NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: UNK
Date: 2013 Sep 17, 16:30 -0700
Stan,
I would not recommend to derive Mer. Pass. from the GHA Sun if it is required to be accurate to the second. We are told on page 261 of the N.A. that the tabulated GHA Sun may be shifted by as much as 0.15' of arc. This makes your method #3 less accurate than #2 because EoT is always accurate to 1 sec of time (I would hope!).
If anything, you may try to derive a "better" value of the GHA Sun from the given EoT. There may be some justification for this when you need the GHA Sun for a lunar distance. However, it should be pointed out that such creative uses of the N.A. are very dangerous because they require a thorough understanding of the design and the implementation of the tables. Both, you and I, have learned from our frustrated attempts to reverse-engineer the N.A. that we don't have this detailed knowledge. Best practice is to use the GHA for GHA and the EoT for EoT.
Herbert Prinz
Herbert
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