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    Re: Moon corrections: how computation order affects the result
    From: Frank Reed
    Date: 2020 Sep 1, 13:46 -0700

    Gary LaPook, you wrote:
    "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."

    I totally agree that the tables in the Air Almanac are a great choice, and I teach that method in my Traditional Celestial Navigation and also my Advanced Celestial Navigation workshops. It's a shame that the standard govt-issue Nautical Almanac has such arcane tables for lunar altitude corrections.

    Just to be clear, I don't think there's any "debate" here over "complex methods"... merely curiosity over the numbers. And there are other motivatiojns. For example, in Stan Klein's case, he was diligently reproducing the exact numbers from the official NA for his app that was used for USPS (Power Squadron) classes.

    You added:
    "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."

    Yes. Absolutely. For common celestial lines of position, this is true. In my Modern Celestial Navigation workshops, I encourage new navigators to work to the nearest 0.01° which is 0.6 nautical miles, just below the system limit of manual celestial navigation. Nonetheless, there are still some circumstance where worrying about tenths can be useful. And it's worth mentioning that some exam questions on USCG licensing exams still depend on that higher level of precision. Differences of a couple of tenths count. They shouldn't, but the test designers are unlikely to change this standard any time soon. 

    "And that is before you allow for the width of the pencil lead that you use to plot the resulting LOPs."

    It's not unusual to have a scale for LOPs of an eighth to a quarter of an inch to a nautical mile. At that scale, your pencil would be more like a crayon if you could not draw with a precision of a tenth of a nautical mile! I do agree that working at that level is normally over-kill, but the pencil is not the problem. Of course if you're using short sight reduction tables (249, 229), you often have intercepts that are 30 miles long, and for plots based on those tables, you would use a much coarser scale, and then you might start sharpening your pencil a bit more obsessively. 

    Frank Reed

       
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