Estimating Distances by Using Your Thumb and Eyes

How can you easily estimate distances between yourself and any object without any lab equipment? With just your eye and thumb, together with some trigonometry, you can confidently judge how far away something is.

Steps

1. Hold your arm straight out in front of you, with your thumb pointed up to the object. Close one eye, and align your thumb with the distant object.

2. Without moving your thumb, switch eyes, now viewing the object with the eye that was originally closed. Your thumb will appear to change position and jump sideways. Now estimate how far it jumped from one position to the other, and multiply this result by 10. The answer is approximately the distance between you and the object.

 
Explanation
The intercept theorem can be used to determine a distance that cannot be measured directly. Knowing the distance between your eyes (A), the distance from your eyes to your thumb (B) and the distance your thumb moves sideways relative to the object (C), enables you to apply the intercept theorem and compute the distance between yourself and the object (D). The ratio of A and B equals the ratio of the according segments D and C: B/A=D/C.

Converting to D, yields the following: D=(BxC)/A.

Your arm is about ten times longer than the distance between your eyes. Plugging in this handy constant transforms this into: