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    Re: Precomputed lunar distances
    From: Bill B
    Date: 2005 Apr 19, 17:01 -0500

    Bill wrote, regarding angular refraction corrections:
    
    > They do not seem to reflect refraction moving along  a straight line to me,
    > where I might expect the corrections to be similar to  a curve derived from
    > refraction values at those altitudes."
    
    Frank responded:
    >
    > I take it that you're suspicious of these results because it seems as if  the
    > correction is just about 0.6 minutes of arc across a wide range of
    > altitudes. Strange, huh? Strange but true... in this case where both stars
    > have  the same altitude (and as long as the altitudes are above about 12
    degrees).
    
    In fact that is not what raised a red flag for me.  I had drilled down too
    far and done a scatter graph with Excel, so every or hundredth or thousandth
    was magnified.  Now that you mention it, I recall a discussion about bodies
    of equal observed altitude and the 0.6' minute figure.
    
    I apologize to the you and the list for rehashing a subject that was
    apparently covered in October.  At that point I was still working with a
    cardboard sextant and H0229, playing with various sailings, Bowditch tables,
    and constructing Mercator plotting sheets.  I did not anticipate the journey
    would take me this far, and did not pay close enough attention to postings
    that seemed above me at the time.
    
    Bill you wrote:
    > "Yes, that is my image.  A two-dimensional representation of three
    > dimensions.  What a camera would see."
    
    Frank responded:
    > OK. And this can be useful so long as you remember that the sides are
    > actually straight as an arrow.
    
    So is a wire-thin hula hoop when viewed from the correct angle.  In
    another special case, a circle.  The rest of the time, and ellipse ;-)
    
    Regarding my question, "Another hypothetical scenario.  If I take the same
    two stars, calculate true separation of 34d 27.7', they have identical Hc's
    of 1d 36.8', and  hypothetical refraction is -88d, what separation might I
    expect to measure with a sextant?"
    
    I asked if for two reasons:
    
    1. As a sanity check to determine if my mental model was workable.  I would
    expect that there would be almost no observed separation given the above
    scenario.  Is that correct?
    
    2.  When I ran the above scenario through the separation refraction
    correction formula, the correction was only a matter of a degree and a half.
    If above paragraph/assumption is correct, I would expect separation
    corrections in the -33 degree range.  Perhaps extreme refraction is out of
    the intended scope of the formula, or one of those lovely rules like "If cos
    X is <0 and it is a Tuesday with an odd date subtract 90" needs to be
    applied.
    
    Bill
    
    
    

       
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