A neat little trick!

Basically, all you need to do to estimate a distance using this classic method is hold your arm out and stare at your thumb with an eye closed. Silly, yes, but it makes sense, because you’ll switch which eye you look through and sort out how far the object appears to have “jumped” to the side:

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), and you get the distance between you and the barn — 5,000 feet, or about one mile.

Estimate Distances with Your Arm and This Rule of Thumb – measurements – Lifehacker.