In my article, “Where Are We?” 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 several faraway objects, including one of our nearest neighbor stars, Alpha 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.

And, as a matter of fact…if we peer into the farthest reaches of the universe, we can look significantly back in time, almost to the universe’s beginning. This is called look-back time, and it’s why we’re able to discover how the universe itself has evolved.

We can literally peer into the universe’s past–all because of the speed of light.