The story I'm writing is about an intragalactic war and is told from several perspectives including the CO of a small warship. Because space is massive, I have run into an issue with presenting distances and speed in a way that makes sense to the reader.

I decided not to use km/s or km/h as that would result in numbers with little frame of reference of speed or are so astronomically large that they lose meaning. I have toyed with the idea of using "light seconds per hour," or something similar/smaller.

Are there any better-suited units of measure I could use? Necessary changes to them to make them more useful within a solar system vs deep space? Any way to abbreviate it similarly to "knots" for nautical miles?

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    $\begingroup$ From a writing, rather than worldbuilding perspective, I would recommend not using numbers at all. What matters to you isn't (probably) the absolute speeds and accelerations, but whether one ship is faster than another, or whether a ship is sustaining more acceleration than it or its crew can tolerate. $\endgroup$
    – Cadence
    Jan 25, 2023 at 22:54
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    $\begingroup$ The TV show Andromeda used light-seconds. It's worth noting (a) most readers won't be reading your story with a calculator in an effort to fully care or even understand what you're saying. @Cadence is right, in most cases it doesn't even matter. After all, what does Star Trek's "warp 2" mean? (b) Just to make matters worse, the closer you get to the speed of light, the more the reference against time (e.g., km/h) takes on an entirely different, relativistic meaning that only astrophysicists would really appreciate. $\endgroup$
    – JBH
    Jan 25, 2023 at 23:01
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    $\begingroup$ @wokopa Generally, it's bad form to answer a question in its comments, though it's sometimes necessary ("I don't have time to flesh this out but think about this..."). But it's really bad form to place a comment on an answer, and then express that answer as your own answer in comments. $\endgroup$
    – JBH
    Jan 26, 2023 at 1:20
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    $\begingroup$ If only astronomers had a suitable distance unit already! Oh wait, what's a parsec? $\endgroup$
    – AlexP
    Jan 26, 2023 at 2:48
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    $\begingroup$ When Star Trek decided that data was measured in quads was when I decided that measurements in scifi should be utterly fictional. $\endgroup$
    – user458
    Jan 26, 2023 at 17:20

12 Answers 12


Frame challenge: Discuss time, not speed or distance

Let's look at the issue here. In space, "speed" is meaningless and the more technically correct "velocity" is always relative to a particular frame of reference. What the captain of a ship needs to know is when things will or can happen.

"The dreadnought will have us in range in 30 seconds."

"The missiles will impact in five minutes and point defence will only be able to engage them effectively in the last minute before impact. We can buy ourselves another thirty seconds if we start accelerating away from them right now."

"It will take us nine days to reach orbit around Frogstar B, or twenty-two days if we want to keep half our fuel in reserve."

"It will take eighteen minutes for our transmission to reach the Mars colony and the same for their return signal to reach us." (If lightspeed is a limit in your universe then savvy readers will be able to infer the distance, but the effect can be understood regardless of education level.)

The computers and the human(?) subordinates using the computers on the warship can be working out distance, velocity and acceleration values in the background - you may want to work them out too as a writer if you are wanting this to be hard(ish) sci fi, - but the reader and the captain just need to know how long things will take - is there time for a breath, a meal or a nap before something is going to happen. The only time distance matters is at more understandable human scales - "Captain, the breach in the hull is 150 metres aft, it will take damage control ten minutes moving through the corridors in zero-G to get a patch in position."

