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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@LISTSERV.WEBKAHUNA.COM] On Behalf Of Dan Allen
Sent: Tuesday, September 24, 2002 9:29 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
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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