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: Sextant Positions versus Map Datums?
    From: Hal Mueller
    Date: 2002 Jan 18, 11:06 AM

    At 12:06 PM -0500 1/18/02, dkruger wrote:
    >The astronomical positions of VSOP87, and all the derived variants are
    >computed first in Heliocentric coordinates, thus the shape of the
    >earth is irrelevant to that part of the computation.
    >Then, it is converted to geocentric coordinates based on the position
    >of the center of the earth.  I don't remember seeing anything about
    >the ellipsoid here, but Hal if you have a page reference in Meeus, I'd
    >like to see it.  What is much more relevant is nutation, because as
    >the Earth "nods" up and down due to the gravitational effects of the
    >moon on the equatorial bulge, the axis changes, and that changes the
    >entire coordinate system.  So the accurate conversion to geocentric
    >coordinates takes into effect nutation, and for distant bodies (beyond
    >Mars) the time it takes light to arrive.  Those are the only
    >corrections I have seen at this stage, and I believe Hal is wrong
    >about the ellipse.
    The conversion to earth-surface based coordinates happens after all
    of this.  The Meeus discussion is in chapter 11, "The Earth's Globe".
    Page 82 has the IAU76 ellipsoid definition.
    He also goes on (p. 83) to give a formula for the difference between
    geocentric latitude (assuming a spherical earth) and geographic
    latitude (using the local vertical; what is plotted on charts).  It
    reaches a maximum of 11'32" at 45 degrees latitude.

    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