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Solar navigation with analog watch & pocket knife

TL;DR – A Russian-style 24-hour watch, aligned with the equatorial plane and aimed with a knife blade, makes a fantastic solar compass with 3° accuracy. Your inferior 12-hour watch will do fine too, with a bit of trickery.

You probably already know some basic solar-navigation concepts, but these are only rough approximations of the truth.

  • The sun rises in the east and sets in the west.
  • The sun is directly south at noon.

These approximations can be plenty misleading, so let’s just call them LIES. Here I present actual truths about the position of the sun and a far better way to use it to find directions. No equipment necessary except an analog watch and a pocket knife.

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Lie #1: The sun rises in the east and sets in the west.

At my home near the 43rd parallel, trusting that the sun rises in the east can mislead a fella by up to 34° in winter and summer. It’s even less accurate at higher latitudes. Here’s how it actually works:

Twice a year, at the equinoxes, the sun rises due east and sets due west. But both the rising and the setting deviate northward as we approach the June solstice, and southward for the December solstice. You’ll get at least 25° of deviation at each solstice, depending on your latitude. Inside the polar circles, the sun won’t rise or set at all, it’ll only circle around you just above or below the horizon.

Lie #2: The sun is directly south at noon.

Yeah, okay, but when is noon? Do you trust the government to tell you? Ha! Today my solar noon came at 13:42, not 12:00.

Don’t forget, of course, that in parts of the world the sun is north at solar noon. And in the tropics, it alternates.

True directions with a reverse sundial

Now that we’ve got those directional lies out of the way, let’s discuss how to find the truth. The hour hand of a 24-hour analog watch follows the apparent path of the sun very well as seen from above the north pole. So with proper alignment, it comes in handy as a solar compass. With a little trickery, even a 12-hour watch will do.

But first you’ll have to know two things: your latitude, and what time on your watch corresponds with solar noon. You can calculate solar noon or look it up. Use 12:00 as a rough approximation if you must, or 13:00 during daylight saving time.

One way to identify solar noon is to observe the shadow of a vertical pole over time. At solar noon, the pole’s shadow will be at its shortest of the day.

Next, you’ll align your watch as a reverse equatorial sundial. Once aligned, you’ll know which direction you’re facing because a sundial only reads the right time when it’s facing a particular direction. Whereas sundials are usually pointed in that direction and then used to determine the time, they work equally well in reverse.

Read on to find out exactly how to navigate with just an analog watch and a pocket knife.

Skip to 12-hour navigation

Navigation is easiest with a 24-hour dial that shows daytime hours at the top and nighttime at the bottom. This arrangement is common in Russian military watches, and it generally allows you to navigate without removing the watch from your wrist.

(when at or north of the equator)

  1. Position the watch dial vertically with the solar noon time at the top.
  2. Tilt the dial forward by an angle equal to your northern latitude.a
  3. Rotate your body till the hour hand points toward the sun.b
  4. You’re now facing southwardc.

a Just eyeball it, aiming to get within 6°.

b If the sun is north of the equator, you can use a knife blade to cast a shadow on the hour hand as an alignment guide. Otherwise, it may still be helpful as an extension of the hour hand.

c If you happen to be on the equator at solar noon on an equinox, then all directions will seem to be south. Wait ten minutes.

(when south of the equator)

  1. Position the watch dial vertically with the hour hand at the top.
  2. Tilt the dial forward by an angle equal to your southern latitude.a
  3. Rotate your body till the solar noon position points toward the sun.d
  4. You’re now facing northward.

d When the sun is south of the equator, you can use a knife blade to cast a shadow on the solar-noon position as an alignment guide. Otherwise, it may still be helpful as an extended marker of solar noon.

Remember, it’s easier with a 24-hour dial. If you dislike bisecting angles or need an excuse to go watch-shopping, keep that in mind. But 12-hour watches are more common, so here’s how to make do.

(when at or north of the equator)

  1. Position the watch dial vertically with the solar noon time at the top.
  2. Tilt the dial forward by an angle equal to your northern latitude.e
  3. Identify the sun’s reference point on your dial. You’ll find it halfway from solar noon to the hour hand, searching leftward from solar noon in the morning or rightward in the afternoon.
  4. Rotate your body till the sun is aligned with its reference point.f
  5. You’re now facing southward.g

e Just eyeball it, aiming to get within 6°.

f When the sun is on your side of the equator, you can use a knife blade to cast a shadow on the dial as an alignment guide. Otherwise, it may still be helpful as an extended marker of the reference point.

g If you happen to be on the equator at solar noon on an equinox, then all directions will seem to be south. Wait ten minutes.

(when south of the equator)

  1. Position the watch dial vertically with the hour hand at the top.
  2. Tilt the dial forward by an angle equal to your southern latitude.e
  3. Identify the sun’s reference point on your dial. You’ll find it halfway from the hour hand to solar noon, searching rightward from the hour hand in the morning or leftward in the afternoon.
  4. Rotate your body till the sun is aligned with its reference point.f
  5. You’re now facing northward.

How accurate is watch-based navigation?

Almost perfectly accurate in theory, and plenty accurate in practice. You can reasonably expect to limit your maximum error to 3°.h As with any other reverse equatorial sundial, accuracy depends primarily on your identification of local solar noon and on fairly good alignment of your watch dial with the equatorial plane.

h Assuming up-to-date knowledge of solar noon. Make that 7° if you use mean solar noon. The distinction is given below.

Solar-noon error

I presume that your watch is set to civil time. 12:00 civil time is the average clock time at which the sun passes a particular government-specified meridian. A useful reference, but it rarely coincides with the actual solar noon of your location. Due to irregular time-zone borders and daylight saving time, civil time can easily be off by two hours, sometimes more.

The time on your watch that corresponds with solar noon will also vary a bit over the seasons due to Earth’s elliptic orbit, so you may just want to remember what time it comes on Christmas Eve. That’s mean solar noon.i It’ll get you within 16 minutes, or 4°.

i Mean solar noon also coincides with solar noon on April 15, June 15, and September 1.

Equatorial-alignment error

For the most accurate readings, the dial of your watch should be parallel to the plane at the equator which bisects Earth into its two hemispheres. If you hold your watch horizontallyj like an amateur, you’ll add up to 45° of error at the edge of the tropics or 21° in northern USA.k But if you can hold it within 6° of proper alignment, a goal easily met by eyeball, then your maximum directional error due to equatorial misalignment shrinks to just 2°.

j Other instruction on navigating with a watch assumes that equally spaced markings are suitable for a horizontal sundial. I’m right and they’re wrong.

k These errors peak on the summer solstice, eastward three hours before solar noon and westward three hours after.

Where to, then?

Although useful anywhere with sun, watch-based navigation is particularly well suited for a summertime polar expedition. The sun is up all day, the equatorial plane is conveniently near horizontal, and the sun is on the right side of it to cast a knife-blade shadow on your dial. Handy, since magnetic compasses are useless inside the polar circles.

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