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 7, 16:29 -0500
From: Courtney Thomas
Date: 2003 Sep 7, 16:29 -0500
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