DISTANCE measurements in science-fiction

NGC 2207 and IC 2163

Space is a big-ass place.

It’s so damn big, in fact, that any relatively decent distance surpasses the capacities of our usual Earthling measurement units. So to avoid saying 588 x 10^8 miles or 9’460’730’472’580’800 meters, we just say 1 ly. Pretty cool, ha.

There are three units of measurement that are predominately used in astronomy, and thus also in science-fiction:

Astronomical units (AU)

An astronomical unit is the average distance between Earth and the sun. And isn’t it obvious we’d take our own planet as a point of reference to measure the galaxy, since it’s by far the most important flying rock out there? What? It is. IT IS. It’s… home. *watery eyes*

The Earth revolves around the sun in an elliptical orbit, which means it’s sometimes closer, sometimes farther away from the sun. Originally, one AU was defined as the length of the semi-major axis of Earth’s orbit, but for simplification purposes (tongue-in-cheek), a bunch of astrophysicists and mathematicians decided Earth’s orbit isn’t ‘perfect’ enough so they replaced our planet with a particle with infinitesimal mass and thus near to zero perturbations in its perfect elliptical orbit, and calculated the average distance between it and the Sun. And ta-da! you get an AU. Which is about 149’597’870’700 meters. *ouch*

AUs are usually used to measure distances within a planetary system. It’s easiest to imagine the position of a planet, asteroid belt or space-ship in a star system if you can compare it to that between Earth and the sun. I guess.

Light-years (ly)

A lot bigger than AUs, are light-years. Much more practical for measuring interstellar distances. Otherwise we’d end up with big numbers again, and that’s migraine-inducing for everyone.

light-year is the distance traveled by light in a vacuum in a year. That’s roughly 9.5 trillion kilometers, or 6 trillion miles. My head just exploded trying to visualize those numbers. See the angry looking cartoon eyes in the picture above? THAT’s how much I hate having to use gigantic numbers. Don’t get me wrong, I like math and physics, but we’re not that close, if you know what I’m sayin’. So no big numbers for me, thanks.

The light-year is the most common used measurement unit in science-fiction precisely because it’s that big. It’s also relatively easy to visualize, even though that visualization is mostly wonky. At least it feels right. You can feel a light-year. It’s a palpable, beefy distance (shut up).

To give you a sense of size, the nearest stars to our good old sun are Alpha and Proxima Centauri, and they’re 4.395 light-years away. That’d be 4.395 x 9’460’730’472’580.8 km = HEADSPLOSION

Parsecs (pc)

parsec calculationA parsec is even bigger than a light-year. In fact, it’s about 3.26 light-years. An ugly number, and it ain’t pretty to calculate, but what can you do, eh… (Except get a degree in advanced astrophysics and then write a paper on interstellar distances and change global measuring conventions. Should be easy enough.)

A parsec is the length of the long leg of a right triangle, whose short leg is 1 AU when the angle between the Sun and the Earth, as seen from outside, is 1 arcsecond (Or 0.000277777778 degrees. And here we have that awesome number-escaping again. Seems even mathematicians hate long numbers!).

To reuse our example, the distance between the sun and our nearest neighbors, Alpha & Proxima Centauri, would then be 1.347 parsecs.

Oh, and the word parsec comes from paralax of an arcsecond which sounds even more science-fictiony than light-year, time dilation, or plasma manifold malfunction.

* * *

          This post is part of the A to Z Blogging Challenge, April 2014.          

     In 2012, my D post was — Description – The Devil’s In The Details

Published by Veronica Sicoe

Science Fiction Author — I deliver the aliens.

18 thoughts on “DISTANCE measurements in science-fiction

  1. Any scientist would tell you that light years are for laypeople and space operas. Science is done in parsecs (Mega, Giga, etc.), or at least in AUs (and, of course, in SI units.) 🙂

    Gotta love them units.


    1. Light-years are too vague (and relative) for scientific papers, that’s true. For science-fiction, however, they’re THE SHIT. 😀


  2. I think units of measure are kind unimportant. People will tend to just accept them without asking too many questions. Which is not to say that the author shouldn’t know what they mean, but they’re nothing to stress over.


    1. I know the grand bulk of readers are uninterested in the actual calculations, but I still did them all just in case the occasional geek checks up on things. Most of my calculations didn’t even make it into the novel, but at least I know I’ve done my homework.

      Yes, I’m that anal. *rolls eyes exhaustedly*


  3. What really annoys me is when people use the term light-year as a unit of time. There’s also a rather famous case of someone using parsecs as a measurement of time (but let’s not go there… Kessel is a silly place).


    1. Oh, yeah, it’s been, like, light years since we last met! *groan*

      Parsecs as a time measurement unit is new to me. It… makes me tick twitch.


  4. As a former sber (spacebattles forums poster), I can tell you that I care about distance calcs.

    Why? Because it let’s be know how fast my FTL drive is! It’s a good yardstick for me to know how long travel takes, because that will effect the story plot.

    For example, I used to think that having my starship travel at 1 LY per hour was a decent speed. That’s actually kinda slow if you are going to far off places. For Earth’s nearest star (Proxima Centauri, the closer star of the binary Alpha Centauri star system), it would take roughly four hours.

    Yet a place a 80 LY away would take a few days. Nonetheless there are oh so many ways to fix that… and I have : )

    I like to understand my tech will enough that the reader would know how to use it IRL. Even dream of using it. Even my artificial gravity system is unique in how it operates. It’s a common scifi trope that gravity generated on a starship is no different than a planet’s with down being down. With mine, down is WHATEVER surface you are standing on upon. It’s all about surface tension. The moment you jump off you are WEIGHTLESS. So be careful as you float to the ceiling and flip your body feet up, because landing on your fingers will likely break them : )


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: