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    A-12 bubble sextant results
    From: Paul Hirose
    Date: 2002 Dec 20, 20:11 -0800

    Near dusk today I pulled out my 1942 Link A-12 bubble sextant and took
    some shots through a window. I made the sights from a standing
    position and tried to make the coincidences rapidly but not sloppily.
    However I did use a magnifying glass to read the vernier carefully to
    the nearest minute.
    Then I used the USNO's online calculator and a GPS-measured position
    to obtain Hc. Right away it was evident the instrument had a +10
    minute index error. Allowing for that, as well as refraction, I got:
        UTC      Ho       Hc    diff.
    00:12:13   4 31.6   4 30.6   +1.0'
       13:42   4 16.1   4 15.3   +0.8'
       15:06   4 01.7   4 00.8   +0.9'
       16:10   3 50.3   3 49.7   +0.6'
    The results are amazingly good. In fact, I don't believe they are
    typical. It doesn't seem likely someone badly out of practice as I am
    (not that I ever was well practiced) should do so well with a zero
    magnification sextant. This was probably just a lucky streak.
    My Ho is still consistently high; a -11 minute index correction would
    have made the numbers look even better. With the large index error and
    rapid change of refraction at these low altitudes, I made the index
    and refraction corrections separately rather than lumping them
    together in the normal manner. There may be nonstandard refraction at
    work too; we just had a storm pass through. Then there's the matter of
    shooting through the window because I was too lazy to go out!
    I plan to repeat the experiment to get a better idea of my accuracy.

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