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    $\begingroup$ I think this is the best route to go in most cases as it neatly sidesteps a lot of issues. Scale is really, really hard to understand, especially in space, and getting bogged down in unnecessary details can cause storytelling problems. I think the detail ought to be kept to what's useful to the reader, and that's (in this situation) the impact on the characters, i.e. the timeframes they're dealing with. $\endgroup$
    – Aos Sidhe
    Jan 26, 2023 at 16:45
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    $\begingroup$ Even when you're not in space, stating distance in terms of [approximate] time can account for a lot of confounding factors that would get really tedious to explain. We use it for terrestrial travel all the time, e.g. "It's a 10 minute drive" is more helpful than "It's 2 miles at 30 mph with four traffic signals." $\endgroup$
    – Theodore
    Jan 26, 2023 at 18:23
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    $\begingroup$ Exactly this. Time is king, it’s what really controls the pacing of your story. Unless you’re going for truly hard sci-fi and discussing things like time dilation and/or not hand-waving the effects of acceleration, speed only matters as a way to translate distance into time. And even if you are going for hard sci-fi, distance only matters as a limiting factor for time. $\endgroup$ Jan 26, 2023 at 20:29
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    $\begingroup$ @AlexanderThe1st "Ship is too big... if I walk, the movie will be over!" $\endgroup$
    – Cadence
    Jan 27, 2023 at 5:16
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    $\begingroup$ @AustinHemmelgarn you can even do this with time dilation. "Captain we can hide near that black hole and let them live to death." $\endgroup$ Jan 27, 2023 at 20:26

Percentage of the Speed of Light (c)

I think this is the one I see used most often. How fast are we going? ".02 c" or "2% light-speed" or "2% c". It's a nice relative frame of reference to use and handles high speeds well.

You could pull a Battlestar Galactica and invent your own words (but I always thought that was a bit silly).

Then again, Star Trek does it. How fast ya going? Warp 3. How fast is that? See the chart.

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    $\begingroup$ Star Trek is really one of the worst examples of worldbuilding generally $\endgroup$
    – wokopa
    Jan 25, 2023 at 23:30
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    $\begingroup$ @wokopa That perspective suggests that you think worldbuilding should be done in only one way and at only one time ("now"). Star Trek changed Science Fiction. You might not like the show, but the development of its world (both in terms of pure worldbuilding and market/application-based worldbuilding, like you might see in video games) was the springboard that gave you the privilege to be contemptuous. I don't use slide rules, but I respect how they changed how humans compute numbers. $\endgroup$
    – JBH
    Jan 26, 2023 at 1:17
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    $\begingroup$ The way Trek did warp worked well for story telling. They had some low number warp speeds that they typically used - 2 and 3. When they were in a hurry they would do warp 5 or 6. Scotty might caution them. Warp 8 makes the ship shake and Scotty yells. When something weird is going on Chekov says "warp... 11?" $\endgroup$
    – Willk
    Jan 26, 2023 at 2:58
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    $\begingroup$ To add, I'd say 1/10000 c as a base measure of combat speed would pretty much do, and that could be named a "knot". Thus, .02c would be 200 knots, pretty satisfying. You need to depict smaller speeds in general, lower the knot's base value. $\endgroup$
    – Vesper
    Jan 26, 2023 at 7:57
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    $\begingroup$ @JamieB The question describes an "intergalactic war". Assuming the OP knows what they are writing about, that means a war within a galaxy and between different star systems, which almost certainly means using FTL travel. So I don't know why you talk about speeds being fractions of the speed of light instead of multiples of it. $\endgroup$ Jan 26, 2023 at 12:55

Let's start with a basic truism: you always want values that will normally be described in two-digit numbers. That's just how humans think. It's ok to go outside of those ranges, but anything three digits are more should be "big" and single digits or less should be "small."

Start by picking the time period that your numbers are meaningful over. Hours, days, weeks, months, years. You'll notice that a car that can go 100mph really averages 60mph, but still doesn't actually travel 1440 miles per day. We have to stop and rest, refuel, get our bearings, slow down for traffic etc., so the number winds up being more like 500-900. This is the difference between burst, instantaneous, and sustained speed, and these numbers aren't generally translatable.

Once you have a time period, then you can identify your distance. Light-years is pretty standard outside of a star system, and astronomical units (AU) is typical within a star system. Parsecs is kinda dumb for measuring travel because it's based on Earth-based observations. Also, it translates to 3.26156 light-years, which isn't terribly useful.

