NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fwd: Principles and Being Practical
From: Phil Guerra
Date: 2003 Sep 9, 07:23 -0500
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 >