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    Re: sextant index error measurement
    From: George Huxtable
    Date: 2006 Nov 5, 10:27 -0000

    Jim wrote-|
    | It still concerns me that although you have compensated for
    | parallax,the effect of refraction when using the horizon would need
    | be considered.The comment that the tables allow for this,in my
    | applies only to the object.It seems to me that adjusting the sextant
    | the horizon would allow for this error.In the past I have adjusted
    | sextant to a nearby roof line and then taken it to the beach and
    | that the true and reflected horizons were not inline.
    Well, the discrepancy, between making a zero-check against a nearby
    roofline, compared with using a horizon, is the effect of sextant
    parallax, nothing to do with refraction. If a sextant has an offset
    between the sightlines of 2 inches, and the roof was 100 yards away,
    the error, compared with lining it up on a distant horizon, would be
    nearly 2 arc-minutes, so it would be very noticeable, and any
    observations which used that as an index-zero would be badly out..
    When you check index error with a horizon, it's true that the
    direction of the horizon is displaced, due to refraction, but that
    displacement is almost exactly the same for the direct view through
    the horizon mirror, as it is for the reflected view through the index
    mirror, from a point only a couple of inches higher up. So that any
    refraction has a quite negligible effect on such error checking. For
    that purpose dip tables are irrelevant.
    However, when you use the horizon line for a quite different purpose,
    for a horizontal reference when measuring an altitude, then any
    refraction of light, in its path from the distant sea-surface to the
    observer's eye, must be taken into account, as precisely as possible.
    This is done in the dip tables, which allow for two separate sources
    of dip. One is geometrical, due to the curvature of the Earth, and is
    exactly predictable. The other ir refraction, for which standard
    atmospheric conditions are assumed. However, if the vertical
    air-temperature gradient near the sea surface differs significantly
    from its assumed value, unexpected values of dip (know as "anomalous
    dip") can result, which give rise to unknown errors in the resulting
    If I have misunderstood what Jim is getting at, I hope he will put me
    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.
    To post to this group, send email to NavList@fer3.com
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