If these numbers aren't convenient for you, pick something that relates to your FTL technology. This gets into technobabble, but can improve your immersion. For instance, you might call something "Warp 2" if that speed causes relativistic distortion that makes stars look half as wide outside your port holes, or whatever.

update: Here's a good discussion I found on the topic on SFFChronicles discussion board. The gist of the discussion is that (a) yes, all units of measure are human, but (b) if you story is translating their language, you might as well translate their units of measure while you're at it. Unless it's something you're using to highlight alien-ness, you'll only confuse your readers.

Update 2: Nosajimiki quite rightly pointed out that having aliens with no reference to human culture talk in miles, or even kilometers, can be quite immersion breaking. Here are four ways I can think of to deal with this problem, in order of my preference.

  1. Rewrite your text to describe the distances in terms of what it means. "It'll take days to get there" instead of "It's thirty light-years away"
  2. Use units that sound like their human equivalents, like "kliks".
  3. Have a foil around to explain the measure to.
  4. Provide copious context so that the reader has something to relate to. This one still probably won't work unless your measure is similar to something in the reader's experience.

I would like to further point out that being vague can often to be a writer's advantage. You can go crazy trying to chart out all of the precise times and distances and velocities so that you avoid continuity issues. Better to just not specify in the first place.

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    $\begingroup$ Light-years (ly) and AU are just as much based on Earth as parsecs. A parsec is effectively defined in terms of an AU via trigonometry, and the conversion to ly follows from that. It is not due to the observer's position. If your world has a home planet with Earth-approximate orbit, all three are equally relevant. $\endgroup$ Jan 27, 2023 at 14:04
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    $\begingroup$ @SarahMesser, good point. I always agonize over how much of a cultural reference I can allow to creep into non-Earth cultures. So many of our words have culture-specific origins that you can't describe a sword without getting cultural. In this case, however, there isn't a single unit of measure that isn't based on an Earth reference. Meters are based on the distance between the poles and the equator. We standardized a second, but it's really just made up and agreed upon. 100 Celsius is the boiling point of water at STP. Every planet has a year. At some point you have to say good enough. $\endgroup$ Jan 27, 2023 at 17:29
  • $\begingroup$ @SarahMesser I had the same thought on AUs being Terra-centric, but I hadn't even considered light-years being based on Earth's orbit. Planet of origin bias is everywhere $\endgroup$
    – Zags
    Jan 27, 2023 at 21:33
  • $\begingroup$ Startrek actually addressed this with the Klingon kellicam. It's a bit longer than a mile or a kilometer, but still in the general range that it makes since as an alien unit of measure. You can make up alien units if you want too, but saying the universal translator handles the unit conversions for you works too. $\endgroup$
    – Nosajimiki
    Jan 30, 2023 at 22:14
  • $\begingroup$ @Nosajimiki, This, I think, is essential when dealing with races that have no human reference. Having the Zigamolts from Proximaton IV talk in terms of miles is quite immersion-breaking. $\endgroup$ Jan 30, 2023 at 22:22

20,000 Leagues Across Space

I wouldn't bother with any real, identifiable standard of measurement.

Basically, people think in miles (or kilometres). They're big, but human scaled big. I'd recommend just bumping that up a couple orders of magnitude, very much like what we do with velocity. MPH for lower speeds, like cars and trains; Mach for super/hyper sonic speeds; Warp for transluminal speeds. Since light is so painfully slow, on a galactic scale, you'll certainly want to come up with a similar scheme for the next order of magnitude or two.

I'd just pick some cool sounding, relevant terms like leagues, knots, clicks, chains, blasts, etc. These would all be understood in context to actually be the supersized units of measure better suited to galactic space travel.

A league could, for example, be 1/100 or 1/1000 of the galactic diameter and a knot could be a tenth or hundredth of that. Smaller units like chains and blasts could be more relevant to solar system astrogation; whilst clicks or ticks could be used on a planetary scale.


