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
    Re: Sextant Positions versus Map Datums?
    From: Michael Wescott
    Date: 2002 Jan 18, 9:26 AM

    >> Sextant positions are referenced to the surface of this ellipsoid,
    >> not to a sphere.
    
    > But was I also wrong in suggesting that common sight-reduction
    > procedures assume a spherical Earth?
    
    There are two places in sight reductions where one can find the
    spherical model assumed. The first is the relation that 1 minute of
    arc is 1 nautical mile, i.e. the offset from the assumed position. The
    second place is in the correction for parallax of the moon.  The
    common element for both is in assuming a certain fixed relation
    between distance and arc. In both cases the errors are small and the
    calculations complex.
    
    > Or are we doing spherical trig with celestial positions calculated for
    > an ellipsoid?
    
    Neither. It might as well be called spatial or 3D trig since it is
    really about the angles between planes and lines in 3 dimensions. It's
    just much easier to draw (and imagine) the problem as it intersects a
    sphere than the full 3D image. This point is made, often too subtly,
    when texts on navigation show the calculations with respect to the
    imaginary celestial sphere rather than the surface of the earth.
    
    
    
    
    
    --
            Mike Wescott
            Wescott_Mike{at}EMC.COM
    

       
    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