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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: September Equinox computation
From: Jay Borseth
Date: 2002 Sep 24, 13:09 -0700
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@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.