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    Re: Sun's instrumental altitude with artificial horizon
    From: Bill Noyce
    Date: 2003 Feb 21, 10:59 -0500

    > Now, about Sun and Moon... what is a preferable way to set images (oil/sky
    > image)through view and just what corrections should I use in order to
    > finally calculate the departure?
    
    I think these are your choices:
    
    #1.  Bring the sun's image down so it is superimposed over
    its reflection in the artificial horizon.  In other words,
    the two disks merge into one.  This is likely to be the
    most accurate way to make the observation.  To convert Hs
    to Ho in this case, first correct for index error in the
    usual way, then divide by two, then correct for refraction
    (using the Almanac table for stars).  There is no correction
    for semidiameter in this case.  For extra refinement, you
    could add 0.1' for the sun's parallax if the altitude is less
    than about 60 degrees.
    
    #2.  Bring the sun's image down so that its lower limb
    just meets the near side of the reflection.  (This is the
    reflection of the lower limb, but it appears on the upper
    side of the image.)  In other words, the two disks look like
    a snowman or a figure 8.  To convert Hs to Ho in this case,
    first correct for index error, then divide by two, then
    apply corrections for refraction, semidiameter (lower limb),
    and parallax using the normal "Sun LL" table.
    
    #3.  Bring the sun's image down so that its upper limb
    just meets its own reflection -- another snowman, but
    upside down from the case above.  To convert Hs to Ho in
    this case, first correct for index error, then divide by
    two, then apply corrections for refraction, semidiameter
    (upper limb), and parallax using the normal "Sun UL" table.
    
    When observing the moon, my guess is that procedure #2
    or #3 (based on whichever limb is visible) will give
    a more precise observation than #1, but I can't claim
    any experience.  Also, in case #2 and #3, the NA tables
    correct for parallax at the same time as for semidiameter,
    whereas in case #1 you have to do it separately, and I
    don't think the NA makes that easy.
    
            -- Bill
    
    
    

       
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