Your arm is about ten times longer than the distance between your eyes. That fact, together with a bit of applied trigonometry, can be used to estimate distances between you and any object of approximately known size.
Imagine, for example, that you're standing on the side of a hill, trying to decide how far it is to the top of a low hill on the other side of the valley. Just below the hilltop is a barn, which you feel reasonably sure is about 100 feet wide on the side facing you.
- Hold one arm straight out in front of you, elbow straight, thumb pointing up.
- Close one eye, and align one edge of your thumb with one edge of the barn.
- Without moving your head or arm, switch eyes, now sighting with the eye that was closed and closing the other.
- Your thumb will appear to jump sideways as a result of the change in perspective.
How far did it move? (Be sure to sight the same edge of your thumb when you switch eyes.)
- Let's say it jumped about five times the width of the barn, or about 500 feet.
- Now multiply that figure by the handy constant 10 (the ratio of the length of your arm to the distance between your eyes).
- Now you get the distance between you and the barn—5,000 feet, or about one mile. The accompanying diagram should make the whole process clear.
With a little practice, you'll find that you can perform a quick thumb-jump estimate in just a few seconds, and the result will usually be more accurate than an out-and-out guess. At a minimum, it will provide some assurance that the figure is in the ballpark—which, in many cases, is as close as you need to get.