Mythbuster

As I’m sure happens with pretty much every subject, bushcraft and survival contains its fair share of “urban myths”.  One that has been perpetuated for many years and seems to regularly do the rounds on social media, is, using your fingers to estimate how long you have got until the sun sets.

Type “using your fingers to estimate sunset” into your favourite search engine and dozens of websites will all tell you how that if you hold your hand out at arms length and line up the bottom of your little finger with the horizon the number of fingers that will fit between the horizon and the bottom of the sun can be used to calculate the time until the sun sets, which is normally quoted as 15 minutes per finger. Sometimes, admittedly, the website will say that this is approximate or an estimate and one or two mention that it only works at certain latitudes (they quote mid latitudes…….a bit vague!!).  It is also referenced in a some books including the “Collins Ultimate Navigational Manual” and the book “Show Me How” from where the following image is duplicated on many of the websites.

onira

The reality is, that, if not completely wrong it is inaccurate and therefore could be potentially dangerous. There are only two latitudes, one north and one south of the equator where it would actually work reasonably accurately and they are not at what I would call a mid-latitudes.  But don’t just take my word for it, below is the science and maths behind what is actually happening that will allow you to judge the amount of daylight you have left far more accurately wherever you are on the planet.

Firstly, the sun appears to move through the sky at a constant rate due to the earth making one complete revolution every 24 hours. This can simply be calculated by dividing 360 degrees (one full rotation) by the number of hours i.e. 360/24 = 15 degrees per hour.

There are several recognised methods for estimating different angles using your out-stretched hand.  The simplest and easiest one to remember is that an outstretched fist will mark out approximately 10 degrees…………so straight away you should be able to start seeing a problem.  If a whole fist which is equivalent to 5 fingers only marks out 10 degrees of the sun’s movement, equivalent to 40 minutes of time how can each finger equate to 15 minutes?

The next factor we need to consider is the angle that the sun actually sets at.  Between the tropics of Cancer and Capricorn the sun will set close to perpendicular to the horizon, that is is sets almost straight down. Which is why in the tropics it seems to go from day light to darkness very quickly.  As you move away from the tropics towards the poles the angle of the sun to the horizon as it sets (and rises for that matter) gets shallower and shallower.  In fact the exact angle can be calculated if you know your latitude, simply take away your latitude, north or south of the equator from 90 degrees and this will give you the angle with the horizon made by the trajectory of the setting sun.

So for London which is 51º30′ North the angle the suns sets at will be 90º – 51º30′ = 38º30′

So with the above information it now becomes quite straightforward to accurately assess how much daylight you have left.

  • Wherever you are you will need to know your approximate latitude.
  • Subtract this from 90 degrees to give you the angle the sun sets from the horizon.
  • Using this you should then be able to reasonably accurately predict the path the sun will take between its current position and sunset.
  • Using your outstretched fist as a measure of 10 degrees or 40 minutes of time you can now easily calculate how long you will have until it gets dark.

You will find that in reality, you will actually have more minutes of daylight than you have calculated, because as the sun approaches the horizon the earth’s atmosphere refracts the sunlight so you actually continue to see the sun even after it has dropped below the horizon. The light is bent by just over half a degree which equates to an extra 2 minutes.  In addition there will still generally by enough light in the sky to see by until the sun has dropped more than 6 degrees below the horizon, this period is known as civil twilight. That’s at least another 24 minutes!

So does the method described at the beginning of this blog work at all, and is so when and where?

Well, yes is does, but we need to go back to school trigonometry to work it out.

In the diagram below the vertical height of the sun above the horizon will form the opposite side of a right angle triangle. The path of the sun as it sets will form the hypotenuse of this triangle which will make the angle  Θ (theta) with the horizon (which will form the adjacent side of the triangle).

Now to put some figures to the triangle.

  • We are saying that one finger width between the sun and horizon equates to 15 minutes of time so to make the maths easier lets give ourselves a whole hour of daylight. This would mean that the sun would move 15 degrees, so in the diagram below the units for the hypotenuse  is 15.
  • For the finger method to work we need a 4 fingers gap between the sun and the horizon, which is roughly equivalent to 8 degrees, so the opposite side of the triangle is 8 units long.

nav pic4

  • With this information we can calculate the angle  Θ using the formula;-  sin  Θ = Opposite/Hypotenuse
  • sin  Θ = 8/15
  • sin  Θ = 0.533333
  • Θ = 32.25 degrees
  • So if the angle that the setting sun is making with the horizon is 32.25 degrees this means that the latitude this occurs at is 90-32.25 or

57.75 degrees or 57º, 45′

So if you were on the Isle of Lewis in Scotland or Gotland in Sweden this would work, as it would in South Georgia in the South Atlantic………….I certainly would not consider these places located at mid-latitudes!

 

Kev

 

References

The Natural Navigator“, Tristan Gooley

How to Navigate Without a Map and Compass“, Harold Gatty

Collins Ultimate Navigation Manual“, Lyle Brotherton

Show Me How“, Derek Fagerstrom