The light-year is mostly used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.

What is a Light Year?

A light year is a way of measuring distance. That doesn't make much sense because "light year" contains the word "year," which is normally a unit of time. Even so, light years measure distance.

We are accustomed to measuring distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live. We know how long a foot or a meter is and we're comfortable with these units because we use them every day. Same thing with miles and kilometers -- these are nice, human increments of distance.

When astronomers use their telescopes to look at stars, things are different. The distances are gigantic. For example, the closest star to Earth (besides our sun) is something like 24,000,000,000,000 miles (24 trillion) away. That's the closest star. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn't a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!

So to measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or:
 

  • 186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year

A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers).

Using a light year as a distance measurement has another advantage -- it helps you determine age. Let's say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so light minutes away. If the sun were to suddenly explode right now, we wouldn't know about it for eight minutes because that is how long it would take for the light of the explosion to get here.

from howstuffworks.com

A light-year is how astronomers measure distance in space. Itís defined by how far a beam of light travels in one year Ė a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with whatís going on in astronomy, itís worth spending a little bit of time understanding what the deal is with this funny unit of measurement.

Have you ever been on your way to meet some friends and, while en route, called to tell them youíre running late? If youíre like me, you might tell them ďIím about ten minutes awayĒ. Youíre using how long it will take you to get there as a substitute for how far away you are. Astronomers do the same thing when figuring out the distance to a star or galaxy, but instead of relying on how fast they could drive or walk, they use a beam of light as their reference.

Why do things this way? First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour. We donít usually think about light traveling anywhere because when we flick on a light switch Ė there it is! We donít have to wait for the room to light up. It happens instantaneously. Except that itís not instantaneous, just insanely fast. In fact, letís pause for a second and reflect on how ludicrously quick the speed of light is.

 

Traveling at that speed, you would encircle the globe of Earth almost eight times in one second. So if you were to travel off the Earth in a straight line at light speed, youíd get pretty far in the same amount of time, right? Actually, since space is so vast, you wouldnít have even made it to the moon. Travel to the moon takes about a second-and-a-half, at light speed. Travel to the sun at light speed takes about eight minutes. Can you even imagine? Traveling at a speed where you cross nearly 200,000 miles every second for eight full minutes would only get you to the center of our solar system.

We only see something once the light from that event actually gets to our eyes. When weíre looking across a room, the time delay is only a few billionths of a second. But when we start looking across large enough distances, the light becomes noticeably delayed.

The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometers we quickly run into unwieldy numbers just measuring the distance to the nearest star: a dim red dwarf called Proxima Centauri that sits a mere 24,000,000,000,000 miles away! Using a longer yard stick, so to speak, helps keep the numbers at least manageable. To an astronomer, the distance to Proxima Centauri is only four light years. Put another way, it takes the light from our stellar next door neighbor four years to get to Earth.

The Milky Way galaxy in which our sun and all the stars we see at night reside spans 100,000 light-years from one end to the other. Putting that into perspective, the duration of recorded human history is roughly 5,000 years. So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end.

Moving beyond our galaxy, itís just over two million light years to our nearest galactic neighbor, the Andromeda galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.

Our last stop takes us to the edge of the visible universe. Itís also here where the trickiness of measuring distance in an ever-expanding universe becomes apparent. The light we see coming from the farthest depths of the universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But thereís a catch: we cannot say that the edge of the visible universe is 14 billion light-years away.

Why? Because the universe has grown larger in that time! A galaxy whose light took 14 billion years to reach our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling. From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles.

And itís getting bigger every day.

Bottom line: How far is a light-year? Itís the distance a beam of light travels in one year Ė a distance of six trillion miles.

from earthsky.org