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    Re: Longitude at noon
    From: Frank Reed
    Date: 2020 Sep 8, 17:00 -0700

    There are a couple of little issues here. First, you have to decide when "culmination" occurred (using the narrow definition of culmination as the time of maximum altitude). As Greg noted, this involves sights before and after local noon. There are various ways of combining and processing these sights. Next, you have to remember that culmination, in this narrow sense, may not be the same as meridian passage. This is especially true for Moon culminations, but even for the Sun there is an offset. You need to apply an adjustment for vessel motion and changing declination, which really amount to the same thing. From here we have the GMT/UT of the body's actual meridian passage.

    Since meridian passage always occurs when a celestial object has the same longitude as the observer, we only need the longitude of the subStar point for that celestial object at that instant of UT. And the longitude of the subStar point is the GHA! GHA is longitude. So finally, yes, that's the quick, modern way to do it. Take whatever steps are needed to determine the UT of the body's meridian passage. Then simply work out from almanac data the body's GHA at that instant of time. Its longitude is your longitude.

    So why don't books on celestial navigation make this simple process more clear? That's a question worthy of some discussion and speculation! 

    Frank Reed

       
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