NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Poor St. Hilaire
From: Andr�s Ruiz
Date: 2007 Oct 29, 14:28 +0100
From: Andr�s Ruiz
Date: 2007 Oct 29, 14:28 +0100
John, Some goods books have mistakes. For example Bowditch use for the calculation of the meridional parts two concepts that are based in none model: a = 21600.0/(2.0*PI) is used in the spherical model of The Earth f = 1.0/298.26 is used in WGS72, but this use a = 6378135.0/1852.0 Andr�s -----Mensaje original----- De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de John Karl Enviado el: domingo, 28 de octubre de 2007 2:57 Para: NavList Asunto: [NavList 3657] Re: Poor St. Hilaire I sure can understand why some of you are wondering what all the St. Hilaire fuss is about. I'm new to the List and 'am finding out how hard it is to carry on technical discussions in this format - as some of you have mentioned. A major, major, part of the problem is definitions, and understanding what each other means - as others of you have said. After starting this topic, now with 36 posts under two topic headings (are topic headings "threads"?), I still haven't got my point across. I'm embarrassed, and apologize for not being able to communicate better. 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 --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---