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    Re: longitude around noon (a twist)
    From: Jeremy C
    Date: 2008 Jun 4, 01:44 -0700

    
    > But when Hc is near 90 degrees (say, above 87 or 88), it's entirely
    > feasible to actually plot a circle of position. � Plot GP of the body on
    > the chart, open dividers (or, better, a draftsman's compass) to D, and
    > draw at least the part of the circle of position near one's DR.
    >
    > Lu Abel
    
    
    > But when Hc is near 90 degrees (say, above 87 or 88), it's entirely
    > feasible to actually plot a circle of position.   Plot GP of the body on
    > the chart, open dividers (or, better, a draftsman's compass) to D, and
    > draw at least the part of the circle of position near one's DR.
    >
    > Lu Abel
    
    
    I have done this many times on paper, and most recently on my last US
    coast Guard exam, and a couple of times at sea with both sun and moon.
    
    It is very difficult to shoot, especially as HS exceeds 89degrees.
    The body literally moves in a sine wave as you rotate yourself around
    across the horizon.  I shot the moon in one case where the body never
    left my 3.5x scope as I moved in a complete circle.  The best thing
    was to shoot it both facing north and south.
    
    Plotting is actually fairly easy, and getting a fix is easy yet
    troubled.  Take 2 sights a few minutes apart, plot the GP, then
    advance or retard the GP to a common time (similar to advancing a
    radar LOP) then draw the arc according to minutes of zenith distance.
    The two circles will intersect in two points, which is the issue.  I
    just picked the nearest to my DR.  On the USCG test, it was a bit
    trickier.  I can post the problem once I get home and look it up.
    
    The very high altitude moon shoot I did (89deg 50+ min) Ho was around
    89deg 57' or so so left me with a small circle of position which was
    in effect my fix.  It is in my old nav notebook and I will post it
    when I get home.
    
    As my rule of thumb, I typically shoot circles when HO exceeds 88
    degrees.  120 miles can be reasonably plotted on universal plotting
    sheets with little trouble.  I tried once with an 87 degree sight and
    it was a bit tricky.  On larger scale charts, it gets more difficult.
    On the exam I was able to plot on PS 923 I believe.  A NM is about 2mm
    on that scale.
    
    It's a fun sight to shoot in the tropics.
    
    Jeremy
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