Last night I awoke to the sound of thunder. How far off I sat and wondered? ... Night Moves, by Bob Seger.
The question as posed by Mr. Seger is an idle late night musing about time and memory, but sometimes the answer to how far off? can be an immediate matter of life or death. Lightning can be lethal and we need to know how close it is and/or if it's moving our way. The answer involves counting and a calculation.
You'll recall that we first see the lightning and then - sooner or later - hear the thunder. That time difference - shorter or longer - tells us the distance of the storm - closer or further away - from us.
Light, the visible part of lightning bolts, zips by at a brisk 186,000 miles per second. But sound, the thunder from lightning, pokes along at a mere mile or so every 5 seconds. Given that speed difference, a factor of almost a million, we can assume for our purposes that the speed of light is instantaneous.
