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    Re: Fwd: Principles and Being Practical
    From: Courtney Thomas
    Date: 2003 Sep 9, 05:22 -0500

    Phil,
    
    Thank you for the book information.
    
    What is the difference between HO 249 and HO 229 techniques ?
    
    Cordially,
    Courtney
    
    
    
    HGWorks - Phil Guerra wrote:
    
    >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
    >>
    >>
    >>
    >
    >
    >
    
    --
    Courtney Thomas
    s/v Mutiny
    lying Oriental, NC
    
    
    

       
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