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What Is a Light Year?

Do you know what it is that a light year actually measures? Do you know why so many people find it confusing? And why they really shouldn’t? Keep on reading to find out.

By
Jason Marshall, PhD,
September 2, 2016
Episode #288

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Andromeda GalaxyHave you ever heard somebody say that something is light years ahead of its time? Or that some product they’re trying to sell you is light years beyond its competition? Or maybe your kids like to complain in the car that the fun and relaxing family road trip you’re taking isn’t so fun and relaxing because it’s taking light years to arrive at the land-o-fun?

I think we’ve pretty much all heard the term “light year” used in one of these ways at least once in our life. And on each of those occasions, the term was used incorrectly. What’s the problem? Where did all of these hypothetical people go wrong with their use of the lingo? And what does the term light year actually mean? Let's find out.

Is a Light Year a Time or a Distance?

When somebody says that something is “light years ahead of its time,” they’re really saying that it’s way ahead of its time … in terms of, well, time. In other words, they really mean that it’s “years and years” ahead of its time. So given that the term “light year” has the word “year” in it, you might be inclined to think that it too is a unit of time. But you would be wrong. Because, believe it or not, the light year is a unit of distance. So why does it have a time in the name?

The light year is a unit of distance.

To understand that and to see how this all works, let’s think about another (totally made-up) unit of distance: the snail-year. As you’ve probably noticed, snails aren’t particularly fleet-footed (what with having only one foot and all). I’m not exactly sure how fast they move, but I’m going to estimate that a typical garden snail might walk around 10 centimeters in 1 minute. If that snail could keep walking at that same pace for an entire year (which, obviously, it couldn’t—but humor me here), it would travel around 50 kilometers or roughly 30 miles in that time interval of one year.

In other words, a snail can travel 1 snail-year in one year … and that distance of 1 snail-year is equal to approximately 50 kilometers. The mathematical idea we’re using here is that a speed times a time always gives you a distance. For example, a car traveling at a consistent 65 miles per hour for exactly one hour will travel 65 miles in that hour. The key thing to realize is that we are free to use whatever speed strikes our fancy to come up with a unit of distance—cars, snails, and (most importantly for us today) light.

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