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: Courtney Thomas
Date: 2003 Sep 7, 04:15 -0500
From: Courtney Thomas
Date: 2003 Sep 7, 04:15 -0500
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