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: Cannot dispense with the assumed position at sea
    From: Fred Hebard
    Date: 2004 Feb 20, 09:43 -0500

    One other point about APs for use with tables.  You use a different AP
    for each body, because they almost always have different LHAs.  This is
    assuming a more-or-less simultaneous round of sights on more than one
    On Feb 20, 2004, at 9:25 AM, Joel Jacobs wrote:
    > I have a couple of questions.
    > 1. I'm receiving only one side of the dialog, and would like to see
    > what
    > "Frank" is saying. Where can I find his comments?
    > 2. Why, at least for those in this country, is there a need to construe
    > definitions that are different from those found in the standard U.S.
    > published navigation texts of which most list members are aware.
    > 3. I have sold mine, but I recall the tables such as HO 214, 229, and
    > 249
    > also had sections with definitions. The Nautical Almanac had
    > definitions.
    > I realize many changes have taken place, but are none of these sources
    > satisfactory?
    > Should we ignore Bowditch as another example?
    > Joel Jacobs
    > ----- Original Message -----
    > From: "Jim Thompson" 
    > To: 
    > Sent: Friday, February 20, 2004 8:02 AM
    > Subject: Re: Cannot dispense with the assumed position at sea
    >> Fred, I still think we are convergent:
    >> One can use a precise DR position in the Ageton-Bayless table, Reed
    >> tables
    >> and computer programs or calculators to do the same thing as a
    > whole-degree
    >> AP does in HO 229: determine an Hc and Zn to plot the LOP.  Those 3
    > methods
    >> can accept the precise longitude to determine meridian angle from a
    > precise
    >> LHA, and the precise latitude in calculating Hc and Zn, as in:
    >> Hc = arcsin [cost x cosD x cosL) + (sinD* x sinL)]
    >> Z = arccos [(sinD* - sinL) x (sinHc / (cosHc) x cosL)]
    >> where
    >> t = meridian angle, precise decimal DMS.
    >> D=declination of the body, precise decimal DMS.
    >> L=DR latitude, precise decimal DMS.
    >> *Note the sign (+ or -): negative if L and D are contrary in name (N
    >> or
    > S).
    >> Which means that one can use a precise DR position as an "AP" in the
    >> sense
    >> that you mean by using whole-degree AP's as an entering argument for
    >> HO
    > 229,
    >> except that one cannot use HO 229 for a precise DMS DR position, and
    >> so
    >> would have to use one of the alternative methods that can.
    >> If a navigator puts an EP box around the point on the celestial LOP
    >> perpendicular to the position used to create the LOP, then that EP has
    > more
    >> significance if the "assumed position" is part of the DR plot.  Of
    >> course
    >> the workaround using a whole-degree AP would be to subsequently drop a
    >> perpendicular to the DR position, I think achieving the same end
    >> except
    > with
    >> extra plotting steps if a whole-degree AP is used as an intermediary.
    >> With respect to semantics, I think my understanding of AP is that I
    >> see it
    >> as a general term for the position for which Hc and Zn are determined.
    > Thus
    >> in my mind any position used for that purpose is an "AP" (Henning uses
    >> "initial position" or IP).  Owing to the whole-degree history of the
    >> entering argument AP, it seems to have traditionally acquired a more
    >> specific meaning.
    >> Jim Thompson
    >> jim2@jimthompson.net
    >> www.jimthompson.net
    >> Outgoing mail scanned by Norton Antivirus
    >> -----------------------------------------
    >>> -----Original Message-----
    >>> From: Fred Hebard [mailto:Fred@acf.org]
    >>> Sent: Thursday, February 19, 2004 11:43 PM
    >>> To: jim2@jimthompson.net
    >>> Subject: Re: Cannot dispense with the assumed position at sea
    >>> Jim,
    >>> This isn't semantics.  As Doug said, they mean different things.  An
    >>> AP
    >>> is used for sight reduction tables such as H.O. 229.  One enters
    >>> these
    >>> tables at a whole degree of latitude, such as 36* N, rather than a
    >>> fractional value, such as 36*16.5'N.   You _can't_ enter the tables
    >>> from other than a whole degree of latitude.  Likewise, the longitude
    >>> is
    >>> chosen to give an LHA in whole degrees; again, one cannot enter the
    >>> tables from a fractional LHA.  One then plots the azimuths and
    >>> distances from that AP.  It also makes locating the latitude of the
    >>> AP
    >>> a bit more convenient.
    >>> The EP and DR are places where you actually reckon you are, so they
    >>> are
    >>> almost never at whole degrees.  In contrast, the AP is not a place
    >>> where you reckon you are, but the closest to where you reckon you are
    >>> in whole degrees of latitude and fractional degrees of longitude
    >>> that,
    >>> combined with the GHA of a body, give an LHA in whole degrees.

    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