8

I've heard this is possible but I'm not sure on the details. Can someone elaborate?

12

Here's the theory: at noon, if you're in the northern hemisphere, the sun is due south of you. At 6am it's due east of you and at 6pm, due west. (Day length may tweak this a little if you're far enough north, but this is not a precise technique, it just needs to beat looking for which side of the tree is mossy etc. That said, you should un-daylight-savings the time by subtracting an hour if you have "sprung ahead" where you are.)

So you point the hour hand of your watch at the sun. Halfway between your hour hand and the noon mark is south. Test it:

  • at noon, the hour hand is pointing at the sun, the halfway make is the same place, that's south
  • at 6am, the hour hand is pointing due east, halfway is 9am which is 90 degrees from where the hour hand is pointing, and that's south
  • at 6pm, the hour hand is pointing due west, halfway is 3pm which is 90 degrees from where the hour hand is pointing, and that's south

You don't need an actual analog watch to do this, just a little logic.

  • 4
    Another way to look at the halfway rule is that it compensates for the hour hand going around twice in a day, when the sun only goes around once in a day. Personally, I find it easier to think about the geometry than to try to remember rules of thumb. – Olin Lathrop Sep 8 '14 at 12:46
  • @OlinLathrop That's obviously the approach of someone who is familiar with maths, periodic phenomena, and the concept of phase! ;-) – Benedikt Bauer Sep 8 '14 at 12:54
  • 1
    Remember to take daylight savings into account. The sun is due south at 1:00 during DST. Also, this is correct only at the center of the time zone. If you are on the western edge of the zone, your watch is 30 minutes fast (relative to solar noon) – Chris Cudmore Sep 8 '14 at 14:51
  • 2
    @ChrisCudmore: It's only a very approximate way of finding north anyway. – Ben Crowell Sep 8 '14 at 21:37
  • @BenCrowell Agreed, but if you know where errors come from, you can mitigate them. – Chris Cudmore Sep 9 '14 at 13:56
5

There is another way that was taught to me in the Boy Scouts.

  1. You need a watch with traditional hour and minute hands, make sure it is showing the accurate time
  2. Take a small stick, like a toothpick, and stand it up vertically in the center of the watch.
  3. Make the shadow of the stick fall on the current HOUR hand
  4. When the shadow is on hour hand, then the 12 on the watch dial will be pointing to north.

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