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Fig. 1 in the article on leap seconds for which I posted a URL a few days ago
shows delta-T between 1600 and 2000 on a linear scale.
-- Richard Langley

On Tue, 22 Apr 2008, George Huxtable wrote:

>
>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.
>
>
>>
>


===============================================================================
 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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