16

I've now heard from a friend and read about a claim online, which I find unlikely:

You can estimate the time until the sun sets by using your fingers. By extending your arm fully and placing your hand above the horizon, you can count the number of finger widths between the horizon and the sun, where each finger width represents approximately 15 minutes until dusk, and a whole hand 1 hour.

I understand that this is just an estimate, but I can't imagine it working for everyone or for most places on the globe. For instance, I live in a more mountainous area of the US and the sunset usually is when the sun hides behind the mountains and not just a horizon. The math seems sketchy at best.

Here's a pictorial representation: Sunset Estimate with Finger Widths from Lifehacker.com

And for citation purposes and further reference: source of picture and explanation of the "lifehack"

  • 2
    My completely uneducated guess (similar to your misgivings) would be that this would vary based on any and all of the following: latitude, time of year, and the ratio of your fingers' widths (sum and individual) versus the length of your arm. – Tory Jun 8 '18 at 18:17
  • 5
    There's nothing magical about the horizon here. You're just estimating the time for the sun to cover the distance across your hand. If you want to know how long until the sun sets behind the mountains, then line up the bottom of your hand with the mountains. Anyway, I guess this is a reasonable question, but I'm not sure how to answer with anything other than a theoretical answer. – Nate Eldredge Jun 8 '18 at 18:25
  • 1
    You might be able to get a good answer on an astronomy StackExchange. The sun definitely moves a certain distance across the sky over some amount of time, but that time definitely varies wherever you live. Someone more into astronomy could probably give a more detailed version(like "number of fingers, plus x if you're above 45 degrees latitude, and minus y if it's winter") . – Giter Jun 8 '18 at 19:17
  • 5
    The sun travels exactly 15 degrees an hour across the sky. A 'Standard Hand' at arms length is -about- 15 degrees. If you have mountains in the way, then that's effectively the horizon. Note that the arc of the sun is rarely directly overhead, so if you want to be more accurate hold your hand perpendicular to the arc the sun is making, not parallel to the horizon/mountains. – BobT Jun 8 '18 at 19:24
  • 3
    I just returned from a place fairly far up to the north of Europe. I was surprised how much, much longer the sunsets were in comparison to those in central Europe. On the other hand, I've noticed that sunset comes, and is gone, in a few minutes e.g. in Barbados. Based on these observations, I'm absolutely certain that at any given day of the year, and for any finger width, there is exactly two latitudes where your finger measures the time to sunset with very high precision. – Schmuddi Jun 8 '18 at 19:48
22

Nearly True

This works, but there is one detail missing, and depending on your latitude the picture could be misleading. The sun moves approximately one finger width across the sky each 15 minutes, but the sun does not necessarily set vertically down onto the horizon. You have to use your hand to measure the distance the sun has to travel along it's path, not the distance above the horizon. At latitudes and seasons where the sun sets at an acute angle to the horizon, it would take a bit of practise to get it right.

How does this work

Sun Path

The earth spins at a constant speed, so the sun moves across the sky at a constant rate. That rate is 360 degrees per day, 15 degrees per hour, or 3.75 degrees in 15 minutes.

Angular size of a finger

A typical human has an arm about a meter long, and a pinky finger about 1.5cm wide (ref, and hopefully you have one you can measure). 1.5cm at one meter is a little over one degree. Three middle fingers is about 5 degrees, and across a closed fist is about 10 degrees. The image in the question suggests an arms length finger is 15 minutes or 3.7 degrees. This seems a bit high, but might work if the wrist is and elbow are bent, bringing the the finger closer to the eye than in the examples in the references. It will also make a difference if the arm is held out straight ahead, or held to the side and the head turned.

Sun path across the sky

Unless you are in the tropics at just the right time of year, the sun does not drop directly into the horizon. The sun sets when it hits the horizon (and it gets dark when it gets 6, 12 or 18 degrees lower, depending on definition). So if you want to measure the distance the sun will travel until it sets, you have to measure along the right path.

Putting it all together

At 15 degrees per hour the actual speed is more like three fingers in 20 minutes for a typical person, but you could be more accurate by "calibrating" your arm length and finger width. And you need to measure along the sun's path, not the height above the horizon. Estimating that path will be easy in the tropics where the path is nearly vertical, and harder in the arctic where it is at a small angle to the horizon, as a small mistake in estimating the angle makes a bigger difference in path length. But if you get that right, it does work.

Caveats

  • Everyone has different hands, fingers, and arms. So to be more accurate, everyone should measure the angular size of their hands and fingers.
  • If there are mountains on the horizon, then this technique can be used to measure the time until the sun goes behind the mountain. Obviously, if you then drive round the other side of the mountain, the time will be different.
  • If you're in the arctic at midsummer, the sun won't set. You can still measure how far along it's path it will go in a given time, but that path never touches the ground.
  • It seems that everything hinges on the ratio of arm length to finger width. I wonder how much this ratio varies between individuals (e.g. mean and standard deviation)? On another note, the angle between the sun's path and the horizon is relevant, but its effect varies as the cosine, which is close to 1 until the angle becomes fairly large. So you can neglect this at low latitudes. One more issue is that due to refraction, the sun remains visible for some time after it's physically below the horizon, by about one diameter (0.5 degrees). – Nate Eldredge Jun 9 '18 at 17:13
  • It's a 'rule of thumb' not an international standard (the platinum hand?)... I would also suspect that if a person's arm is shorter then their hand would be somewhat proportionally smaller, so the angle would (more or less) be the same. – BobT Jun 10 '18 at 2:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .