NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Poor St. Hilaire
From: Geoffrey Butt
Date: 2007 Oct 28, 15:30 -0000
From: Geoffrey Butt
Date: 2007 Oct 28, 15:30 -0000
John If you are correct that your examples A,B and C are the only explanations given .... then I am truly surprised. Surely an assumed position is used because any body's altitude angle and azimuth can be found quickly from Reduction Tables: the parameters of the assumed position having to correspond to the restricted increments of entries in those tables. No other tables, mathematical knowledge or calculator are then required to plot an LOP. The accuracy of the plot was adequate to meet the needs of aircraft flying increasingly long distances and at increasing speeds in the mid-20th century. The estimate of speed over the ground introduced a potentially larger error due to assumptions which had to made about the wind vector. I was talking recently to a school friend who flew BOAC VC10s on long-haul routes in the 60/70s. He described his transatlantic navigation qualification flights as a non-stop round of sight-taking, plotting and application of wind vector estimates. The need for a rapid and simple method for plotting estimated LOPs was paramount. Geoff Butt John Karl wrote: : .............. : I was speaking from the viewpoint of a modern author introducing the : St. Hilaire method and the reader trying to understand. I'm talking : about how the method is introduced in any number of commonly used : books: books by Blewitt, Bowditch (2000 edition), Cunliffe, Dutton : (2004 edition), Howell, Letcher, Meyrier, Moody, Schlereth, Turner, : and Toghill, to name a few. These authors all are discussing using HO : 229, HO 249, or a calculator. They are all talking about finding a : single LOP - not a fix, not an iteration of fixes. They're not taking : about older log-trig methods or the Sumner method. : : My statement was that all of these authors either don't attempt to : explain the REASON for the assumed position, or they explain it : incorrectly. For example, : : A. Some say that an assumption is necessary because the distance : between the sun's GP and the ship is too great to plot. While that's : true (for most sights and charts), it's not the reason for the assumed : position. : : B. Some say the AP is necessary because there's insufficient : information to plot the LOP. That's false. We know everything : necessary to plot the LOP : : C. Others say the AP is necessary because we don't know how to plot : the exact LOP. That's also false. They are several ways to plot the : LOP exactly. : : I mused about WHY so many authors either don't explain the reason for : the AP, or they have it incorrect - I think that the term "assumed : position" has misled these folks. But of course, that's only my : guess. : : I wonder what List members think the reason is for their : misunderstanding, particularly when they're supposed to be teaching : others! No one has addressed this - yet. : : John : : : : : : : : -- : No virus found in this incoming message. : Checked by AVG Free Edition. : Version: 7.5.488 / Virus Database: 269.15.10/1091 - Release Date: 24/10/07 14:31 : : --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