Increase the numerical prefix used in front of -meters to whatever is the most useful for the speed of your ships.

  • 1 kilometer = 1000 meters
  • 1 megameter = 1000 kilometers
  • 1 gigameter = 1000 megameters
  • 1 terameters = 1000 gigameters
  • And so on if necessary (here is a numerical prefixes chart)

With this system, you can express speeds in a way that is familiar to your readers while staying relevant to what your ships are capable of. For instance, as 1 light-second = 299 792.458 kilometers, the speed of a ship that can travel this distance in an hour could be written as slightly less than 300 megameters per hour.

3600 times that is equal to a little over 1 terameter, so the speed of light is roughly 1 terameter per hour with this system. As you can see, its enough to cover all speeds below this supposedly unreachable limit ; if you decide to go the Elite Dangerous way and make it not only reachable but breakable, you can start using the speed of light itself as a unit (1c, 5c, 25c…) but then again, you could simply keep using terameters at this point.

I'm unsure about abreviation conventions, but I imagine you could shorten those with Mm/h, Gm/h and Tm/h (or Mm/s, Gm/s and Tm/s if those apply)

(Edited a lot of times because of calculus mistakes. Math is hard and my head is empty.)

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    $\begingroup$ The speed of light is not 300,000 m/s. It's 300,000 km /s, or in your example, 300 megameters per second. $\endgroup$
    – BMF
    Jan 25, 2023 at 23:37
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    $\begingroup$ @BMF Woops, well there you go, even with 15 edits I couldn't get it right. Fortunately I think my answer stands nonetheless! It's corrected now. $\endgroup$
    – Kubler
    Jan 26, 2023 at 0:12
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    $\begingroup$ @M.A.Golding I think your description is better called interstellar. I interpret inter- as between. Ergo, intergalactic means between galaxies. Like how interstate means between states. So that's even bigger than your description, by ten to hundred times. The average distance between galaxies is 1 million lightyears. You'd need speeds in excess of 100 million c to travel that distance in days, not even hours. You can drop it to 10 million if you want a more colonial "months to destination" kind of story. $\endgroup$
    – user458
    Jan 26, 2023 at 17:36
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    $\begingroup$ Your last comment shows why you can't do this. More than a factor of 10 people get easily confused. How many hours to go 200 km at 40 km/h? Now roughly how many hours to travel 2 megameters at 60 km/h? You had to stop and think. $\endgroup$
    – user71659
    Jan 27, 2023 at 3:16
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    $\begingroup$ @user71659 How much time to transfer all the content of a 2 To hard drive at 60 Mo/s? People may stop and think, I would probably make mistakes again because I know for a fact that I'm bad at calculus, but we already use this system nonetheless. The point of my proposition is to have one that is merely an extension of what we're already familiar with, and allows for a wide [speed] range of uses. $\endgroup$
    – Kubler
    Jan 27, 2023 at 13:04

"The distance between the home planet and the first habital Star - called 'kerf' represented a distance of some 3 earth Light Years. This distance was used as the basis of Interstallar travel. A Kilo-Kerf being 1,000 times this distance.

The related unit of Kerfacs was the number of Kerfs travelled in a standard home planet day.

A commercial grade space freighter in good order could be expected to do 6-7 Kerfacs. Whereas the elite Military vessels were rumoured to be able to break 100 Kerfacs in emergency situations"


I decided not to use km/s or km/h as that would result in numbers with little frame of reference of speed

Give them a reference. Realistically, Earthly references aren't that useful in space. Trying to relate a car ride to interplanetary (never mind intragalactic) distances will result in those large numbers you're trying to avoid. Describe the impact and relevance to the situation. 10 km/s, for example, is slightly too low to escape Earth's gravity from near its surface. It's also practically a dead stop for something capable of traversing light seconds in hours, and it's fast enough that each kilogram has kinetic energy equivalent to 12 kg of TNT, so you really don't want to hit something at such speeds.

