Today's block of instruction concerns the skill-set popularly called "flash-to-bang time" and its use in SHAG* range estimation.
"Flash-to-bang time" requires two things: 1) an event which produces both an effect that you can see -- the "flash" -- and a sound that you can hear -- the "bang" -- and 2) some way to reliably count seconds.
Now, light travels at about 186,000 miles per second. As far as us knuckle-draggers are concerned, this is close enough to instantaneous as to make no nevermind.
Sound, on the other paw, travels about one mile every five seconds, or 375 yards every second. More or less. This is quick-and-dirty ranging -- you want precision, get a laser rangefinder.
For those on the far side of the pond, one mile every five seconds comes out to about 340 metres per second. More or less.
Obviously, we're going to use the difference between the speed of light and the speed of sound, but to do so, we have to be able to reliably count seconds. This is easily done using the "One thousand" or "Mississippi" technique. Since "One thousand" and "Mississippi" both take just under a second to say, they are both fairly reliable -- as long as you remember two things:
First: The number of seconds goes at the end of the word. You say, "One thousand-one, one thousand-two, one thousand-three" or "Mississippi-five, mississippi-six, mississippi-seven" -- NOT, "One, one thousand; two, one thousand, three one-thousand" or "One mississippi, two mississippi, three mississippi".
Second: Don't count past ten. One through ten are nice one-syllable words. Once you get off into those multi-syllable teens, the number takes longer to say and throws your timing off. If you find yourself needing to go to eleven or higher, mark the first ten and start over again at one thousand-one.
Let us say we have witnessed a flash of lightning. We immediately say, "One thousand ONE, one thousand TWO, one thousand THREE, one thousand FOUR, one thousand FIVE", we hold up an index finger, then we start counting again at, "One thousand ONE, one thousand TWO" and we continue, holding up another digit each time we get to one thousand FIVE and starting the count all over again until we hear the bang of thunder.
Remember the speed of sound? About one mile every five seconds? The number of piggies you are holding up is the range to the lightning bolt in miles.
Of course, there are some of us who are simply too cool to hold up piggies. In this case, count your total number of seconds -- remembering to start over if the count goes past ten -- and divide by five to get the range in miles.
"But, Professor LawDog," I hear you say, "What if the range is less than a mile?"
Nae problemo, grasshopper.
As a creative articulation, let us say that we have a boat-load of
Fortunately, the boat has a steam-whistle, and when you see the plume of white stuff (the flash), you start counting seconds. Two seconds later, you hear the shriek of the whistle (the bang).
Sound travels about 375 yards per second ... two seconds ... 375 times two ... carry the little piggy ... about 750 yards away.
Oops, your new friend is French. Okay ... 340 metres per second ... ummm ... call the range 680 metres.
Using this method, any event which you can both see (flash) and hear (bang) can be ranged. In the past I have used a pile driver -- starting my count at the sight of the drivers impact and ending the count at the distinctive "whomp"; I have used the dust raised by the muzzle-flash of a prone rifleman together with the sound of his shot; and I've used the sight and sound of a man splitting cord-wood with an axe.
Thus endeth the lesson.
*Scientific Hairy-Arsed Guess