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From: Courtney Thomas (no email)
Date: Tue Sep 09 2003 - 06:22:59 EDT
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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