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    Re: September Equinox computation
    From: Dan Allen
    Date: 2002 Sep 24, 09:29 -0700

    On Tuesday, September 24, 2002, at 04:43 AM, Pierre Boucher wrote:
    
    > Which method would you use to PRECISELY compute (hh-mm-ss) the
    > September
    > equinox?
    
    This is actually a fairly hard problem.  We usually know that the
    equinox
    has a declination of zero and a right ascension of 12:00:00 hours in
    September.
    
    My computation calculates the position of the sun using the Meeus
    formulas and
    iterates with a secant root finder to find when the declination is
    zero, or when
    the right ascension is 12:00:00.  The hard part is that our
    mathematical models
    do not always have a time when these two quantities have these two
    desired values.
    
    I always seem to be a few minutes off of what the Naval Observatory
    says is the
    beginning of Fall and I have discussed this with a friend at the Jet
    Propulsion
    Laboratories (JPL) and he says that what I need to get the "right"
    answer is the
    full DE200 emphemeris package, which is huge and largely in Fortran.
    Alas, I
    am a C programmer.
    
    Dan Allen
    
    PS - My computations for this year put the equinox at 4:52:04 AM GMT on
    23 Sep 2002.
    
    
    

       
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