NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: September Equinox computation
From: Dan Allen
Date: 2002 Sep 24, 09:29 -0700
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.
