Every bright person knows that light travels at an incredible rate of speed –186,282 miles per second to be precise, when you're measuring light in a vacuum. Bright folks also may know that a light-year measures just under 5.9 trillion miles. So why do we measure the speed of light in miles per second and light-years in miles? Bright stargazers know the answer to that one, too: it's because a light-year measures the distance that light travels over the course of one year in the vacuum of space.

Bright word lovers may wonder whether it's proper to use light-year to mean a very great distance, as in this sentence: Only minutes from downtown, we found ourselves light-years away from the bustle of modern Japan.

The answer is yes: such usage is both widespread and established. That may be because the distance referred to is typically more cultural than physical, or it may reflect the fact that the figurative sense of light-year followed the literal one with the speed of light.

But there is one figurative sense of light-year that is not yet common in standard writing. That's when light-year is given the temporal sense a particularly long period of time. We can't explain why this usage excites disapproval, but we suspect it has something to do with the sense shift from distance to time.

To me, at least, the reason the use of the term 'light-year' to refer to a very long period of time is unacceptable is that it DOESN'T---it refers to precisely ONE YEAR, the period of time it takes light to travel just under 5.9 trillion miles.(*grin*)

One light year is equivalent to:

9,460,753,090,819,000 meters.
5,878,639,427,505.244 miles.

You say -186,282 miles per second. I thought it was +186,282 miles per second

I think that's a dash there, not a negative sign.