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    Re: Moon meridian passage
    From: Frank Reed
    Date: 2020 Sep 28, 18:01 -0700

    Yes, you can do a Moon "meridian" sight (*). Navigators sometimes try this out as a novelty or a challenge. Some people like to call this a LAM or "Local Apparent Moon" sight as a pun on LAN for "Local Apparent Noon". It's considerably more difficult than other meridian sights because you have to have skill correcting Moon sights, and you need to deal with the Moon's relatively rapid changes in position on the celestial sphere. For example its Dec can change as rapidly as 15 minutes of arc per hour while the Sun's maximum rate of change is 1 minute of arc per hour. This is equivalent to correcting for vessel motion. If you're travelling due south at 15 knots during sights of the Sun around LAN, then you face similar issues as you would for sights of the Moon near meridian passage even when you're not moving. It's all manageable. It can be done.

    Moon sights like these have never achieved much popularity because they're relatively hard work, and they don't offer much advantage. Sights of the Sun at and around noon will always be easy and valuable. Sights of the Moon around meridian passage are mostly a novelty.

    *Do we really mean a meridian sight? Usually when people discuss this type of sight, and this actually applies to noon sights though it's less significant, they're not really dealing with meridian sights. Instead they're talking about maximum altitude sights, also known in the jargon as "culminations" or maximum altitude sights. Because the Moon or Sun has changing Declination, and because the observer may be in motion, maximum altitude does not occur when the Sun is on the meridian. Life would be easier if you could insist on actual meridian sights for the Moon, and this is possible on land.

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

       
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