current date.
>
>Any difficulties resulting from such a change, in freezing leap-seconds,
>would be felt by the ordinary person in real-life, way into the far future,
>who would become aware that the time given by his clock on the wall no
>longer corresponded with his time-by-the-Sun. Would that matter? We already
>have got so used to tinkering with our clocks these days, first with mean
>time, then with daylight saving and time-zones, that it's no longer sacred
>any more for the Sun to be overhead at noon.
>Perhaps an all-change in the time zones by an hour, a millenium or so from
>now, would be called for. From then on, the interval between such changes
>would get less and less.
>
>Should such things matter? We should ponder hard about the consequences of
>our actions before making changes. What's a few thousand years in the future
>when we consider how long civilisation has been going? How far ahead was
>Julius Caesar thinking, when he reformed the calendar? And then, Pope
>Gregory?
>
>Freezing leap-seconds at some date would create a bit of a mess, anyway. We
>would end up with three different "scientific" time-scales, running in
>parallel, with constant differences between them. First, there's Ephemeris
>Time, (or TDT), a constant measure of time frozen from Greenwich Time as it
>was in 1900, . Then there's GPS time, which was effectively Greenwich Time
>but frozen at some date around 1980. And then we would have a new measure of
>time, frozen from Greenwich time at some date a few years from now. There
>would be a range of several tens of seconds spanning these three
>time-scales. What chaos!
>
>I have tried to offer a balanced view of this matter, being somewhat
>undecided about it myself. I would resist any hurry to change.
>
>A curious fact has struck me, in thinking about all this. Nobody seems to
>have coined a word for delta-T, except "delta-T", sometimes expressed as a
>Greek symbol. Isn't that a bit of a surprise? It doesn't really describe
>well what it represents, and it certainly gives no clue as to which
>time-scale is lagging on which. Delta-T , as defined, is normally positive,
>but can be negative, as it was for a few years near 1900. Is there another
>word for this difference, used in any other language? In many ways, it's
>similar to the confusion that surrounds another time-difference, that
>between mean and apparent time, which is so confusingly expressed by the
>words "equation of time".
>
>George.
>
>contact George Huxtable at george@huxtable.u-net.com
>or at +44 1865 820222 (from UK, 01865 820222)
>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
>
>
>
>
>>
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

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