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    Re: Appeal to forum members to collect data
    From: George Huxtable
    Date: 2007 Oct 25, 22:50 +0100

    Jim wrote-
    | I would like to collect some data from a variety of observers using a
    | variety of marine sextants taken in a uniform manner.
    I have no wish to pour cold water on such an admirable aim, but I think
    Jim's attention should be drawn to a serious difficulty.
    The biggest error in measuring altitudes above the horizon is in
    establishing the direction of that horizon. The dip of the horizon can often
    differ by a significant fraction of a minute, and occasionally by much more
    than that, from its "book" value. This is the effect known, when it becomes
    serious, as "anomalous dip". It can occur at its worst in coastal waters,
    particularly semi-enclosed waters, as Alex can testify from recent
    experiences in Kiel. It can't be predicted, and its effects can be of the
    same order as any errors that are likely to be found in a sextant, and will
    mask or confuse those errors. How will Jim become aware of any dip errors,
    and disentangle them?
    By the way, Jim didn't ask for an estimate of the observer's height of eye,
    and that can be a significant matter.
    But I would like to ask a slightly-cynical question. Unless you are going to
    use your sextant to measure lunar distances, does the possibility that it
    might be out by a minute or two matter a scrap to your navigation, in a
    small craft? Just thought I'd ask...
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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