NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Out of Date Almanac
From: Frank Reed
Date: 2009 Dec 8, 00:26 -0800
From: Frank Reed
Date: 2009 Dec 8, 00:26 -0800
Gary, you wrote: "It's an exact correction to GHA Aries" OK, but does it really do you any good? What you really need is a precession table for each of the navigational stars. And we don't even need to think of it as a pure precession table. If there's a longer interval involved, it could include proper motion. But it's nothing complicated and not hard to produce. The point is that it's some number of minutes of arc for each star's SHA and Dec. And then you throw in the correction for GHA Aries and add them up as usual to get the position of the star. But correcting GHA Aries alone while leaving out the corrections for the individual stars doesn't help. So instead, the Aries correction can just be lumped in with the other tabulated corrections for the individual stars. Algorithmically, we can do either: Dec*(2010) = Dec*(2006) + dDec* GHA*(2010) = [GHA Aries(2006) + dGHA Aries] + [SHA*(2006) + dSHA*] or: Dec*(2010) = Dec*(2006) + dDec* GHA*(2010) = GHA Aries(2006) + GHA*(2006) + dGHA* It's not much of a difference --saves one step. But no matter what, we still need a table of dDec* and either dGHA* or dSHA* for all the navigational stars. And again, it's not hard to make one. Or you can just accept errors of a few minutes of arc, or just not use the old almanac for the stars at all. -FER -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com