NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2020 Sep 1, 12:48 -0700
This is why I use the corrections from the Air Almanac. With an altitude of 80° and on a day when the moon's semidiameter is 16.2' and the P-in-A correcton for the 80° altiude is 11', the correction adds to 27', (for a LL observation) just like the much more complex methods you are debating. In the AA, both of those correction for the Moon come from the bottom of the Moon daily page, see attached. The refraction correction and dip corretions comes from the inside of the back cover, see attached. Since the sigma of the accuracy of marine sextants observations is greater than one minute, you guys are argueing about "how many angels can dance on the head of a pin." For real, on board a ship, enroute between two ports, out on the real sea, you guys are attempting to compute much more accurate LOPs than are practical or that make any difference. And that is before you allow for the width of the pencil lead that you use to plot the resulting LOPs.
gl
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Re: Moon corrections: how computation order affects the result
From: Eric Fernandez
Date: 2020 Sep 1, 06:36 -0700
Thank you David. I understand what you are saying and did some further computations for various values of HP and Ha. Indeed, as you mention, correcting SD before P-in-A (serialising addtions) or using uncorrected SD after P-in-A (so purely additive) lead to almost the same result. For instance for HP = 61 and Ha = 80 (maximising parallax and augmentation corrections, and minimising refraction) give me those correction results:
- Ha - R + P(Ha) + SDgeo = 27.0403' <--- uncorrected SD
- Ha - R + P(Ha) + SD + SDaug = 27.3307' <---- purely additive P + SD + augmentation
- (Ha - R + SD + SDaug) + P(Ha-R+SD+SDaug) = 27.0401' <--- serialised: P-in-A calculated at the end from the corrected Ha, which is as you mentioned close to the result obtained with the uncorrected SD.
What troubles me, is that Norie's tables give a total correction of 27.3'. Capt Khan's guide or navsoft website also describe a purely additive method (would lead to 27.3'). The Nautical Almanac also adds all terms calculated from Ha and table xxxv gives for HP=61 and H=80 a correction of 20.5+6.4=26.9', which is closer to either an uncorrected or a serialised calculation. I am wondering if there is not some inconsistency in various sources on how to use or properly add both the augmentation and parallax?
