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The variations in the Earth's rotation due to the exchange of angular momentum
between the atmosphere and the crust are not that small. The peak-to-peak
effect on the length of day of the seasonal oscillation is about 1 ms. But the
effect is not cummulative as so we don't see a significant contribution on
decadal or longer time scales and perhaps this was George's point. See
 and in particular
. The large spike
in the early 1980s is due to a particularly strong El Nino.
While a post-doc many moons ago, I used lunar laser ranging data to establish
the existence of a 40-60 day quasi-periodic variation in the Earth's spin due
to atmospheric processes.

-- Richard Langley
   Professor of Geodesy and Precision Navigation

On Thu, 2 Dec 2004, George Huxtable wrote:

>Omar Reis asked, about calculating delta-T (the difference between Atomic
>time and Greenwich time) for future years-
>
>>I wonder why can't this value be deterministic,
>>like everything else in astronomy ?
>>
>>I mean, the earth rotation is slowing down, as much
>>as a second per year. But why is this amount variable
>>and umpredictable ?
>
>======================
>
>Not everything in astronomy is deterministic. Even in a system, such as the
>solar system, in which more than two bodies have a mutual gravitational
>interaction, there's a limit to the long-term predictability of it all.
>This is the famous "three-body problem".
>
>In the case of the Earth, a small part (and a rather constant part) of the
>Earth's spin-momentum is transferred to the Moon each year, dissipating a
>certain amount of its spin-energy to the tides in doing so.
>
>But a greater transfer of momentum is between the Earth's crust and
>currents in the liquid parts of its interior, which show a slow turbulence.
>Because the overall spin-momentum has to be conserved, changes due to these
>currents must be matched by equal-and-opposite changes in the crust. This
>effect can work either way, to slow down the speed of the crust or even to
>speed it up. Even if we understood the basis of these turbulent motions
>within the Earth, I doubt whether their future details would be
>predictable.
>
>Even changes in the East-West circulation of the atmosphere have to be
>balanced against corresponding changes in the spin of the Earth, but I
>think this effect is a small one.
>
>Delta-t bumbled along without great dramatic changes in the two centuries
>prior to 1900, since when it has been increasing rather steadily at about 1
>second per year. However, over the last few years that regular increase
>seems to have switched off, rather suddenly and dramatically. There are
>things going on there, far beneath our feet, of which we know little.
>
>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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