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    Re: DeltaT
    From: Omar Reis
    Date: 2004 Dec 3, 18:32 -0200

    > Michael Dorl wrote:
    > The table in the Astronomical Almanac giving delta T going back to 1620
    > contains the following note....
    >
    > This table is based on an adopted value of -26"/cy**2 for the tidal term
    > (ndot) in the mean motion of the Moon from the results of analyses of
    > observations of lunar occultations of stars, eclipses of the Sun, and
    > transits of Mercury. (See F.R. Stephenson and L.V. Morrison, 1984, Phil.
    > Trans. R. Soc. London, in press.)
    >
    > and then goes on give an equation for corrections for different (ndot)
    > assumptions.
    >
    > So presumably there is a theory involving the Moon's interaction with the
    > Earth for predicting delta T.
    
    Even considering that DeltaT  is rather small change in
    earth rotation (~1 sec/year = 3,17E-08 ) I wonder if it
    could be atributed to magma flow, or atmosphere exchange
    or even el ni?o, as it was suggested in previous messages,
    for the earth angular momentum must stay constant
    and the DeltaT is increasing since 1900.
    
    Interaction between the Moon and the sea water,
    generating the tides, seems more likely to be
    the culprit (at least in the long term).
    
    Omar
    
    P.S. I made a DeltaT chart since 1580, using Meeus AA formulas.
    (actualy I have adjusted the current epoch values to be more in line
    with the observed data). The chart can be found at:
    
    http://www.tecepe.com.br/nav/nav/DeltaT.gif
    
    
    

       
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