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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@jimthompson.net
www.jimthompson.net
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-----------------------------------------

> -----Original Message-----
> From: Navigation Mailing List
> [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of George Huxtable
> Sent: Saturday, May 17, 2003 3:05 AM
> To: NAVIGATION-L@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@huxtable.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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