NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Extracting the HP of the Moon from the Nautical Almanac daily pages
From: Hewitt Schlereth
Date: 2015 Feb 4, 21:07 -0800
From: Hewitt Schlereth
Date: 2015 Feb 4, 21:07 -0800
Stan, I always went to the NA line for whole hour of the shot. Which I guess is what is meant by "for the whole UT hour previous to the time of the observation."
I can see if you have a moon sight at say XX: 59:59 a case can be made for going to hour XX + 1. Presuming you can use 0.1' precision. I never felt the need. For me NA data to the nearest arc minute was plenty close enough.
Hewitt
We are working on revisions to the Power Squadrons Navigation course, and this question has come up.
In older versions of the Student Manual, it says to extract the HP of the Moon for the whole UT hour previous to the time of the observation. This makes sense, in that all Moon data (GHA, V, Dec, d, and HP) are taken from the same line.
However, in a later version, it says to use the whole UT hour closest to the time of the observation. This is in agreement with page 280 of the Nautical Almanac, "Methods and Formulae for Direct Computation". This would appear to be more accurate, but less "convenient".
I have over a dozen references, but none of them spell this out. Only one shows an applicable example, which uses the previous hour. All the other simply give the HP, or are on the hour, or have no difference in HP for the two hours in question.
In "real life" this is pretty unimportant, since the HP never varies by more than 0.1' from one hour to the next, but is one more right than the other? And right or wrong, what is the common practice?
Opinions?
Stan
