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    Re: Sumner's Line (Navigation question)
    From: Bill B
    Date: 2006 Feb 16, 16:21 -0500

    George wrote:
    
    "There's an important factor that Sumner says nothing about, but which a
    navigator in those waters neglects at his peril; the effect of the strong
    local tides around those rocks and headlands."
    
    George raises yet another interesting point that's been kicking around in
    the back of my mind but had not yet thought through.  But I feel he has just
    hit the tip of the iceberg-all the fuzzy variables.
    
    1.  By Sumner's recounting, his true position (as later determined) was 51d
    45 N, 5 d 53' W.  To calculate his true position under the circumstances he
    would have to have used CMG/COG, SOG, and time to a *known* position.
    Thanks to the list's brain trust, we can see no evidence to date (other than
    Plate III) that places Small's above 51d 45' N.  Even if it were at 51d 48'
    N, depending on its longitude (5d 38' or 5d 36' W) I can't match his LOP
    angle with 3' lat and 15-17' long.  He would have to have had an unaccounted
    for set and drift of 12 to 22 degrees depending on the scenario.  Not to say
    that could not happen, but his longitude was off 1 in 20 miles, and latitude
    1 in 75 miles over the 600 mile run to that leg.  He seemed too skilled to
    miss set and drift by that much, but stranger things have happened.
    
    2.  The known position was then used to determine that the DR latitude was
    8' south of known position, and 31.5' east of the time-sight longitude at
    the DR latitude.
    
    3.  The differences in #2 depends on #1 being spot on, and the anchor point
    for the LOP being spot on.  When we look at the time sight input we see
    latitude as 51d 37'N, declination of 23d 23' S, Ho of 12d 10', GMT 10:47:13
    and Eqn of Time 3:37. Let the chronometer be 2 or 3 seconds off, or Eqn of
    Time be 1 second off, or Hs/Ho be 0.5' off, declination be 0.1 off, etc..
    It's not unlikely that at least one, or more, of the above were so.
    
    Putting it to the common sense test, given a top-quality modern sextant,
    time signal, almanac, and calculator or computer, how close might a skilled
    practitioner hope to come in Sumner's circumstances?  Maybe 2 miles?  Some
    texts would suggest regarding measurements made under marginal conditions be
    treated as EP's.
    
    All said and done, unless there is a copy of Sumner's log (or notes) where
    he addresses all of the calculations and Small's position, and/or his
    charts, I feel I've have hit a dead end as far as solving the puzzle is
    concerned.
    
    Bill
    
    
    

       
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