The Elusive Light Year



In my last post, I showed you just how huge astronomical distances really are. There’s a reason people say that incredible things are “astronomical!”

The image above illustrates how far Earth is from the nearest star, Proxima Centauri. But what does it mean for two objects to be 4.3 light years apart?

The light year is a unit of distance, used to measure distances that escape traditional units on Earth. It’s impossible to measure the universe in kilometers or miles; many thousands fit into one planet alone.

Even “astronomical units,” the distance between the Earth and the sun, are too small. That distance, as we saw in my last post, is barely a fraction of the distances in our solar system alone.

So what exactly is a light year?

A light year seems, by its name, to be a unit of time. This is perhaps a failing on the part of the astronomers who named it; they never anticipated the public’s reception of their new measurement.

It’s actually a unit of distance, and it describes the distance light travels in one year.

This works as a measurement because the speed of light is constant at 299,792 km/sec. That’s really fast.

To get better idea of what that means, imagine shining a flashlight on a nearby surface. Notice the way the light seems to “jump” straight there, as if it didn’t travel at al?

The light wasn’t beamed or transported, or anything you might guess after watching Star Trek. It just moves that fast. In fact, if you’ve ever watched an astronomer use a laser to point out the stars in the sky, you’ve seen the speed of light at work.


One instant the light is in the flashlight, and the next, poof, it’s there! And it looks like it’s actually touching the stars in the sky.

The light from the laser travels a great distance before being dispersed by the atmosphere, but it’s far enough to look like it’s reached beyond Earth itself. And all this happens within an instant of turning it on.

That’s how fast light travels. In fact, light travels faster than anything else in the universe. And according to Einstein, it’s the upper limit on how fast anything can travel.

Light travels far enough to appear to reach the stars in a fraction of a second, too fast for our eyes to follow. So if it stays at the same constant speed, how far can it go if we give it a whole year?

Answer: one light year. One fourth of the distance to the nearest neighboring star in the galaxy. The light year is abbreviated “ly.”

It’s easy to understand why a word that sounds like a unit of time is used as a unit of distance if you consider miles/kilometers per hour.  Mph/kph measures how far a vehicle travels in one hour. That gives us speed.

(Although, I really don’t recommend that you confuse mph and kph. As this image illustrates, they’re really quite different…)


But that’s not where the fun of the light year ends.

Consider this. Proxima Centauri, our nearest stellar neighbor, is 4.2 ly from Earth. That means it takes light from that star 4.2 years to reach Earth. In other words, the light we see now left the star 4.2 years ago.

So when we look at Proxima Centauri, we’re actually seeing the star as it was 4.2 years ago. We’re looking back in time.

That’s the difficulty of astronomy—why it’s such a difficult frontier to explore. Not only is most of its science and what we know about the universe theoretical, but most objects in the sky are never seen in their present-day condition.

We can’t see what any object looks like right now, because even looking at the moon means seeing how it looked 1.29 seconds ago.

That may not seem like much of a difference, but the margin for “error”—or, to be more accurate, the margin for looking back in time—just increases as you look out farther. The moon is the nearest object to Earth, and it’s far enough away that we can’t see it as it looks right now.

We can’t see what the moon looks like right now. We can’t see how the sun looks right now, or watch its flares and prominences right now. And we definitely can’t see what’s going on with Proxima Centauri right now.

But we can infer what’s going on right now from finding patterns in our observations about the past. We just won’t see if we’re right for another 4.2 years.

That mystery is the foundation of astronomy. It’s the science of wondering what’s out there and exploring the universe with the technology we have. But, until we leave our solar system, we’re dependent on mathematics to make projections about what we see.

That’s why we explore space. We want to see what’s happening—right now. Because we can’t see it all from telescopes at home. And we want to see it all—we want to see everything.

So we continue to explore.

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