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The wristwatch compass trick, and why it isn't as good as you might think

There's an old technique, taught in boy scouts, US army ranger training, and the like for using a simple, analog wristwatch to find your direction like a compass. It's a simple thing to do and has found quite some popularity on "survival" type websites and blogs. While I assume the potential pitfalls are mentioned when taught properly, not many of the "survival" sites mention them so I'm going to. Yes, someone is WRONG on the internet!

How to do it

The trick is simple. Take your wristwatch (or pocket watch I suppose), and look at the sun. In the northern hemisphere, align the hour hand with the sun, and the point in the middle of the smaller distance between the hour hand and 12 o'clock should be due south. In the southern hemisphere, align 12 o'clock with the sun, and the point in the middle of the smaller distance between the hour hand and 12 o'clock should be due north.

When it won't work at all

There are some obvious issues if you intend to rely on this. First, is that you need line-of-sight to the sun. So it's not going to work at all if it's cloudy, if there are trees in the way, or buildings, mountain ranges, elephants, etc. And it's definitely not going to work if it's night.

There are also places where it won't work. Because the earth is at an angle in relation to the sun, depending on season, the sun could be in exactly the right position for this trick to give you wildly misleading directions. These places are 23.5 degrees north and south of the equator, above 66.5 degrees north, and below 66.5 degrees south.

When it kinda works

Your watch will say it's 12 o'clock at the same time every day. The sun, however, cares not for our hallucinatory concept of "time of day" and will turn up when it feels like. More accurately, apparent solar time, where the sun actually is, differs from mean solar time, which people use. This difference changes throughout the year but has a maximum of 16 minutes 33 seconds. This can cause the wristwatch compass trick to be out by slightly more than 4 degrees.

This isn't the only problem though. Enter everyone's favourite thing, timezones! Your watch is not set to mean solar time, it'll be set to your local time zone. In theory time zones are 15 degrees each with the centre matching the mean solar time. This in itself would result in an error of 7.5 degrees but it gets worse. Time zones in practice weave alongside national borders which could make this wildly more inaccurate.

There's also daylight savings time to take into account. Your watch will be set an hour out from what the mean solar time in the middle of your time zone is which will affect how accurate this trick is.

In conclusion

Using your wristwatch as a compass is a party trick at best. While it is better than nothing at all don't rely on it for accurate navigation. Compasses aren't that expensive.