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    Re An "alternative" sextant?
    From: Scott Owen
    Date: 2008 Dec 18, 23:37 -0600

    George Huxtable wrote:
    The following idea may be a bit daft, and if it is I hope Navlist
    members will identify the snags and point them out.
    I'm not sure I can point out any snags but I have some questions and
    comments, so your patience is requested.
    What follows is an alternative way to reassemble the basic components of
    a sextant in a new geometry, which will give it different
    characteristics from the standard instrument. It's difficult to
    illustrate the notion without making a perspective drawing (at which I
    am hopelessly bad), so instead I'll try to paint a word-picture.
    With my feeble brain a flat side view would be very helpful and a
    perspective view wouldn't necessarily be needed.
    Start by taking a normal sextant and rebuild it. I'm not asking you to
    demolish your favourite instrument; just imagine it in your head, which
    is all I've done, so far.
    The angling of the index mirror has to change. Instead of its plane
    being perpendicular to the frame, it's now placed at exactly 45� to the
    frame, in such a way that incoming light from the observed body is now
    reflected to travel along the pivot line of the arm. The horizon mirror
    is shifted and fixed to a bracket that places it so that the pivot line
    passes through its centre, with a few centimetres spacing between the
    two mirrors. The horizon mirror, too, is angled by exactly 45� to the
    plane of the frame (about a vertical axis). In that way light from the
    observed body, travelling along the pivot line, is reflected into the
    telescope, which is aligned to point to the horizon as normal, and
    relocated to centralise the horizon mirror in its view. The horizon
    mirror is half silvered, as normal, so the observer sees, in the
    telescope, a direct view of the horizon and superimposed, a view of the
    celestial object, reflected via both mirrors, just as with a
    The "shifting" of the horizon mirror position and the resulting location
    of the telescope seems to be the tricky part, at least for me anyway.
    If I understand this correctly, and I don't think that I do, you want to
    relocate the horizon mirror to a position a few centimeters "down" and
    connected to the index arm then relocate shades and move the telescope
    "back".  Is that right? or are you envisioning something like a dual
    frame with the index arm "inside"?
    FWIW, a good link with lots of pictures regarding many different celnav
    instruments is here:
    and the only thing I found which seems close to your alternative sextant
    is the "reflecting half circle" though your geometry is different still.
     A picture of the reflecting half circle is here:
    The essential feature of the Hadley 2-mirror invention is then retained;
    that after two reflections the image of the observed body will shift
    about in the view exactly the same as the direct view does, so that
    motion of the instrument in a seaway will not cause any relative motion
    between the two views.
    No problem there you still have two mirrors.
    At the index-arm position when the two mirrors become exactly parallel,
    it should be at its zero mark, and the two images of any distant object
    should coincide, allowing checking of index zero-error as with the usual
    I'm guessing the "zero mark" to be near the 3 or 4 o'clock position, is
    that right? or is it near the 6 o'clock position?
    However, there's another "essential" feature of the normal sextant that
    will no longer apply. Normally, as the angle of the index mirror
    changes, the deflection of light changes by twice that angle. But that
    law is only true when all light paths are kept in the same plane. With
    this alternative instrument, the light paths are in very different
    planes (except at the zero-check position), such that the deflection of
    light becomes equal to, not double, the angling of the mirrors. (If
    anyone can offer disproof, please do so).
    This seems correct to me but are the light paths in different planes
    because you  "tilted" the mirrors 45deg or because you've moved the
    horizon mirror and tilted it 45deg too?  Sorry if some of these
    questions seem rather stupid, but as this experiment intrigues me, the
    only way for me to fully understand is to ask some stupid questions.
    Horizon shades will be placed in the direct view line through the
    horizon glass, as usual. Index shades will go into the space between the
    two mirrors.
    What would be the consequences of the new geometry? Most obvious is the
    doubling of the angular motion of the arm, that's called for to achieve
    the same angular range. A standard sextant frame, only 60� wide, would
    only be able to measure up to 60� of arc, not 120� as with the standard
    sextant, so this would clearly be insufficient. The arc length would
    need to be doubled to subtend 120�, or even (for reasons we will see)
    180�. That would lead to a large, clumsy instrument, except for the
    With an arc of 180degs doesn't the index arm get in the way of the
    telescopic observation as the index arm approaches the horizontal position?
    Because each measured degree now corresponds to a whole degree subtended
    on the engraved arc, and not half a degree as with the normal sextant,
    the degree markings, and the teeth of the rack,  now become twice as far
    apart as they were. Or, to put it another way, the same precision as
    before can now be achieved by halving the radius of the arc. So a
    standard sextant with a 60� arc and 18cm. radius could be replaced by
    one of 180� arc and 9 cm radius, which would be no less compact.
    There would be a few advantages, if only minor ones. Because each mirror
    always reflects through 90�, there's no shrinkage in the view to a
    letter-box shape, as you get with a sextant at large angles. That
    implies you could use the instrument right up to 180�, wall-to-wall
    between horizons. to measure dip, as long as one proviso has been met.
    That is, that the sideways displacement between the two sight-lines is
    enough to allow incoming light from astern to miss the ear of even the
    most jug-eared navigator. That's why there has to be a certain minimum
    spacing between the two mirrors.
    Won't this "sideways displacement" between the sight lines affect the
    split horizon mirror and effectively shrink the sight picture "side to
    Another minor advantage would be that there's no vertical offset between
    the two view-lines, only a horizontal one instead. So instead of calling
    for a distant horizon to zero-check on, to avoid parallax errors, that
    job could be done with something quite close-up. Also, it means the same
    instrument could be used for on-land close-up angular measurements for
    surveying purposes, which presently call for a theodolite because of the
    parallax error that offset causes..
    I haven't yet discovered any major drawbacks to such a redesign, and
    hope that Navlist members will point out any there may be. For example,
    is its calibration unduly sensitive to the exactness of those 45�
    twists? Drawbacks there must be, I presume, if in all the years of
    sextant development, nobody has tried, or even proposed, such an
    alternative construction. Perhaps someone has, that I'm unaware of;
    dicovered the snags, and dropped the notion.
    I've no more questions but that picture sure would come in handy about
    now.  --Scott
    The era of the sextant has been and gone; now is not the moment to be
    proposing a redesign, and that's not what I am doing. I am just asking
    the question; could sextants have been made in quite a different way,
    and if not, why not?
    If no serious objections emerge to the principles of what's been
    suggested, I can proceed to the practical details of how such a
    very-different instrument might have been constructed, which I've been
    pondering on, a bit.
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