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    Re: Longitude at noon
    From: Bruce Cutting
    Date: 2020 Sep 08, 21:45 -0600

    The shot pamphlet that I got with my first Davis Plastic sextant said
    it exactly that way - bought the sextant in 1982 as I remember
    Quoting Frank Reed :
    > 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
    > [plain text auto-generated]
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