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    Re: Artificial Horizon.
    From: Geoffrey Kolbe
    Date: 2002 Apr 1, 08:57 +0100

     Rodney Myrvaagnes wrote:
    > Has anyone tried making a prismatic horizon that dangles like a damped
    Yes, I used a Davis plastic sextant as the basis for a novel (as far as I
    know) design of sextant incorporating an "artificial horizon" mirror. This
    mirror was balanced on a single point in such a way that it remained in the
    horizontal plane regardless of any modest tilt or rocking of the sextant.
    The movement of the mirror was damped by virtue of a copper strip attached
    to the mirror mount. As the mirror tilted, the copper strip moved in the
    field of a small rare earth magnet fixed to the sextant frame, and so
    damped the movement.
    The way the sextant worked was as follows. The image from the index mirror
    was reflected down through an image inverting dove prism, to a partially
    reflecting mirror in place of the usual horizon mirror. At this point, part
    of the image was reflected back to the telescope in the usual way, and part
    of the image went through the mirror, where it was reflected off the
    "artificial horizon mirror". Another fixed, fully reflecting mirror then
    reflected the image coming up from the horizon mirror back through the
    partially reflecting mirror and into the telescope.
    In this way, two images of the same object were seen in through the
    telescope. The sextant was rocked sideways until both images were in the
    same vertical plane and the angle of the index arm was adjusted until the
    two images were superimposed (for stars) or touched in the usual way for
    sun or moon. The virtue of this arrangement was that image was invariant to
    sextant tilt, just as with a normal sextant, and no other illumination was
    required as with a bubble sextant.
    The sextant worked quite well, but the artificial horizon mirror mount was
    difficult to make. I never did manage to make the mirror mount so that the
    mirror stayed perfectly horizontal as the sextant was tilted. Though the
    principle was proven, I did not see any way to overcome this problem in any
    simple way, so I gave up on this project. I did try a small bowl of mercury
    as a horizon mirror, but the mercury would not keep still! The slightest
    knock or movement would send shivering waves across the surface of the
    Geoffrey Kolbe.
    Dr Geoffrey Kolbe, author of "Long Term Almanac 2000-2050" for sun and
    selected stars, with concise sight reduction tables.
    Available online from www.pisces-press.com

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