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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.
    >> Now, about Sun and Moon... what is a preferable way to set images
    >> 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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