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
    Magnetic Variation - Lewis and Clark
    From: Kieran Kelly
    Date: 2004 Feb 17, 07:22 +1100

    I sent the post below recently as a way of showing people how land explorers
    calculated compass variation in the days before modern maps. I was intrigued
    about the Gregory method shown below and went hunting for the maths.
    Surprisingly I found the answer in a Bowditch of 1977 Vol 1 as follows:
    Hav(180dd-Z)= sech secL cosS cos(S-p)
    Z = Azimuth angle (not Azimuth)
    h = altitude
    L = latitude
    p = polar distance (visible pole)
    S = 1/2 (h+L+p)
    I am not a mathematician but it appears he was trying to solve for Z - the
    Azimuth Angle - and compare it to the compass to get the difference between
    true and magnetic north. In this case Gregory was using the south visible
    pole so substituted 360dd in his formula for 180dd. Note that this is not a
    universal formula because it relies on the restricted navigational triangle
    based on the visible celestial pole.
    This is how it was done in Australia. How did Lewis and Clark calculate Mag
    Variation in the United States?
    Kieran Kelly
    >Kieran stated he has a way to find local variation using " a
    >sextant, compass and amplitude". How can this be done? Am I missing
    >or are my thoughts to confined i.e.. ship board application only?
    The Australian explorer Augustus Gregory used several techniques for finding
    variation (i.e. difference between true and magnetic north. I will quote one
    here using an early morning sun altitude. This is verbatim from the field
    28 AUGUST 1856 at Camp LVII
    Alt sun symbol 7dd 30'          az N 279dd 30' e mag
    Polar Distance          99dd 30'
    Lat                     18dd 00'           sec .021794
    Alt                      7dd 30'           sec .003731
                         2)125dd 00'
            1/2 sum         62dd 30'          cos 9.664406
            diff            37dd 00'          cos 9.902349
                            77dd 25'   log sin sq 9.592280
                           282dd 35'
                           279dd 30'
                             3dd 05'   var E
    So there you have it. Mag var with a compass, sextant and log tables. In
    this case he did not use amplitude tables. Also because he is on land with
    no ship nearby, deviation is presumed to be zero (I think). This was only
    one technique he used and is based using haversines for a solution of the
    celestial triangle originating at his visible pole i.e. the south pole.
    I can post some of his other techniques if you are interested..
    Kieran Kelly

    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