Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Poor St. Hilaire
    From: Gary LaPook
    Date: 2007 Oct 29, 01:40 -0700

    Gary writes:
    You asked a while back about the use of the word "assumed" and whether
    that implied some inaccuracy. What you are actually saying is: "I know I
    am not there, but assuming, for the sake of the computation that, if I
    were there, then  this is the altitude I would have measured." The
    assumed latitude and longitude are exact but are only used to feed in
    the values need to compute an altitude for comparison with the actual
    measured altitude. Using the Sumner method one also assumed a latitude
    for the computation because you need this value to plug into the
    computation. There was no advantage to using your D.R. latitude. Even if
    you used your D.R.  its latitude was actually assumed for the
    computation and  the other latitude was some convenient latitude most
    likely one of the lines printed on the chart.
    In response to your "A" statement, because the distance is too great to
    plot on the chart at a sufficiently large scale to be useful (except for
    very high altitude shots for which you actually plot the circle of
    position around the GP) you must calculate where to draw a segment of
    the circle of position on the chart near where you think you are and the
    AP gives you exact values to feed into the computation for St. Hilaire
    or an exact latitude to use for Sumner.
    John Karl wrote:
    >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
    >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
    >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.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site