11

Is there a relatively reliable way of calculating the time until sunset, without any specialist equipment, when I'm in the wilderness?

5
  • 1
    A watch. You check when does the sun set. Subtract the current time from that and that's it. I guess error could be up to one hour because of the landscape (mountains). – Vorac Jul 10 '12 at 10:01
  • @Vorac "You check when does the sun set" - and how exactly if you're in the wilderness? – berry120 Jul 10 '12 at 11:29
  • 1
    First way - in the internet, before departing to the wilderness. You can also check how many minutes a day does that change. Second way - the first evening see at what time it sets. Of course, all this is provided you have a watch. – Vorac Jul 10 '12 at 11:45
  • @Vorac The first way doesn't work if you've forgotten to check - and the second isn't much good on the first evening! – berry120 Jul 10 '12 at 12:38
  • I live in a valley and I look at the shadow on the east mountains. When it is at the base it takes about 20-30 minutes for the mountains to be covered. At that point, I have about 15 minutes of "good" twilight that I can still ride at the skatepark with out it being dangerous. – BillyNair Jul 16 '12 at 21:33
10

One method I've found to be particularly reliable is the finger method - hold your arm straight out in front of you, facing towards the setting sun like so:

Sunset diagram

Place your hand so your little finger is level with the horizon, and your fingers are stacked on top of each other. Each finger represents around 15 minutes of sunlight before the sun sets behind the horizon.

This method can be used relatively accurately up to two hours before sunrise, by using both hands.

See also this page.

7
  • 4
    Which latitude have you tried this? It will have different results depending on where you are, eg where I grew up (quite far north!) a hand's breadth could give you hours before the sun set. – Rory Alsop Jul 4 '12 at 8:39
  • @RoryAlsop This works well for me in the UK - I would imagine unless you're at the extremities, it works reasonably well. I've just checked the collins ultimate navigation manual where I got it from, and for this particular technique it makes no reference of it working better / worse in any particular place. – berry120 Jul 4 '12 at 10:10
  • 1
    In Orkney at midsummer, the sun doesn't go down so much as around - only briefly dipping under the horizon to the north. And north of the Arctic circle it won't go down at all in the summer. I wonder if it works at the equator... – Rory Alsop Jul 4 '12 at 10:27
  • Winter vs summer and closer/farther from the equator has an impact on the accuracy of this technique (in the summer here it almost seems to hover above the horizon and skim to the right for a while before dipping lower). @berry120 - start with this technique and test it a few times in early then mid summer and then in the fall and winter to see the differences at your latitude. (fall and spring will be similar) – BillyNair Jul 16 '12 at 21:29
  • 2
    I'm in the Philippines (9.1 deg north) and it works out about 4 fingers to half an hour all year round so it does make quite a difference with different latitudes. – Andy Gee Mar 26 '16 at 8:55
1

Another method I've grown accustomed to over countless sleepless nights is the "amount of light" metric.

If you spend a long enough time at the same latitude and environment, you will get an idea of how bright the sky is at any given time of day and year. Angle of attack of sunlight? Intensity? White, yellow, red, blue or white? Experience.

It's easiest to spot when half-dead awaiting the morn but still works for normal situations. Colour of the sun and clouds, height, angle, nearby obstructing peaks - pull all those in the organic neural network and hope for the best. If you survive - readjust.

PS: I use @berry120's algorithm daily when hiking!

1
  • 1
    This. It is especially important to operate flexibly in mountain/forest terrain. Winters in BC can have sunsets around 4, but effectively you start getting in the dark 30-45 minutes before that under forest cover in a valley. Add a bit of clouds - customary - and, if you what you are interested is your ability to see without lights rather than a formal sunset, you are going to have a hard time judging it from observing anything else than the ambient light. – Italian Philosophers 4 Monica Oct 1 '20 at 0:28
1

There are a number of influencing factors, and the further you are from the equator, the more important they get:

  • season/time of the year. The further you are from the equator, the more prominent is this effect.

  • longitude and local time zone: very much as a rule of thumb, your position within the current time zone makes a difference of ±30 min.
    Some places of course use time zones other than the "naturally closest one" and/or daylight saving time.

  • latitude has a huge influence on how long the sun takes to travel a given vertical angle (declination):

    • at the equator, 12 h day/night is a decent approximation all year round. At the polar circles, that's true only at equinox - at solstice you have 24 h of day or night.
    • useful twilight at the equator is maybe 20 min, but more than an hour at 50° and all night long in summer in Scandinavia.
  • weather obviously plays a role, too. Dark rain clouds can cost you several hours here (50 °N), snow or bright moonshine can make the whole night useful twilight. Again, I'd expect the effect to be the stronger, the further you are from the equator.

  • topography: e.g. horizon in the Soča valley can be +20° - +25° up.


Local rules of thumb

I live at about 50°N (quite central in Germany). Here,

  • the sun takes about 4 min from first hitting the horizon till being completely down.
  • In addition, we get a more than an hour of decently useful twilight* (until the sun is ≈10° below the horizon) all year round (± weather, vegetation, light pollution, obviously).
    (At 40° N (north side of the mediterranean) twilight is only about half as long. This caught me many times, even when I was living in Northern Italy! Lighting conditions were reminding me rather too late to look for a place to stay for the night.)
  • The daily shift in sunrise/sunset is about 3 min around equinox (and approximately 0 at solstice)

Tools

  • If you have been at roughly this location for a while, knowing yesterday's sunset, maybe an approximate adjustment (like the 50° numbers I gave above) and the approximate duration of twilight shoud give you a decent idea.

  • If you go somewhere far from your usual home, consider looking up sunrise/sunset and twilight (and maybe know the current moon phase and what that means in terms of moonrise and moonset) as part of planning your tour.
    Since daily changes are not that huge (not even far from the equator) and you anyways cannot plan day tours to the minute, noting maybe one day per week should be sufficient - that's a piece of paper that shoud fit into any outfit.

  • Being in a deep valley

    • End/beginning of civil twilight is defined as the sun being 6° below the horizon. You can use this together with twilight duration (from tables) to approximately adjust sunset for being in a deep valley (this approximatin becomes worse at high latitudes in summer - but then you have anyways very long days).
      Actual twilight will be somewhere between the sun 6° below your apparent horizon (mountain range) and 6° below azimuth (astronomic horizon).

    • More precise numbers can be read from a sun path diagram: sun path diagram where you can see when (time & date) the sun is at what elevation (angle).
      Some calculators calculate sunrise and sunset based on a so-called shadow file that gives the apparent horizon. There are tons of such diagram generators available nowadays (they are used e.g. to evaluate locations for solar power)

Once printed out, this may be sufficiently low tech to fulfil your needs.


* Decently useful for outdoor purposes for me is somewhere in between civil and nautical twilight, I's say until -10° elevation. Adjust according to your night vision.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.