NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: DR by any other name....
From: Peter Hakel
Date: 2010 Dec 31, 10:51 -0800
From: Brendan Kinch <Kinch@Telefonica.net>
To: NavList@fer3.com
Sent: Fri, December 31, 2010 7:14:42 AM
Subject: [NavList] Re: DR by any other name....
[rest deleted by PH]
From: Peter Hakel
Date: 2010 Dec 31, 10:51 -0800
In his book, NavList member John Karl writes in Chapter 3: The St. Hilaire Method (italics are as in the original text):
"It finds the bearing and distance from any preselected point, defining the region of interest, to the nearest point on the LOP."
…
"Traditionally this point has been called the assumed position, the AP of the ship. But that's misleading; we are not assuming the ship is there - to the contrary, we usually expect it is not."
…
"For continuity with the rest of the celestial navigation world, as a concession to history, and against all sensibility, we'll continue to call this point the AP."
I am curious now, what was the original term St. Hilaire himself used in the original French: "point rapproche"? I got this last term from the following link, found by a google search:
http://www.i-DEADLINK-com/lists/navigation/0107/0020.html
This article also pertains to a recent debate here about whether this method is or is not iterative; I'd rather not get into that, I'm just trying to get an answer to Kinch's question.
As for the difference between AP and EP, the use of certain printed tables for sight reduction allows only integral latitude and LHA as input, thus making AP necessarily different from DR or EP in almost all cases. I think of the AP is the origin of a convenient local coordinate system in which we plot LOPs according to the intercept method of St. Hilaire.
Peter
"It finds the bearing and distance from any preselected point, defining the region of interest, to the nearest point on the LOP."
…
"Traditionally this point has been called the assumed position, the AP of the ship. But that's misleading; we are not assuming the ship is there - to the contrary, we usually expect it is not."
…
"For continuity with the rest of the celestial navigation world, as a concession to history, and against all sensibility, we'll continue to call this point the AP."
I am curious now, what was the original term St. Hilaire himself used in the original French: "point rapproche"? I got this last term from the following link, found by a google search:
http://www.i-DEADLINK-com/lists/navigation/0107/0020.html
This article also pertains to a recent debate here about whether this method is or is not iterative; I'd rather not get into that, I'm just trying to get an answer to Kinch's question.
As for the difference between AP and EP, the use of certain printed tables for sight reduction allows only integral latitude and LHA as input, thus making AP necessarily different from DR or EP in almost all cases. I think of the AP is the origin of a convenient local coordinate system in which we plot LOPs according to the intercept method of St. Hilaire.
Peter
From: Brendan Kinch <Kinch@Telefonica.net>
To: NavList@fer3.com
Sent: Fri, December 31, 2010 7:14:42 AM
Subject: [NavList] Re: DR by any other name....
[rest deleted by PH]
Of further interest to me is the use of AP (assumed position), regularly used here on NavList. This is a term that I don't recall encountering at sea or college. Is this a U.S. term or just a more modern term again? My reading of the term is that it is a position used in a hypothetical exercise - not in actual navigation. If I am wrong, then what is the difference between AP and EP?
Kinch
