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    Re: DeltaT (was Tycho Brahe Mars oppositions)
    From: Michael Dorl
    Date: 2004 Dec 3, 12:48 -0600

    At 08:32 AM 12/3/04 -0200, Omar wrote:
    >As F. Reed said in another message, the DeltaT is a difficult value.
    >I'm not sure how the formula for early DeltaT values in
    >Meeus book was constructed. Methods for the DeltaT
    >calculation from the same book are already diverging just 20
    >years from publication. How someone could estimate what
    >the value of DeltaT was 500 years ago scapes me.
    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)
    So presumably there is a theory involving the Moon's interaction with the
    Earth for predicting delta T.

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