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Sorry for the tardy reply. Yes, I've got Mary Blewitt's book, as well,
although, I've misplaced it. I found it to be a really good reference, and
it's compact size made it easy to take to work for reading on my breaks. If
I remember correctly, she uses the H.O.249 to do sight reductions. Other
methods are given some mention, but not really examined.
The book I really worked through was Susan Powell's Practical Celestial
Navigation. It's more like a workbook giving lots of examples and
solutions. She uses the H.O. 229 for her sight reduction work. I think,
the method you use depends on your specific needs. What's most important is
that you know your method down pat, and have a backup method or two.
I know many of the list's group could tell you more, I've no real experience
in actual on-board CN. I'm just in awe and admire all who are able to do
it. I enjoy the mechanics of the process of CN because it emcompasses so
many of my interests into an area that uses them all. Good luck to you.
Phil Guerra
www.hgworks.com
----- Original Message -----
From: "Courtney Thomas" <>
To: <>
Sent: Sunday, September 07, 2003 4:29 PM
Subject: Re: Fwd: Principles and Being Practical
> Phil,
>
> Thanks so much for the fulsome reply.
>
> I am a non-armchair sailor and am trying to find the best, i.e. easiest
> that meets real world navigational needs, CN technique rather than a
> more abstract interest but thank goodness for such.
>
> I suspect Newton would've probably been a poor farmer but gratefully so.
>
> For now I just don't want to waste time/energy learning one technique to
> later learn that it was not the most suitable.
>
> It's not that it is intrinsically uninteresting it's that my agenda is
> reversed, at this time.
>
> Incidentally, are you familiar with Mary Blewitt's book ? If yes, what
> do you think of it ?
>
> Cordially,
> Courtney
>
>
>
> HGWorks - Phil Guerra wrote:
>
> > The Ageton method is not discussed in Bennett's book. It is really a
> > compact treatment of the subject designed for use on-board. As far as
the
> > best explanation of the method, I never really found anything more than
his
> > book, "Manual of Celestial Navigation" in print. I found the book by
chance
> > in a used book store, but have seen it offered on Ebay for around an
average
> > price of 10-20 dollars (US). Unfortunately, the book is not really a
> > 'teaching guide' but probably was used to supplement classroom
instruction.
> > Another, offshoot of the method was put forward by Allan E. Bayless,
> > "Compact Sight Reduction Table", again using a slight modification of
> > Ageton's method. This book is out of print as well, and I found a copy
on
> > Ebay.
> >
> > My expanding CN library includes, Bowditch, Dutton's Navigation &
Piloting,
> > which all refer to the method, but really do not give it much clarity,
at
> > least for me coming in as a novice. This lead me to ask questions on
this
> > list about it. I did find a good description on a referenced web site
> > http://home.t-online.de/home/h.umland/page3.htm, by Henning Umland,
which
> > cleared up most of the questions regarding how to use it, as his
authored,
> > "The Ageton Tables", gives some good description of the method,
examples,
> > and solutions. Umland did expand the method a bit by providing a new
set of
> > tables to give it more accuracy. The site is a great starting point
> > information regarding CN in general, and he has a lot of very useful CN
> > links. After going through Umland's article, I was able to go back to
the
> > Bowditch and Dutton books and understand the terse descriptions and work
the
> > examples yielded the solutions.
> >
> > I've begun work on using the information gleamed from all of my sources
to
> > produce a web site to teach the method, but it's stalled at present due
to
> > other responsibilities. However, if you need help understanding it, let
me
> > know via my existing web site www.hgworks.com using the Contact Us page.
I
> > found that building the web application to use Ageton gave great
accuracy
> > with the mathematical model, and using the table values gave it such
> > accuracy that it was, I believe in use for over 30 years, before falling
out
> > of favor, due to technological advancements. There are questions of
> > accuracy in Azimuth calculation, and it is documented.
> >
> > Although, I'm a 'deskbound navigator', others who I've come into contact
> > with on this list, indicate that the methods and books are still used
> > on-board, which is testament to the value of the work done.
> >
> > Hope this helps,
> >
> > Phil Guerra
> > www.hgworks.com
> >
> > ----- Original Message -----
> > From: "Courtney Thomas" <>
> > To: <>
> > Sent: Sunday, September 07, 2003 4:15 AM
> > Subject: Re: Fwd: Principles and Being Practical
> >
> >
> >
> >>Is Ageton's method described in Bennett's book ? If not, where is the
> >>best exegesis of it, please ?
> >>
> >>Thank you.
