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    Re: Loss of accuracy using Sun over Sun with a mercury artificial horizon?
    From: Fred Hebard
    Date: 2016 Jul 18, 21:12 -0400
    The Nautical Almanac conveniently tabulates the SD of the sun on its daily pages.  You can compare those data to your results for index error obtained from touching disks. Those results also allow computation of SD, to refine your technique.

    Fred Hebard

    On Jul 18, 2016, at 5:58 PM, David C <NoReply_DavidC@fer3.com> wrote:

    A very relevant thread for me. I find it difficult to decide when the sun's limbs are just touching.

    A few days ago I took 18 sights. I used my GPS position as my AP. 17 of the intercepts were < 1.5 ', with 13 < 0.5' .   16 of the sights were Away which suggests to me a systematic error.  The IE is 0'. One thought I had was that if the IE was 1' off there would be an even towards/away distribution. However I had carefully checked IE using the two limb method.

    I have concluded that I am not bringing the sun down far enough before  reading the sextant. Next time the sun appears I plan to take another series of sights, this time deliberately nudging  the vernier screw just past the point where the limbs seem to touch.

    Another thing for me to try is sun over sun.  This would give me two limbs to observe, rather than just the one.

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