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    Re: Checking a sextant calibration.
    From: George Huxtable
    Date: 2003 Oct 7, 01:30 +0100

    I agree with much of Fred Hebard's recent posting, but with a few quibbles.
    >...not all on this list are small
    >boat navigators.  Some, indeed, are BIG ship navigators!  Apparently,
    >they routinely can get their positions to under one mile, and even
    >under 0.2 miles.
    I concur with the one-mile figure, but would be reluctant to accept the
    claim of any big-ship navigator to routinely achieve positions within 0.2
    miles. This might be achieved on occasion, however, but only by lucky
    accident. Anomalous-dip errors, alone, are likely to exceed 0.2 miles.
    >That is true also of land-based observers.
    A land-based observer using a good liquid reflector is much better off than
    the mariner, because his observations don't involve the horizon, and
    because his errors (with his sextant readings) will be halved.
    >I found the quest to determine errors in my sextant to be useful in
    >motivating me to use the instrument enough to where I became proficient
    >with it.  For me, the key element of proficiency in taking altitude
    >shots was accurate timing of the sight, while for measuring
    >interstellar angles, it was accuracy of measurement alone.  There are
    >significant differences between the two.  Ultimately, I chose to take
    >altitude sights since that is the common use of a sextant in navigation
    For checking sextant calibrations, using the altitude of a body near the
    meridian can bypass the need for accurate timing.
    >I don't think errors of 10-30" of arc could be detected by an observer,
    >but grosser errors, of over a minute, should be easily detectable from
    >land with an artificial horizon.
    I certainly agree with Fred's view here..
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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