"The Rule Of Thumb"

A book I am currently reading, called "Finding Your Way Without A Map Or Compass", the abridged version of a longer tome by Harold Gatty originally published in 1958, explains that people find it harder to judge distances stretching away from them than guessing the size of distant objects, most of which are known, such as a building, a tree or a car.

So you can use parallax errors to help you. You keep one eye shut, and stretch out an arm with your thumb held upright and align it with said object of known size. You then shut that eye and open the other one and estimate how far the object appears to have moved laterally relative to your thumb.

You then multiply that lateral distance by ten and hey presto, that's the distance between you and the object. If you have particularly long arms or close together eyes, the multiple is a bit lower, maybe nine, and vice versa. But apparently the British Army used it in the First World War and it worked for them.

According to Wiki, the origin of the expression Rule Of Thumb is unknown, but for want of a better explanation, I shall henceforth assume that this is how it originated.