NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tom Sult
Date: 2014 May 12, 11:29 -0500
Alan, you asked:
"Would one then obtain a valid "fix" or might the moon's movement, it moves at a rapid, irregular rate, mess things up."As long as you work each sight correctly, there's no problem. The Moon is different because its SHA and Dec change much more rapidly than other objects. For the stars, the SHA and Dec scarcely change even over the course of a week or two. That means that if you worked a star sight at 9pm and then a different sight of the same star at 3am, you would not need to look up new values for SHA and Dec. You could skip that "almanac" step with the second sight. For the Sun and most of the planets, the SHA and Dec also change relatively slowly so the error would not be too large if we used the same almanac data later in the same day for a second sight. But for the Moon, its rapid motion across the celestial sphere implies large changes in SHA and Dec even in just one hour. So we have to be careful and do a full interpolation of the almanac data for each and every Moon sight. But it's all in there in the almanac data. There's no extra uncertainty in the Moon's tabulated position beyond what you can see there in the pages of the Nautical Almanac (or equivalent).
In an earlier post, you mentioned something about shooting UL and LL sights of the Moon every time. I'm not sure that's what you really meant. At any given time, you should only be using the "sharp" fully illuminated limb. On a typical day, that implies that you're doing UL sights for about half the time that the Moon is in the sky and LL sights for the other half. The details of the Moon's altitude correction add another wrinkle and another place to screw things up. But so long as we follow the rules correctly for each and every sight that we work up, any Moon sight is just as good as another and usually just as good as any other celestial altitude (since the oblateness correction for altitudes is typically ignored, there is, in fact, a small error on the order of a quarter of a minute of arc that can be found in Moon sights and not others, but that's usually well below the limits of good observations anyway).
-FER
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