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    Re: The Moon and Parallax
    From: David Pike
    Date: 2019 Oct 15, 11:47 -0700

    I think I’m beginning to understand the Moon Altitude Correction Table in the back of the Nautical Almanac.  Please tell me if this is correct.  The tables assume you’re observing Moon’s lower limb.  The upper table assumes HP is its lowest possible value, 53.0’ say.  They choose this so that the correction from the lower table is always added. The top table appears not to follow the cosine App Alt formula exactly, because the kind people at UKHO/USNO make life easy for us by correcting for standard atmospheric refraction and the Moon’s semi-diameter at the same time. 

    The lower table is to correct for the fact that on any particular day the Moon’s HP might be any value between 54’ and 61.5’.  There’s only one more question to ask now.  Does all this super 0.1’ accuracy go out of the widow when you subtract 30’ if you observe the Moon’s upper limb or 15’ if using a bubble sextant?  Shouldn’t you subtract 2 x or 1x the Moons actual semi-diameter for the day of the observation?  In asking this question I’m assuming the lower table correction allows for any difference in semi-diameter for a lower limb observation, because you enter with HP, and thus actual SD can be calculated and added by the USNO computer.   DaveP

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