Recently we introduced the whole concept of the mil-dot (or milliradian) system for rifle scopes in Mil-Dot Made Easy. In that article, we got into the practical application of how mil-dot scopes can be used to figure out how to aim at a distant target. With simple math, you can figure out how much adjustment to make to your optic to account for bullet drop at longer ranges. The same system can be used to calculate windage adjustment to account for crosswinds and figuring out how much to lead a moving target.
That’s all fine and cool, but what I like most about the mil-dot system is that it can be used to figure out the distance to a target, just by looking through your optic. Hey, when the Zombies come, batteries are going to be in short supply, and those fancy laser rangefinders will only work for so long.
In fact, one of the earliest uses of the mil-dot ranging system allowed submarine commanders to figure out how far away an enemy ship was. This knowledge, used with some basic math that factored in the speed of their torpedoes, told them where to aim in order to intersect the path of the ship. If you’re ancient enough (like me) to have played that old arcade game Sea Wolf, you’ll know the concept. Except back then, you had to guess when to launch the torpedo, and it took a lot of quarters to nail the timing consistently. If you don’t know what an “arcade game” is, count me as envious of your youth.
Likewise, mil-dot markings in your scope can easily be used to figure out how far away a person, animal, or object is from your current position. It’s a matter of proportion. For example, have you ever tried to help someone spot a planet in the night sky by telling them something like, “Look two thumb widths over from the moon and you’ll see it?” Obviously, Mercury is somewhat farther than two thumbs away from the moon. Your thumb width is just represents a proportional distance relationship. The width of your thumb two feet from your eyes represents some millions or billions of miles of distance far out in space.
The concept of determining how far away something is using mil-dots is similar. Because the proportional size of an object is constant with distance, you can use some basic algebra principles to figure out the range. Don’t freak out because I used the word “Algebra!” I hated that class too, but there is one important thing we all learned that applies here. Remember “solving for X?” All that really boiled down to was knowing that if there are three pieces of information, and you know two of them, you can usually solve for the missing third one. In this case, you can always solve for the missing piece of info at the relationships are proportional.