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I was watching one of those Discovery Channel wilderness survival shows last night and one of the "survivors" made a compass from a piece of wire tie and an earbud's magnet.

He stripped the plastic from the tie and rubbed the magnet in a single direction on one end of half the wire, then suspended it from a thread.

How would he know which end of the wire was north? As best I can tell, if you don't know the orientation of the magnet, you can't determine the orientation of the needle. Did magnetizing only one end of the wire make a difference?

Edit:

In their scenario, it was daytime, but in a jungle. The view of the sky was pretty obscured and I believe it was somewhat overcast. I figured if they couldn't actually determine east from west (they wanted to go east), they couldn't tell north from south to calibrate the compass by the light/sun. I believe it was midday.

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  • Another survival technique is that moss would normally grow more on north side on trees in the northern hemi. Depending on location, any compass could be off by quite a bit because of declination. In the US the max is about 20 degrees.
    – topshot
    Sep 16, 2016 at 22:30
  • Honestly, I think they cheated. They sort of stated they couldn't navigate by the sun, nor mentioned the moss, so they needed a compass. But didn't offer an explanation for calibrating the compass. To be generous, maybe they just didn't explain the difference and value between having a vague notion of north vs. a precise compass.
    – rrauenza
    Sep 16, 2016 at 22:47
  • If a compass can know which end then a needle can. A magnet has two ends. Pick one end and test. If the earbud magnet is not marked then that is a problem. But I they all is installed the same orientation.
    – paparazzo
    Sep 17, 2016 at 1:39

3 Answers 3

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Having watched what I believe to be the episode of the program in question (and if not, it was something very similar), I would say that in my recollection the specific direction of travel was not important (the two survivors did not know where in the jungle they were, nor which direction had the best chance of rescue/encountering civilization.

The point of the crude compass, per the dialogue/explanation, was not to have them traveling in any specific direction, but rather to prevent them from getting lost and traveling in circles.

One direction, regardless of which direction it was, is what they needed, and what they achieved. It doesn't matter if their "North" was actually "South" so long as it was the same direction that they were traveling before.

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  • In this episode they wanted to head east to the coast -- Brazil, if I recall. They were in the "Araucaria forest of South America"
    – rrauenza
    Sep 16, 2016 at 22:43
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Even in an overcast jungle, the sun still cast's a shadow, they'll just be more diffuse and not as ray traced, but if there is light, it is possible to determine which direction it is coming from. The only caveat being at high noon right on the equator during equinox when the sun is literally directly overhead. Having the sun directly overhead doesn't help you, but even then, only 15 minutes later you can have your bearings.

Use a tree, or use a stick planted in the ground. A clear floor of any size will make things easy for you, plant your stick, then look for whatever you might think looks like the darker side of it. Mark this with a stone or leaf or something. Wait 15 minutes, then do the same thing again. You'll have two different points, and easily be able to determine which hemisphere of the sky the sun is in.

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  • Unless you don't know if you are in the northern or southern hemisphere. This should rarely occur, but is still a possibility in serious survival situations after your boat sunk, or plane crashed, and you as a passenger have no idea if you crossed the equator.
    – Peter1807
    Sep 18, 2016 at 11:50
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    If you are that close to the equator you will still know which is east or west by where the sun rises
    – Rory Alsop ♦
    Sep 18, 2016 at 13:53
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What you would have to know beforehand, is which way is generally north, by other methods, like the North Star, or from the direction of the sun. I don't mean that you could tell which way is north precisely without a compass or gps, but one of the ends should be obviously wrong. For instance in the northern hemisphere, the end pointing towards the sun is the south end.

Then you could mark which end of the improvised magnet pointed north.

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  • I added some more details to the question...
    – rrauenza
    Sep 16, 2016 at 21:07

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