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    Re: Fwd: Principles and Being Practical
    From: Phil Guerra
    Date: 2003 Sep 9, 07:23 -0500

    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
    >
    
    
    

       
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