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    Re: Star-star distances for arc error
    From: Bill B
    Date: 2009 Jun 28, 05:14 -0400

    > From: "George Huxtable" 
    > This attention being given to appendix G distracts from the real value of
    > John Karl's book, and I understand that in the new edition, this, and a few
    > other matters, are corrected.
    What I do get from this is star-to-star distances have been published in at
    least two texts I know of, all which seem to fall a bit short of actually
    doing the calculations for the moment, and adjusting refraction for
    sea-level barometric pressure, altitude, and temperature; but close to the
    theoretical value, although not always spot on.
    Discussion of this star-to-star nonsense (as I have recently learned) is
    waste of electrons, George.
    "Accuracy of Measuring Inter-Stellar Distances.
    page 129,  refers to methods of determining index error, and specifically
    mentions inter-stellar angles as a means of checking sextant accuracy.  He
    concludes it is impracticable as the superimposition of stars cannot be made
    accurately enough,  and "the error in the readings is unacceptably large".
    He places the error as large as 5 minutes of arc, which even I find
    surprising and considerably larger than I would have thought.  He also makes
    the point that I have mentioned of different eye acuities having an effect:
    namely the ability of judging continuity of a line as opposed to
    superposition of points."
    Personally when swinging the sextant I perceive I am judging lines, or at
    least as I was taught in visual perception (the key word here being
    "perception," or why a movie flickering at 28 frames per second looks like
    fluid motion.)
    Thanks to terms like "rigorous and scientific vs. "anecdotal evidence" I see
    the light. I would have preferred an MIT, Princeton etc. study over Yale or
    Harvard (lawyers and MBA's vs. the sciences) but how can one argue with hard
    scientific evidence like, "Observations were made on land; on a small boat
    yawing at anchor; reaching in a force 3 wind at sea;  Beating into force 4
    wind etc ...
    "I note the error in most cases including the 'ideal' conditions was a sigma
    of around 0.33 minutes of arc.  This means an accuracy range of 0.66 minutes
    of arc is to be expected in 68% of observations;  and 1.32 minutes of arc in
    95% of all observations."
    The results are not really too bad under field conditions IMHO.  Just for
    openers, rigorous?  What size (length and beam) is a small boat? Light,
    medium or heavy displacement?  Keel type?  Wind direction/duration and fetch
    or frequency and amplitude of waves/swell?  Test subjects experience and
    instruments?  On board measurements of pitch, roll and yaw? Measurement from
    the top of a wave/swell? Anchored??
    Oh well, back to the coconuts ;-)
    Bill B.
    Navigation List archive: www.fer3.com/arc
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