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    Accuracy of position
    From: Geoffrey Kolbe
    Date: 1999 Oct 19, 1:10 AM

    I would welcome some input on the accuracy of various aspects of celestial
    navigation using a sextant. I apologise if this has all been discussed
    before, (it almost certainly will have been).
    The first question concerns the absolute accuracy obtainable using a
    sextant to determine an altitude. I have briefly done some experiments (on
    dry land) where I used an artificial horizon (Freiberg) carefully levelled
    to better than 0.1'. The purpose of using an artificial horizon was to use
    the doubled altitude angle to effectively halve the errors in the arc and
    in reading the angle. I was able to obtain readings that were consistently
    within 0.2' of what they should have been. Has anyone else any experience
    of what kind of accuracy in the altitude is practically obtainable using a
    The next question is, what sort of practical working accuracy would you
    expect using a marine sextant at sea? Would 1.0' be a fair estimate or
    would you reasonably expect better (or worse) than this?
    The final question is, in your experience, what sort of accuracy in the
    computed position have you come to expect from the observation of two or
    more celestial bodies? In other words, having computed your position, what
    is the radius of the "comfort zone" circle that you would draw around that
    position, such that you would want known dangers to be outside this circle?
    (Note that this is a different question to what sort of theoretical
    accuracy should you expect.)
    Dr Geoffrey Kolbe, Border Barrels Ltd., Newcastleton, TD9 0SN, Scotland
    Tel: +44 (0)13873 76253  Fax: +44 (0)13873 76214

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