Thursday, October 13, 2016

Outdoorsy Hack: Measure distance using your thumb

I know the old hand trick for measuring how much daylight is remaining, and I know how a string of beads can be used to measure distance. But this is a new one for me: Estimating distance with your thumb:

Estimating distance by using only your finger is based on this known fact about human anatomy: Your arm is about ten times longer than the distance between your eyes.

The distance between your eyes is about 2” apart and the distance from your eye to your extended finger is about 20” apart. How can this be useful for estimating distance?

Read the instructions in detail here, but the quick and dirty version is as follows:

  1. Hold up your, thumb up, elbow locked open
  2. Close one eye
  3. Note where your thumb is placed
  4. Switch which eye is open
  5. Estimate how far your thumb has jumped in feet
  6. Multiply this value by 10
  7. That's the rough distance the object away is in feet

Step (5) is the dicey one. The article suggests picking an object in the distance that has a known size (such as a building you're pretty sure is 100' wide) and then estimating how many of those objects fit within distance your thumbs jump.

This all works because your eyes are in two different locations:

When you hold out your thumb and view it with one eye open, then with the other eye open, your finger seems to shift relative to the object background. This makes it appear that the object has “moved” from side. This phenomenon is known as parallax. The parallax of a distant object is the angle between its directions of view from the two ends of a baseline.

Math to the rescue!

Incidentally, the same approach is used to determine how far (some) stars are away from our planet.

Can't wait to give this a try.

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