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    Beginner Meridian Passage Question
    From: Bill B
    Date: 2004 Sep 1, 14:51 -0500

    I am a bit confused on meridian passages.
    
    So that we using the same frame of reference, assume a position of 40N 90W.
    
    As I understand it the observer's meridian in the Northern Hemisphere runs
    from the north horizon, through the celestial pole, through the observer's
    zenith, through the celestial equator, continuing directly south to the
    horizon.  An unseen portion of this (great?) circle continues on through the
    nadir and back to the northern horizon.
    
    First, are there terms for the line segments from the north horizon to the
    pole, pole to zenith, zenith to equator, equator to southern horizon, and
    the unseen semicircle though the nadir? I suspect much of what follows is
    matter of semantics/terminology, hence the above queries.
    
    Second, the concept of meridian passage for objects with a declination
    greater than the observer's latitude causes me some confusion as they never
    dip below the horizon, so under proper viewing conditions Polaris, Dubhe et
    al could be observed crossing the meridian twice in a sidereal or solar day.
    (They do this in fact, whether I can see it or not.  While not observable
    from my position, the Sun would cross the unseen part of my meridian 12
    hours after my meridian passage.)
    
    When looking at times of meridian passage for various bodies with navigation
    software only one time of meridian passage is listed, even for bodies with
    declinations greater than my latitude that would cross over my observable
    meridian twice.  A spot check for meridian passage of bodies that don't dip
    below the horizon seems to indicate that meridian passage is when the Hs/Ho
    is greatest.  Put another way, when it intersects the meridian to the south
    of the north celestial pole.  Yet another look, when its LHA is 0.  I am
    guessing the same is true for Hs/Ho for the Southern Hemisphere, but
    direction is opposite.
    
    Would anyone be so kind as the slightly shrink my large sphere of ignorance?
    
    As a side bar, Susan Howell/Practical Celestial Navigation's chapter on
    meridian sights states that in a lifeboat situation the position of Polaris
    relative to the north celestial pole can be estimated by the position of
    Ruchbah, as it is on the same side of the PN and directly in line with
    Polaris and the PN.  While published in 1976, it has been revised since.
    Inspection of the current almanac star pages would indicate the SHA of
    Polaris is 320+, while Ruchbah is 338+.  Inspecting the rate of change in
    one year, it appears that Ruchbah's SHA is changing faster than Polaris's,
    so the relationship may be quite different in 2004 than it was in 1976.  Am
    I missing something?  If not, any current rules of thumb (other than finding
    a visible star with a like or 180 opposite SHA) for estimating the angle of
    Polaris to the North Pole?
    
    Thanks,
    
    Bill
    
    
    

       
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