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    Re: Navigation without Leap Seconds
    From: George Huxtable
    Date: 2008 Apr 22, 09:05 +0100

    Geoffrey Kolbe and I seem to agree well, now, about the difficulties in
    predicting future changes in delta-T.
    
    However, his posting ended with-
    
    | George's Delta T = 0 for the epoch of 1900 for a current value of 38
    | seconds for Delta T seems to be adding to the chaos which he feared with
    | different time systems. In such literature as I have read on the subject,
    a
    | value of 24.349 is used for the 1900 epoch, with a current value of about
    | 65 seconds. See, for example, ftp://maia.usno.navy.mil/ser7/deltat.preds I
    | would be interested to know where George got his value for Delta T.
    
    Sorry about that.
    
    Geoffrey asks where I got that "current value of 38 seconds for Delta T"
    from, and I am rather ashamed to say that I simply pulled it out of my
    memory, and got it quite wrong. I should have known better. A current value
    for delta-t, for 2008, will be somewhere round 65 seconds, not 38 seconds,
    just as he says.
    
    However, Geoffrey and I seem to disagree seriously about its earlier value,
    around 1900, which he states to be 24.349 seconds. I'm not sure where that
    comes from, but not from the web page that he quotes, which deals only with
    recent years. I have in front of me the "Explanatory Supplement to the
    Ephemeris", 1974 amended impression , which gives, in table 3.1, delta-t
    values until 1972. The value for the year 1900.5 is -3.90 sec, and values
    were negative from 1869 to 1902, reaching an extreme value of -8.23 sec  in
    1892. If the deceleration of the Earth was steady, then delta-t would follow
    a parabolic curve, but it's vastly perturbed by those short-term
    fluctuations. There may well have been recent reassessments of the
    historical changes in delta-t, but surely not enough to account for the big
    discrepancy between that table and the number that Geoffrey quotes.
    
    Between the two of us, we seem to have added rather a lot to the chaos that
    I warned against...
    
    By the way, if anyone chooses to look up that volume, they should be careful
    about the graph on the facing page to the table, fig 3.2, which purports to
    show "general trends of delta-T, 1660-1972". This was taken from a paper
    intended to show up other matters, in which much readjustment of the numbers
    had taken place. In a later edition, this was replaced by a version of fig
    3.2 that was completely revised, and now accords with the numbers in the
    table.
    
    A completely revised "Explanatory supplement" appeared in 1992, edited by
    Seidelmann, excellent in many ways, but this doesn't tabulate delta-t
    changes, and its graph is on a compressed log scale, from which it's
    impossible to extract numerical values.
    
    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.
    
    
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