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    Re: Northern Limits of Magnetic Compass Usefulness
    From: Bob Goethe
    Date: 2019 Feb 4, 14:32 -0800

    Thank you all who joined in on this for your help in clarifying the uses of a magnetic compass up north.  

    1.  David, phoning the airport in Clyde River is a simple step - and easily done with my anywhere-in-Canada long distance plan - but I confess that it never occurred to me.  Great suggestion.  I will certainly do this.

    2.  Gary, the maps of reduced reliability, erratic compass behavior, and non-functional compass behavior are terrific.  

    3.  Greg, I think a sun compass could be useful as part of this virtual journey/thought experiment.  I am going to see if I can't make a sun compass for my own home latitude.  I could print out the face of a sundial and tape it to a DVD.  I am aware of making a gnomon at the same angle as my latitude.  I am going to experiment and see if I can use a single vertical cord as the gnomon...suspending the DVD from a piece of parachute cord.  When I look at the design of the Bagnold Sun Compass at https://www.sundials.co.za/THE%20SUNDIAL%20GOES%20TO%20WAR%20web.pdf it is clear that it is using a vertical gnomon...but I have no way of knowing if the hour lines of the Bagnold Sun Compass were set up according to the equation in wikipedia:  \tan H_{H}=\sin L\tan(15^{{\circ }}\times t)

    ...where t = the number of hours before or after noon, and Hh = the angle between the given hourline and the noon hourline, and L = the observer's latitude....

    Whoa!  That sentence kind of got away from me.  Anyway, I see that the equation will give me usable hour lines if I have a triangular gnomon.  I don't yet know what will happen with a vertical cord as the gnomon.  I'll play around with it.

    4.  Seeing the pages from Pub. 216 reminded me that Bowditch has a chapter on Polar Navigation.  I have never done more than skim it in the past, since it was not relevant to me.  But it points out that the magnetic poles are more regions than points on the map, as they wander 50 or 100 miles on a daily basis.  I am supposing this is from molten iron in the core sloshing about as the earth spins.  Bowditch also talks about how solar storms that produce great Northern Lights, plus mineral deposits, plus other sources of magnetic disturbance, can introduce compass variation of up to 40°.

    Clearly, once you get north of the Arctic Circle, the Russian proverb "Doveryai no proveryai" becomes relevant in compass use:  Trust but verify.

    Again, thanks for all your help on this.


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