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    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
    body.
    
    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{at}jimthompson.net
    >> www.jimthompson.net
    >> Outgoing mail scanned by Norton Antivirus
    >> -----------------------------------------
    >>
    >>> -----Original Message-----
    >>> From: Fred Hebard [mailto:Fred{at}acf.org]
    >>> Sent: Thursday, February 19, 2004 11:43 PM
    >>> To: jim2{at}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.
    >>
    >
    
    
    

       
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