Distance Estimation: Miraculous!
I've just been reading an old, old book about shooting, and in it I found something that I wish I'd known about years ago, while I was still competing.
Although I've been moderately successful in practical rifle exercises, I have never been particularly good at distance estimation. Therefore, I almost always missed badly with my first shot at an unknown distance, and only hit with my second or third after making mental or actual-sight-setting corrections.
To make a long story short, If you know the width of an object, you can very closely estimate your distance from it!
Here's how it's done:
Fully extend your right arm. Grasp your right fist within your left hand, left arm also fully extended. Make your arms rigid.
Extend your right index finger.
Close your left eye, and open your right eye.
Visually place the tip of your right index finger at the left edge of the object of known width, that is an unknown distance away. (You may have to swivel your body slightly, to do this.)
Without moving anything at all, close your right eye and open your left eye.
Using your knowledge of the width of the object, estimate how far to the right from its left edge your finger has "moved" when you switched eyes.
Multiply this estimated distance by 10.
That is your distance from the object!
The ratio of the width between your eyes to the length of your arm is just about 1:10.
Sitting in my reading chair, my fingertip "moves" about a foot of the 18" width of the DVD player.
Thus, the DVD player is pretty close to 10 feet from my reading chair.
(Turns out, it is so close to 10 feet away that it isn't worth noting the difference.)
(I'm going to be posting this same technique on a couple of other forums, so you may see it again.)