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: Geogebra?
    From: Brad Morris
    Date: 2018 Dec 21, 18:16 -0500

    When I had a problem very similar to this one,  I realized a very interesting trick you may be able to use.

    My issue was to draw the outlines on the continents on the sphere and then to create illustrations using that three dimensional sphere as projected onto the two dimensional paper.  

    The trick is simple.  To project the X,Y,Z of the sphere onto the X,Y of the paper, simply eliminate Z!  For all Z positive, the pen is down (drawing). For all Z negative, the pen is up (not drawing).  The XY plane that bisects the sphere at its center has Z=0.  Suddenly, all those three dimensional points lose the 3D attribute and become 2D.

    I was able to draw lines of latitude and longitude on a sphere tilted by 23° or so out of the Z=0 XY plane.   The appearance of the globe and continents on the paper was very professional looking.  It appeared as if you were looking at a true globe of the earth.

    So any line you want to draw is feasible, as long as you know XYZ, you can determine XY.  This trick may not be suitable for purely mathematical constructs, but for simple illustrations, its very simple and accurate.


    On Fri, Dec 21, 2018, 4:03 PM Tony Oz <NoReply_TonyOz@fer3.com wrote:

    Dear Lars, Seasons Greetings!

    May I ask you if you are familiar with the Geogebra tool?

    I am creating explanations and illustrations for Haversine table - I want to include the properly drawn PZX triangle. The PX and PZ sides are easy to draw - they are just ellipses. I'm stuck with the ZX side - have no idea on how to draw a great circle not passing through poles of a sphere...

    Here is where I got so far.

    I want to use the same sketch for all possible (same-name, contrary-name) allocations by moving Z and X around - with minimum editing where absolutely required.

    Could you please give a mathematical advise here?

    Thank you in advance.

    Warm regards,

    Tony 60°N 30°E


    View and reply to this message

    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