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    Re: QMOW Days work in Navigation
    From: George Huxtable
    Date: 2010 Jan 9, 13:30 -0000

    Peter Hakel wrote-
    " ...experience has apparently shown that doing a running fix is preferable
    to getting longitude from the time of LAN.  This is very interesting.  Does
    anyone know how this rule was established?  Has anyone tried both methods
    and compared their accuracy?  It is also conceivable that nobody has really
    tested this and the reasons are historical: i.e. the LAN is used to give
    latitude ONLY and thus the running fix is the next best thing to establish
    Peter has it right. Although much attention has been given on this list to
    the determination of longitude from observations around Local Apparent Noon,
    mentions of its use at sea in actual chronometer-navigation are rare. That
    is because its defects were recognised, as a method which would require
    extended and precise observational data, to achieve a mediocre result; there
    were better ways of doing the job. The moment of Local Apparent Noon was
    indeed calculated, but that was to provide the best monent to take an
    altitude of the Sun to determine latitude, which would (unlike the maximum
    observed altitude) be unaffected by the vessels course and speed.
    Before Sumner / St. Hilare position lines were adopted,
    Chronometer-navigation under a clear sky, using only the Sun, involved the
    following procedure-
    1. Several hours before noon, take an altitude of the Sun. On that basis,
    and the best guess of latitude, based on dead-reckoning from previous
    observations, compute morning longitude.
    2. From that longitude, using dead reckoning, compute when the moment of
    Local Apparent Noon will be, and at that LAN moment take another Sun
    altitude, to provide noon  latitude. From that latitude, using dead
    reckoning, work backwards to check whether the latitude assumed at the time
    of the morning observation was reasonable. If it was out, recalculate that
    morning longitude on the basis of the newer latitude.
    3. From that morning longitude, using dead reckoning, work out the longitude
    at noon. Announce noon lat and long.
    4. Take another Sun altitude, several hours after noon. Use dead-reckoning,
    from noon, to estimate latitude at that moment. On the basis of that
    altitude and latitude, compute afternoon longitude.
    In that why, whenever a clear sky served, a navigator would interleave
    observations for latitude and longitude, bridging the gaps between them by
    dead reckoning, and keeping the errors within bounds..
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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