The term light-year looks like an adjective-noun combination naming a kind of year. Instead it denotes a distance, how far light travels in a year. Can the deceptive name be justified?

Of the three quantities time (in the sense of time-interval), distance, and speed (assumed constant), any two determine the third. If I go 6 miles in 2 hours, my speed is 3 miles per hour (mph); if I go 6 miles at 3 mph, the time it takes is 2 hours; if I go at 3 mph for 2 hours, the distance covered is 6 miles. Yet we privilege the first two as basic, assigning them units and determining the third only in terms of these. Celestial phenomena provide years and days, which divided give hours and seconds; more artificially we introduce feet and miles, meters and kilometers;[1] then speeds are measured in such units as miles per hour and meters per second.

What if it were otherwise? Imagine that there is a common animal, the mooph, which always moves at the constant speed we call 1 mph. Then we may choose its speed as a primary unit; and just as the human foot names the unit of distance it measures (approximately), and the moon the unit of time it measures (month), so the mooph can name that primary unit of speed: let it too be called the mooph.

On this assumption, and keeping the usual units of time, what unit of distance will we derive? A convenient one is the mooph-hour, the distance covered by going at 1 mooph for 1 hour. Like the familiar unit of electrical energy the kilowatt-hour, this is not a kind of hour; rather, the juxtaposition of units indicates multiplication. Kilowatt-hours = kilowatts × hours; likewise mooph-hours = moophs × hours. If I go at 3 moophs for 2 hours, the distance covered is 6 mooph-hours.

Now although there are no moophs,[2] there is something that always moves at the same speed, at least in empty space, and that is light. Just as the mooph lent its name to its characteristic speed, so can light. Then a light-year is quite analogous to a mooph-hour: it is a unit of distance. Of course it can be expressed in terms of the units of distance (mile, meter) usually regarded as basic; but for some purposes it is more convenient not to do that. As with the mooph-hour and the kilowatt-hour, the name light-year suggests multiplication. In this case the speed called light happens to be the greatest of all speeds, so we cannot speak of going at 3 lights, as we could of going at 3 moophs; but we can use fractions of the speed. If I go at 0.3 light for 2 years, I go 0.6 light-years.

I don’t say that this interpretation of the name was in the mind of its originators, merely that thinking of it in this way can be helpful.

  

[1] See my series of posts “Wretched matter and lame Meter” beginning Jan. 7, 2010.

[2] Do not confuse them with the well-known moops discovered by George Costanza.