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I'd like to reiterate that, over the long term, there is nothing very
remarkable about the recent period during which the Earth's mean rotation rate
has increased slightly. Attached are 4 very small files to illustrate this.
1) The first is a plot of UT1-TAI from 1962 to the present using IERS data.
The instantaneous slope of the UT1-TAI curve gives us the relative difference
of the Earth's rotation rate from some standard or mean value. Multiplying
the slope by -86,400 gives us the length of day in excess of 86,400 seconds
(LOD). Over the past 40 years, or so, the slope has been fairly constant at
about -0.7 or -0.8 seconds per year but since around 1998, the absolute value
of the slope has been getting smaller.
2) The relative rotation rate for the corresponding period is shown in the
second plot.
3) LOD for the corresponding period is shown in the third plot.
4) A plot of historical LOD values which shows frequent variations in the
Earth's spin of even greater significance than what we have observed over the
past few years.
The first three plots were produced using the IERS's on-line facility:

And the fourth plot was produced using:

and selecting JPL's combined series.

-- Richard Langley
   Professor of Geodesy and Precision Navigation

On Wed, 28 Jul 2004, George Huxtable wrote:

>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.
>
>================================================================
>contact George Huxtable by email at george@huxtable.u-net.com, by phone at
>01865 820222 (from outside UK, +44 1865 820222), or by mail 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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