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    Re: Sun's instrumental altitude with artificial horizon
    From: Robert Eno
    Date: 2003 Feb 21, 12:10 +0000

    As a long time and frequent user of artificial horizons of all types I
    have to put my two bits' worth in.
    
    I respectfully disagree with your contention that # 1 -- that is,
    superimposing the sun over its own reflection in the artificial horizon --
    is the most precise method. In fact it is likely the least precise of the
    three options that were presented. If a machine were doing the observing,
    then maybe that would be so, but it much more difficult to judge perfect
    superimposition than it is to judge when the suns' limbs touch.
    
    I strongly recommend options 2 or 3. I'll add one more bit of advice: do
    not attempt to physically bring the limbs into tangency. Allow the sun
    to "rise" or "fall" into tangency with its reflection while gently rocking
    the sextant back and forth.
    
    regards,
    
    Robert
    
    
    >> 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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