  • $\begingroup$ @Christpher James Huff The question describes an "intergalactic war". Assuming the OP knows what they are writing about, that means a war within a galaxy and between different star systems, which almost certainly means using FTL travel. So I don't know why you talk about speeds being fractions of the speed of light instead of multiples of it. $\endgroup$ Jan 26, 2023 at 12:58
  • $\begingroup$ @M.A.Golding The question describes an intragalactic war, and specifically mentions light seconds per hour as an example of the scale they're thinking of. $\endgroup$ Jan 26, 2023 at 15:22
  • $\begingroup$ It's a little convoluted, but I have been working on this project for the better part of the last decade, I'm using a mix of "star gates," Alcubierre drives, and Krasnikov Lanes. With in-universe reasons for having all three and limitations to each. I was advised to use "light seconds per hour" instead of hours per light seconds for clarity purposes. $\endgroup$ Jan 26, 2023 at 16:43

I'm not sure if this actually constitutes a helpful answer, rather it's two excerpts that seems rather useful.

"Well, have you any conception of the sort of thing you're setting yourself to?" The Space Viking snapped his lighter and puffed. "You know, of course, how big the Old Federation is. You know the figures, that is, but do they mean anything to you? I know they don't to a good many spacemen, even. We talk glibly about ten to the hundredth power, but emotionally we still count, 'One, Two, Three, Many.' A ship in hyperspace logs about a light-year an hour. You can go from here to Excalibur in thirty hours. But you could send a radio message announcing the birth of a son, and he'd be a father before it was received. The Old Federation, where you're going to hunt Dunnan, occupies a space-volume of two hundred billion cubic light-years. And you're hunting for one ship and one man in that. How are you going to do it, Lord Trask?"

-- Page ~26 of The Space Viking by H. Beam-Piper

A second, which I'm going to have to somewhat abridge (a travesty), comes from Gray Lensman by E.E. Doc Smith:

"I never could see how you deep-space men can really understand what you're doing—either the frightful speeds at which you travel, the distance you cover, or the way your communicators work. In fact, a professor told us that no human mind can understand figures of those magnitudes at all. But you must understand them, I should think ... oh, perhaps—"

"Or maybe the guy isn't human?" Kinnison laughed deeply, infectiously. "No, your professor was right. We can't understand the figures, but we don't have to—all we have to do is to work with them. [...]

"I can explain it best, perhaps, by analogy. You can't visualize, mentally, the size of North America, either, yet that fact does not bother you in the least while you are driving around on it in an automobile. What do you drive?

"We'll have to pretend that you drive a Crownover sedan, or some other big, slow jalopy, so that you will tour at about sixty and have an absolute top of ninety. Also, you have a radio. On the broadcast bands you can hear a program from three or four thousand miles away; or, on short wave, from anywhere on Tellus—

"Well, change 'miles' to 'parsecs' and you've got the picture of deep-space speeds and operations," Kinnison informed her. "Our speed varies, of course, with the density of matter in space; but on the average we tour at about sixty parsecs an hour, and full blast is about ninety. [...]

"Ultra-waves are faster than ordinary radio waves, which of course travel through the ether with the velocity of light, in just about the same ratio as that of the speed of our ships to the speed of slow automobiles—that is, the ratio of a parsec to a mile. Roughly nineteen billion to one."

"Nineteen billion!" she exclaimed. "And you just said that nobody could understand even a million!"

"That's the point exactly," he went on, undisturbed. "You don't have to understand or to visualize it. All you have to do is to remember that deep-space vessels and communicators can cover distance in parsecs at practically the same rate that Tellurian automobiles can cover miles. So, when some space-flea talks to you about parsecs, just think of miles in terms of an automobile and a radio and you won't be far off."