> >>
> >>Dr. Geoffrey Kolbe wrote:
> >>
> >>
> >>>George Huxtable has pointed up a potential problem with the azimuth
> >>>
> > tables
> >
> >>>in George Bennett's book "The Complete On-board Celestial Navigator".
He
> >>>has shown that there can be errors in computed azimuth of (at least) 15
> >>>degrees where the celestial body is that sort of distance away from the
> >>>prime vertical.
> >>>
> >>>Peter Fogg tells us that this is "nit-picking" and that in any case,
the
> >>>book tells us that, "In extreme cases the table should be interpolated
> >>>
> > when
> >
> >>>observations have been made in the vicinity of the prime vertical."
> >>>
> >>>I do not have the second edition, only the 1999-2003 edition where this
> >>>phrase is not present. Perhaps Peter can tell us just what "extreme"
> >>>
> > means
> >
> >>>in this context? When do we know we are in an extreme case? George also
> >>>posed some other pertinent questions to Peter and I too would be
> >>>
> > interested
> >
> >>>to see the answers...
> >>>
> >>>I also wonder just how much of a problem it would cause having your
> >>>near-prime-vertical azimuths off by around 15 degrees? For a cluster of
> >>>star sights, say, a prudent navigator would also be taking sights from
> >>>objects far away from the prime vertical (to get useful angular
> >>>
> > separation)
> >
> >>>and this would tend to mitigate any problems due to bad
> >>>
> > near-prime-vertical
> >
> >>>azimuths. The inaccuracy of the tables near the prime vertical are also
> >>>mitigated by being able to assess independently (in many cases) in
which
> >>>azimuth quadrant the celestial object sits.
> >>>
> >>>If your estimated position is pretty close (say, within 10 nautical
> >>>
> > miles)
> >
> >>>to your actual position then I cannot think of any circumstances where
it
> >>>would significantly affect the sort of accuracy we would expect from CN
> >>>
> > in
> >
> >>>a small boat at sea, which is the sort of user the book was aimed at in
> >>>
> > the
> >
> >>>first place. I have not thought deeply on this problem and I would
> >>>appreciate the thoughts of other listers who will have greater insight
on
> >>>this problem than I.
> >>>
> >>>The "short" method of sight reduction used by Bennett is popular
because
> >>>the computed altitude can be arrived at quite quickly. But a different
> >>>procedure is required to calculate an azimuth and this rather takes the
> >>>gilt off this method. Ageton's method, by contrast, requires more steps
> >>>
> > to
> >
> >>>get to the calculated altitude, but the azimuth then drops out very
> >>>
> > quickly
> >
> >>>and is accurate. Azimuth quadrant ambiguities are also easily resolved.
> >>>Too, only one set of tables is required for the Ageton method.
> >>>
> >>>Geoffrey Kolbe
> >>>
> >>>
> >>>-------------------8<---------------------
> >>>From: George Huxtable
> >>>The problem with these azimuth tables ...
> >>>is not in their ambiguity, but in their inaccuracy, and that inaccuracy
> >>>
> > is
> >
> >>>exactly what I have complained about. And there is not one word, not
even
> >>>
> > a
> >
> >>>hint, in the book that major errors in azimuth can occur, for certain
> >>>observations in a VERY wide swathe around East or West.
> >>>-------------------8<---------------------
> >>>>From Peter Fogg
> >>>
> >>>Inserted in second edition is . "In extreme cases the table should be
> >>>interpolated when observations have been made in the vicinity of the
> >>>
> > prime
> >
> >>>vertical and/or LHA, declination and latitude require substantial
> >>>
> > rounding
> >
> >>>off before using the table. When in doubt use the Weir diagrams.
> >>>
> >>>In practice you could happily sail across an ocean and never notice
this
> >>>supposed problem, particularly by following the common sense approach
> >>>outlined previously. With nav. it it often a case of one system
checking
> >>>another. In fact taking sights and working out a fix is a check on the
> >>>basic tool of running a DR.
> >>>
> >>>If the whole book has been subjected to the same searching criticism
and
> >>>this rather inconsequential nit-pick is the only flaw found, then it is
> >>>really a back-handed compliment to the book as a whole. A ferocious
> >>>
> > critic
> >
> >>>seems to think the rest works just fine.
> >>>
> >>>Border Barrels Ltd., Newcastleton, Roxburghshire, TD9 0SN, Scotland.
> >>>Tel. +44 (0)13873 76253 Fax. +44 (0)13873 76214.
> >>>
> >>>
> >>>
> >>--
> >>Courtney Thomas
> >>s/v Mutiny
> >>lying Oriental, NC
> >>
> >>
> >
>
>
> --
> Courtney
> s/v Mutiny
> lying Oriental, NC
>
