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    Re: Altitude correction using Artificial Horizon?
    From: Michael Wescott
    Date: 2000 Sep 05, 9:33 PM

    > Is it necessary to apply any correction for altitude when reducing a sun
    > sight taken with an artificial horizon?
    > If the two sextant images are superimposed, I could presume that the sight
    > is the center of the disc and no correction is necessary.  However, the
    > correction tables in the Nautical Almanac show that the correction is
    > skewed between UL and LL.  For example, at an apparent altitude of 20
    > degrees in September, the corrections are +13.5(ll)/-18.3(UL).  The total
    > difference is 31.8' which is exactly twice the SD of 15.9'.  So I might
    > argue that I should use a correction "in the middle" of -2.4'.
    > Any clarification on this point would be appreciated.
    > Similarly, if I take the sight by aligning the LL of the sun image with the
    > UL of the image in the AH, should I just correct the sight by the SD?
    >From Bowditch (1984):
        By means of the sextant, the double-rflected image is brought into
        coincidence with the image appearing in the liquid. In the case of
        the sun or the moon the bottom of the double-rflected image is brought
        into coincidence with the top of the image in the liquid, if a
        lower-limb observation is desired. For an upper-limb observation, the
        opposite sides are brought into coincidence. If one image is made to
        cover the other, the observation is the center of the body.
        When the observation has been made, apply the index correction and any
        other instrumental correction, as well as any correction for personal
        error. Then take half of the remaining angle and apply all other
        corrections except dip (height of eye) correction, since this is not
        applicable. If the center of the sun or moon is observed, omit, also,
        the correction for semidiameter.
    The Almanac Altitude Correction Table for the sun includes corrections for
    SD, Refraction, and Parallax. So it should be used for a LL over UL sight.
    For a center-of-the-body observation, add the SD to the LL correction or
    just use the Stars and Planets correction.

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