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    Re: Northern Limits of Magnetic Compass Usefulness
    From: Frank Reed
    Date: 2019 Feb 6, 13:30 -0800

    Bob,

    Since we're talking about a sundial-in-reverse, what you need is a tool for drawing sundial arcs. One that you will enjoy is the Shadows sundial app, which you can download at shadowspro.com. It will draw all the curves you need for any set of conditions that you specify. Some features require the "pro" version. Constructing sundials is a good side-hobby for a celestial navigation enthusiast, and it's an excellent way to improve one's understanding of the concepts of hour angle, local apparent time, the equation of time, and also learn how these depend on latitude and other factors.

    There's another way to draw all the curves for a sundial. Get yourself a little light bulb on a stick (I have used handheld Halloween toys for this), some thin tape, and a wide, short clear plastic cylinder (like the ones that 25-50 CDs are shipped in). With those tools and some simple computation, you can produce a nice analog computer which can project the lines required for a sundial onto any surface facing in any direction. Want to set up a sundial on a lumpy rock in your backyard? You can do that with a simple analog computer. Given the components that I have described, some of you reading along may be able to figure out the design of this. Feel free to speculate! I'll post photos of a real one that I made for a sundial class when I get a chance... 

    Frank Reed

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