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    Re: Delta-T: was [NAV-L] Tycho Brahe Mars oppositions
    From: Richard B. Langley
    Date: 2004 Dec 2, 14:43 -0400

    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---.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---.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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