Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    On the Mischief of Geographical Position.
    From: Herbert Prinz
    Date: 2002 Jan 21, 9:18 AM

    Nine out of ten popular books on celestial navigation define the
    Geographical Position of a celestial body as  that point on the Earth�s
    surface where the latter is intersected by a line from the body to the
    center of the Earth. If this definition were correct, the body would not
    be in the zenith of its Geographical Position on a non-spherical Earth.
    The navigator would be in a worse situation than the proverbial man who
    hunts his own shadow: the sub-stellar point of his zenith, i.e. his own
    position, could be as far as 12 nm away from him.
    
    To remedy this situation the author will (sometimes after admitting that
    the Earth is in reality a spheroid) tacitly assume that the celestial
    navigator uses a spherical Earth at any rate and will in particular
    present the concept of latitude as if there were only geocentric
    latitude. Hence the author has to make believe that it is the latter
    that is related to the declination of the stars and therefore of
    interest to the navigator.
    
    Even the most reputable books such as Dutton�s engage in this kind of
    deceptive oversimplification, leaving many readers wondering, why it all
    works on an elliptical Earth. Indeed, it would not. But as we are, in
    fact, knowingly or unknowingly, performing all sight reduction on the
    CELESTIAL sphere, which is a sphere by definition, our sight reduction
    methods based on spherical trigonometry are strictly rigorous. It is the
    ZENITH that the navigator is attempting to locate in the celestial
    sphere. Once that has been accomplished, he projects it down via the
    local vertical, simply by renaming declination latitude, and GHA
    longitude, and calls it his �position�. This works on an egg, a pear, a
    lemon or an onion.
    
    Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    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:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    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