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    Re: Northern Limits of Magnetic Compass Usefulness
    From: Frank Reed
    Date: 2019 Feb 5, 15:04 -0800


    There are two simple extremes in "sun compass" design. If you think about it, a "sun compass" isn't really a separate "thing"...

    First, it's a sundial run in reverse. Take any sundial design that suits you, and set it for your latitude. As in your experiment, this usually means tilting a shadow stick (gnomon) until it is inclined at an angle equal to your latitude relative to local horizontal. Then calculate the local apparent time. Local mean time will be a close enough proxy in most cases, but don't use zone time. To get LMT, take GMT(UT) from some standard source and subtract longitude/15° (add in east longitudes). Adjust that by the equation of time to convert to Local Apparent Time (but as I say, LMT will be close enough for most scenarios). Then just place the sundial on a flat surface and rotate until it reads the correct Local Apparent Time. When it reads right, the gnomon is pointing north. Ta-da: sun compass!

    At the other extreme, we can just compute the Sun's true azimuth (or that of any other celestial body). This is easily done with myriad apps including my GPS.anti.Spoof, and it is also not difficult using standard old-style celestial navigation tables if that seems more "authentic" for the scenario you're playing with. Note that this, like the "sundial in reverse" requires GMT and latitude for the computation. You can't live without that. Once you know the azimuth of some celestial body, you are essentially done: you face it, and you know which way you're facing. However if you want to add a "device" to it, you can make a simple shadow stick with a compass rose on a disk around it (shadow stick perpendicular to the disk). Then if you know the true azimuth of the Sun is, say, 220°, you know that the shadow points to 40°, so you hold the disk level and rotate until the shadow is on 40°. The zero line on the disk is then pointing north. Easy.

    I can help you with your CD case experiments... Rather than drilling through brittle plastic, melt you're way through. You can make wonderful, smooth circular holes by running a high-speed drill bit in reverse or by using a pointed sanding or polishing bit (common in the standard kit for a Dremel tool). The plastic heats up, melts, and the bit slides right through. Just be sure to keep it running as you slide back out so that the plastic doesn't harden in place around the drill bit.

    Frank Reed

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