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    Re: Sextant Accuracy
    From: Fred Hebard
    Date: 2003 Mar 17, 23:12 -0500

    I don't know that I'm not too upset by recent war-precipitating events
    to respond well, but here goes.  Perhaps we can find rational refuge in
    our abstruse meanderings about an _almost_ obsolete technology.
    It was very gratifying to hear from our professional sea officer, Doug
    Royer.  I might imagine that my attempt to get as accurate as possible
    is related to his comments about real-world conditions.  I also wonder
    how close the winner of one of those jackpots generally was to the GPS
    position?  I recall our correspondent from the arctic saying his shots
    were within 0.2', and Bowditch saying that an experienced observer
    could shoot to 0.1'.  I doubt I'll ever get to the level of experience
    to which Bowditch refers, which I would expect comes from taking rounds
    of shots five times a day or more.
    George Huxtable trotted out his old hobby horse of anomalous dip, which
    I was pleased to see.  In that regard, I have recently come into
    possession of the 1962 edition of Bowditch, which is a delight for us
    sextant lovers, as it very much concentrates on navigation by sextant,
    being written before electronics had thoroughly permeated our art.
    Although I have not had an opportunity to study it at length, I did
    peruse the section on anomalous dip.  One interesting point was that
    measurements of three or four bodies more-or-less equidistant in
    azimuth would cancel the deleterious effects of anomalous dip.  Another
    was that backsights of an object combined with normal sights would
    allow one to estimate the magnitude of anomalous dip.
    George was also curious about some of my data.  I am probably an
    optimist when it comes to examination of data, always looking for the
    most favorable (to me) interpretation.  To let him judge for himself,
    below are summaries of my more recent observations; I would provide
    more detail on request.  Remember that the standard deviations would be
    twice the reported size if I were not using an artificial horizon.  In
    that regard, I remove the glass over my pool of oil when the wind is
    not blowing, but have not yet examined the effect of the glass
    systematically; it may be important. Note that I also have not yet
    systematically trained myself by watching a body pass through a preset
    altitude while listening to the time.
    Regarding Bruce Stark's comment on irradiation, several of the objects
    are Sirius or Jupiter, and they fall within the same range as the sun.
    However, irradiation is one phenomenon I ought to examine more closely.
      Most of the sights are between the east and southwest, as that is the
    most favorable aspect from my house.
    In response to Gary Harkins, my house is about 2200' above sea level.
    That does not markedly affect parallax, perhaps 0.01' to 0.1' for the
    moon (consider that 2200' is not a large fraction of the earth's radius
    of about 3400 nautical miles).  But elevation does markedly lower
    atmospheric pressure (by about 2 inches of mercury here), which is a
    main component of refraction.  In winter, the other main component of
    refraction, air temperature, is low and tends to cancel out the effect
    of the reduced air pressure.  Now that it's spring, at least as far as
    the thermometer is concerned, I'm often adding 0.1' or 0.2' to my
    altitudes, per Table A4 of the Nautical Almanac.  Since I'm using an
    artificial horizon, dip is not a factor.
    My sextant only has a 2.5-power telescope.  I would very much like to
    have a more powerful scope, as that may be what is keeping me from
    greater accuracy, especially with star shots.  I might add that
    although I am new to celestial navigation, I am very used to optical
    devices, having used microscopes professionally for many thousands of
    hours, as well as binoculars to a lesser extent.  So I definitely had a
    leg up in that regard compared to many beginning navigators.
    I have plotted altitude against time for a few rounds of these sights,
    and have been struck by how straight the line was.  The graphs have not
    been useful tools for detecting bad shots, compared to knowledge of the
    true altitude.  I'm sure they could be helpful in spotting bad data
    points if one did not know one's position.  I have settled on three
    observations of most objects, so that I don't tire myself out too much
    before observing a round of three or more.  Perhaps here, however, it
    would be better to get more observations per object.
    Fred Hebard
    36o 46.8'N, 81o 50.7'W
    Obsvtn         Rise                    Ho - Hc
    #    Object    or Set   N   Ho dd    mean   std dev
    73   Sun       RISING   3   20.615   -0.28   0.29
    74   Sun       RISING   3   30.537   -0.12   0.49
    75   Sun       RISING   3   48.384    0.30   0.10
    76   Moon      RISING   3   24.663   -0.01   0.56
    77   Jupiter   RISING   3   44.132   -0.69   0.63
    78   Sirius    RISING   3   34.326   -1.01   0.56
    79   Moon      setting  3   54.659    0.04   1.46
    80   Moon      setting  3   22.442   -1.30   1.66
    82   Jupiter   RISING   3   46.829   -0.58   0.30
    83   Sirius    RISING   3   35.166   -0.58   0.24
    84   Sirius    setting  3   26.507   -0.52   0.22
    85   Moon      setting  3   47.242   -0.58   0.26
    86   Sun       setting  3   46.959   -0.32   0.21
    87   Sun       setting  3   17.388   -0.52   0.31
    88   Moon      RISING   3   52.570   -0.44   0.10
    89   Moon      RISING   3   56.003   -0.81   0.31
    90   Sirius    setting  3   29.114    0.18   0.27
    91   Saturn    setting  3   42.991    0.59   0.29
    92   Moon      setting  3   61.565    0.70   0.23
    93   Moon      RISING   3   53.312    0.71   0.93
    94   Sun       setting  4   51.092   -0.55   0.41
    95   Moon      RISING   5   19.061   -0.28   0.12
    96   Sirius    RISING   3   35.827   -1.21   0.32
    97   Jupiter   RISING   3   52.750   -0.31   0.12
    98   Moon      RISING   3   58.330    1.01   0.99

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