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    Re: September Equinox computation
    From: Jay Borseth
    Date: 2002 Sep 24, 13:09 -0700

    There is a C version of Novas which can use the DE200 ephemeris
    available at http://aa.usno.navy.mil/AA.  Pocket Stars
    (www.nomadelectronics.com) uses the C version of Novas and DE405.  You
    can look on the bibliography section on the download page for more info
    on merging JPL and Novas.
    The results I'm getting put the equinox at:
       4:51:54 AM GMT on 23 Sep 2002, crossing at E 105 degrees 9' 56".
    - Jay
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Dan Allen
    Sent: Tuesday, September 24, 2002 9:29 AM
    Subject: Re: September Equinox computation
    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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