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John Karl has continued his attempts to tease a bit more yarn from this
thread.

| I sure can understand why some of you are wondering what all the St.
| Hilaire fuss is about.  I'm new to the List and 'am finding out how
| hard it is to carry on technical discussions in this format - as some
| of you have mentioned.

Yes, that's so true. A list such as ours has the advantage of immediacy. But
where the author of a book or a paper has space and time to build up to a
topic, here it all has to be compressed into a posting, definitions and all.
Some readers will respond off-the-cuff, without reading carefully enough (I
know. having done just that about another topic).

  A major, major, part of the problem is
| definitions, and understanding what each other means - as others of
| you  have said.  After starting this topic, now with 36 posts under
| two topic headings (are topic headings "threads"?), I still haven't
| got my point across.  I'm embarrassed, and apologize for not being
| able to communicate better.

John, you shouldn't need to apologise or feel embarrassed about our failure
to grasp the point you are making, a point that clearly, you feel strongly
about. Do try again.

| I was speaking from the viewpoint of a modern author introducing the
| St. Hilaire method and the reader trying to understand.  I'm talking
| about how the method is introduced in any number of commonly used
| books: books by Blewitt, Bowditch (2000 edition), Cunliffe, Dutton
| (2004 edition), Howell, Letcher, Meyrier, Moody, Schlereth, Turner,
| and Toghill, to name a few.  These authors all are discussing using HO
| 229, HO 249, or a calculator.  They are all talking about finding a
| single LOP - not a fix, not an iteration of fixes.  They're not taking
| about older log-trig methods or the Sumner method.
|
| My statement was that all of these authors either don't attempt to
| explain the REASON for the assumed position, or they explain it
| incorrectly.  For example,
|
| A. Some say that an assumption is necessary because the distance
| between the sun's GP and the ship is too great to plot.  While that's
| true (for most sights and charts), it's not the reason for the assumed
| position.
|
| B.  Some say the AP is necessary because there's insufficient
| information to plot the LOP.  That's false.  We know everything
| necessary to plot the LOP
|
| C.  Others say the AP is necessary because we don't know how to plot
| the exact LOP.  That's also false.  They are several ways to plot the
| LOP exactly.
|
| I mused about WHY so many authors either don't explain the reason for
| the AP, or they have it incorrect - I think that the term "assumed
| position" has misled these folks.  But of course, that's only my
| guess.
|
| I wonder what List members think the reason is for their
| misunderstanding, particularly when they're supposed to be teaching
| others!  No one has addressed this - yet.

Well, I have, from that list, Letcher, Bowditch, Dutton, Blewitt but maybe
in editions that differ from his. I have many others, not on his list, some
of very doubtful quality. But the trouble is that John has taken a
scatter-gun approach. He has listed a number of texts, and three perceived
errors. But he doesn't associate which with which. If I pick up Letcher, for
example, I can't be sure which, of A, B, or C, John is pointing out to be
its defect, in this regard.

I have the advantage of now owning a copy of John's own book, "Celestial
Navigation in the GPS age", and perhaps already some list members have their
own. and I wonder whether he brings some of the difficulties of definition
upon himself. For example, he defines his use of "estimated position" to
mean a point on a celestial position circle that's as close as can be to the
DR position. Other authors might use those words to describe the best
estimate of position before any celestial observation is taken into account.
That's either the DR position or the DR modified by any other information,
such as a sounding, perhaps, or a tidal stream correction (if the DR had
been defined in such a way as to exclude that, as sometimes it is).

And (reluctantly) he accepts the name Assumed Position, or AP to refer to
"any preselected point defining the region of interest", quibbling somewhat
about that name by saying "... that's misleading; we are not assuming the
ship is there- on the contrary, we usually expect that it is not". But (to
quibble back) isn't that what the word "assuming" implies? You "assume"
something to be the case, whether or not it actually IS the case. Usually,
"assumed position" has been used for a position which is deliberately
displaced from the DR, to allow whole-degree entries in the alt-az tables
(and an unnecessary distraction when using a calculator, as John Karl
recommends). Later in the book, he uses AP in that special context.

What I'm saying, really, that his use of DR, AP, EP can be different from
the usage you may find elsewhere, which is fair enough, when those meanings
have been carefully defined. Indeed, you may find in other works the terms
chosen position, trial position, just to add to the complicated mix.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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