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    Re: Northern Limits of Magnetic Compass Usefulness
    From: Bob Goethe
    Date: 2019 Feb 5, 13:52 -0800

    Attached is my first effort at making a sun compass...and I have just verified that it works fairly well.  Line the shadow up on local solar time, and the outer ring gives you true bearings.

    I determined that a sundial with a vertical-post gnomon is called an analemmatic sundial...and the position of the vertical post needs to change with the time of year, which makes it complicated for a home-built sun compass.

    Turns out it is easier to build a compass using a gnomon that is at the same angle as my latitude.

    I built an excel spreadsheet to determine where to put the hour lines on the face of the compass...but once I did this, I was able to confirm that http://www.anycalculator.com/horizontalsundial.htm works just fine and is quicker to use.

    As for figuring out where to drill into my CD case to get the string to an angle of 53.5° (i.e. the same angle as my latitude), I used the equation of SunCompass3.png, attached.

    If I do this again, I need to work harder on drilling holes.  The plastic of the CD case is so brittle, it wants to shatter.  Further, I would use a cord that is smaller diameter than parachute cord.

    Bob

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