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    Re: How did Sumner navigate in 1837?
    From: Jim Thompson
    Date: 2003 May 17, 07:46 -0300

    Thanks for this, George.  All I've had so far in preparing my draft (and my
    understanding, as I grope up the CN learning curve in general) is the brief
    excerpt from Sumner in Bowditch.  So as you astutely point out, I took
    literary license and read between the lines.  I appreciate the academic
    reminder, and the helpful pointers.
    
    Jim Thompson
    jimt{at}jimthompson.net
    www.jimthompson.net
    Outgoing mail scanned by Norton Antivirus
    -----------------------------------------
    
    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of George Huxtable
    > Sent: Saturday, May 17, 2003 3:05 AM
    > To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    > Subject: Re: How did Sumner navigate in 1837?
    >
    >
    > More about Sumner's observation in 1837.
    >
    > Jim Thompson said-
    >
    > >He was pretty damn gutsy to have sailed ENE in poor
    > >visibility toward the rocks, assuming that his longtitude was west of
    > >Small's Light.  When Small's Light popped out of the mist, he
    > must have been
    > >both immensely relieved and incredibly gratified.
    >
    > Jim says "in poor visibility", and "out of the mist". He may know
    > more than
    > I do. All I have to go on at present is Cotter's account of the
    > event, with
    > a copy of Sumner's chart and some quotes from Sumner's text. But I find no
    > mention there of "in the mist". Indeed, Cotter managed a Sun
    > altitude at 10
    > am, and to do that he would need a clear horizon, several miles away.
    >
    > Cotter's plan was not then particurly "gutsy" in my view, but it was
    > logical. As the Smalls rocks were well marked, except in thick weather it
    > would be quite safe to approach them as Sumner did in the prevailing
    > Southeasterly gale, knowing he could always bear away when the light-tower
    > was sighted. He might well first sight the Welsh coast (near Milford Haven
    > entrance), which was a few miles further on. What he was trying to do was
    > to keep up to windward as far as he possibly could until he knew exactly
    > where he was, to be sure he could keep clear of the rocks off the
    > Southeast
    > corner of Ireland, which was a lee shore. Prudent, yes. Inventive,
    > certainly. "Gutsy", no.
    >
    > So, when Jim says- "It will make an interesting lecture, if I
    > >can reduce the elements sufficiently to lay terms and spice it up with
    > >information and graphics about a shipboard navigator's life in those
    > >days.", I hope he won't spice it up any more than is justified.
    >
    > Jim said-
    >
    > >If that old DR latitude was way off, then his longtitude was too -- which
    > >was one of the points that navigators in those days might not have
    > >appreciated, because they did not commonly understand the concept of a
    > >celestial LOP.
    >
    > Well, that's the whole point, really. Sumner had shown that
    > WHATEVER his DR
    > latitude was,  based on an old noon Sun several days earlier, he
    > simply HAD
    > to be on the oblique position line he had calculated entirely from the one
    > Sun observation made at 10am on that day.
    >
    > It's true that navigators "did not commonly understand the concept of a
    > celestial LOP", but they were certainly aware that the longitude they
    > derived from a "time sight" was crucially dependent on knowing an accurate
    > latitude.
    >
    > There were several ways of deriving the longitude from a
    > time-sight and the
    > latitude, but what he probably would use would be-
    >
    > cos (P/2) = sqrt( sin s sin (s - ZX) / (sin PZ sin PX))
    >
    > where P is the local hour angle, ZX is the Sun's zenith distance,
    > PZ is the
    > co-lat, PX is the polar distance, and s = 1/2 (ZX + PZ + PX) .
    >
    > It's an interesting question, why it had to wait until 1837
    > before mariners
    > had the commonsense to realise that an oblique position line
    > could be drawn
    > from a single altitude of any body with a known position in the sky.
    > Looking back, it seems such an obvious step. I hope the new book on "Line
    > of Position Navigation" will go some way to enlighten us.
    >
    > George Huxtable.
    >
    >
    >
    >
    > ================================================================
    > 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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