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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fwd: Principles and Being Practical
From: Courtney Thomas
Date: 2003 Sep 9, 05:22 -0500
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