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.

Jason Marshall, PhD
4-minute read
Episode #288

How Far Is a Light Year?

As you might imagine, one of the keys to defining a good unit of distance for all the world to use is making sure it is actually well defined. The problem with our brand new made-up snail-year distance unit is that not all snails travel at the same speed. So people in different parts of the world might come up with different distances for the snail-year based upon the fact that their local snail populations are either super-speedy or particularly sluggish (pun!).

Which is why the speed of light—which is a universal constant speed—is such a great choice to use when defining a unit of distance based upon speed. The speed of light is really, really fast: approximately 300,000 km/s through vacuum. To gain a bit of perspective on just how fast this is, if you could run at the speed of light, you’d be able to run around the Earth 7.5 times every second.

So the idea then is to take this really fast speed and figure out how far light can travel in a given amount of time. The answer is:

  • In 1 second, light travels a distance of 1 light second.
  • In 1 minute, light travels a distance of 1 light minute.
  • In 1 year, light travels a distance of 1 light year.

And so on. If you want to know how far this is in more familiar units, 1 light year is around 9.5 trillion kilometers or roughly 6 trillion miles.

How Big Is the Galaxy in Light Years?

While a light year seems like an incredibly huge distance, on the scale of the galaxy and the universe, a light year is actually rather tiny. But before we contemplate these huge distances, let’s start a bit closer to home.

The Moon is around 1.3 light seconds away from Earth. That means that if you send a radio signal to the Moon or shine a laser at it (both of which are different kinds of light), that light will arrive a little over 1 second later. As I mentioned earlier, light could travel around the Earth around 7.5 times in 1 second, which (putting this all together) means that the Moon must be at a distance equal to roughly 10 times the circumference of the Earth.

The Sun is around 8 light minutes away from Earth.

Heading out into the solar system, the Sun is around 8 light minutes away from Earth. The distance to Mars varies from between around 4 and 25 light minutes depending on where the planets are in their orbits. And the Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are on the order of several light hours away from Earth.

The next nearest star after the Sun, Proxima Centauri is located around 4.3 light years away. So stars are typically separated from one another in our galaxy by a few light years (which makes the light year a very convenient unit to use when talking about these kinds of distances). And finally, the diameter of the Milky Way galaxy is on the order of 100,000 light years across. Or, if you prefer, 100 light millennia in diameter.

Which goes to show you that indeed, on the scale of galaxies and the universe itself, even the light year is relatively tiny. The universe is a really big place.

Wrap Up

Okay, that's all the math we have time for today.

For more fun with numbers and math, please check out my book, The Math Dude’s Quick and Dirty Guide to Algebra. Also, remember to become a fan of The Math Dude on Facebook and to follow me on Twitter.

Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!

Galaxy image courtesy of Shutterstock.


About the Author

Jason Marshall, PhD

Jason Marshall is the author of The Math Dude's Quick and Dirty Guide to Algebra. He provides clear explanations of math terms and principles, and his simple tricks for solving basic algebra problems will have even the most math-phobic person looking forward to working out whatever math problem comes their way.