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Jan Kalivoda appears to be enjoying his stay in Paris and making proper use
of the local amenities.

He wrote-

>By the way, sitting in Paris to the end of this week, I had read Howses's
>"Greenwich time" in the evenings. I have returned it already, but I remember
>that in the chapter about UT1, UT2, ET, UTC and so on, the author gives very
>tiny values for the yearly slowing of the Earth' rotation (in the graphs,
>not in the text, where no values stand, if I remember it O.K.), absolutely
>incomparable with the amount of cca 0,7 sec per year, valid up to the 1995
>or so, according to your discussion.
>
>Is it the mistake of my memory or didn'n I understand the text well? (Red
>wine is my hobby here, this can be the cause of my being so dull.)
>
>Yours, Jan Kalivoda

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

I have here the 1997 edition of Howse's "Greenwich Time and the Longitude",
an updated version of his 1980 "Greenwich Time and the Discovery of the
Longitude".

On page 168 he provides a plot, covering the last 300 years, showing rather
sudden changes (over a decade or so) in the spin rate of the Earth, which
cover the range between gaining about 1 sec per year, and losing at most
1.5 seconds per year, compared with his "reference standard". which is
based on the average length of the day in the 19th century.

In the text he refers to-

1. Secular changes; the progressive slowing down of the earth as a result
of tidal friction combined with "glacial rebound", the melting of ice caps
since the last Ice Age,, which amount to an increase in the length of the
day of 1.7 milliseconds per century.

2."Irregular (and unpredictable) fluctuations, probably caused by
differeing rates of rotation between the molten core and the slid mantle of
the Earth, which may result in the length of the day increasing or
decreasing by up to 4 milliseconds in a decade."

3. Seasonal changes, which need not concern us here.

It's interesting that the combined effect of items 1 and 2 in the text does
not even seem to approach the sudden changes in rate that Howse's plot on
page 168 shows, and comes nowhere near the change in rate of ebout 0.7 sec
per year, which appears to have occurred over a very few years towards the
end of the decade 1990-2000. So it seems that the sudden change  that
occurred then was indeed dramatic and unusual (though not unprecedented),
and greatly exceeds the magnitude of the fluctuations that would be
expected from Howse's text.

George.

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