These two do a better job of conveying the matter than I could. Fundamentally, humans can't process the stupid numbers, they just pick a unit and/or multiple that results in reasonable quantities and forget the rest. In practical terms, both these works resort to light-years per time unit (can be shortened to lights) for general speed and switch to light-seconds for shorter distances. Without looking it up I think it's something like 8 light seconds to the Moon and interplanetary distances are all measurable in light minutes.

  • $\begingroup$ I forgot to mention... the text for both books is curtesy of project gutenberg transcription of publications in the public domain ... very worth reading. $\endgroup$
    – Kaithar
    Jan 28, 2023 at 19:01

Earth human characters should probably give distances in light years. A light year is the distance traveled by light in a Julian calendar year 365.25 Earth days or 31,557,600 seconds long. Long distances could be given in kilo light years or mega light years, thousands or millions of light years.

Speeds can be expressed as so many light years, per year, day, hour, or other time unit. 5 light years per hour is 120 light years per day, for example. Or speed can be given in multiples of c, the speed of light, as in 1,000 c or 1,000 light speeds, for example.

Those units are for interstellar navigation, and for strategic considerations. Combat might happen at much shorter distances and at speeds much slower than light, as it does in some space opera type war stories, in which case different units would be used for tactics. But since I don't know the space war tactics in your story, or what units would be useful in those tactics, I am considering the units used to describe how far away a destination star might be and how long the voyage there might take.

If you are writing about an intragalactic war, which is a war between different governments in the same galaxy, usually called an interstellar war, you probably need to have spaceships which travel faster than light, or at least arrive at the destination in much less time than light would take to travel the distance.

Part One: Average Speeds.

In our region of the galaxy, which may be the setting of your story, especially if the story involves Earth humans who only had faster than light (FTL) interstellar travel for a few decades or centuries, the average separation between stars is about 5 light years. Distances to stars are usually given in parsecs, preferred by astronomers, and in light years, preferred by science fiction writers because they make, time, distance, and speed calculations easier.

So if your characters travel between two stars, the distance will probably be at about 5 light years if those two are the closest neighbors, and of course if those two stars are not the closest neighbors to each other the distance can be many times as great.

Suppose that the spaceship travels between two stars five light years apart. If the voyage takes one Earth year, the average speed will be a 5 times c, the speed of light. If the voyage takes one Earth day the ship will travel 5 x 365.25 light days, or 1,826.25 light days, in one Earth day, or at an average speed of 1,826.25 c.

If a spaceship travels 100 light years in 1 Earth year, it travels at an average speed of about 100 c. If a space travels 100 lightyears in 1 Earth day, it travels 100 X 365.25 light days or 36,525 light days, in one Earth day, or at an average speed of 36,525 times c.

Part two: Accelerating to a Steady Speed, and then Decelerating at the End of the Voyage.

Note in my examples I talk about average speeds. Suppose that it takes one week, or seven Earth days to accelerate to the ships travel speed, and that travel speed is 1,000 c. Halfway during acceleration, the ship will be travelling at 500 c, and 500 c will be the average speed of the ship during acceleration. So during the acceleration phase the ship will travel a distance of 7 X 500 light days, or 3,500 light days, or 9.582477755 light years.

With these assumptions the acceleration and deceleration phases of an interstellar voyage will total 14 Earth days and a distance of 19.16495551 light years will be travelled in those two phases of the journey. So if a space travels 100 light years it will travel 80.83504449 light years at 1,000 c, for a total of 0.0808 years or 29.5122 days, plus 14 more days for acceleration and deceleration, making the total voyage last for 43.5122 days.

Part Three: Accelerating for Half the Voyage and Decelerating for Half the Voyage.

What if a spaceship constantly accelerates for the first half of the voyage, and then constantly decelerates for the rest of the voyage. Suppose it travels 100 LY in one earth year. So its average speed is 100 c, reached after 0.25 Earth year of acceleration.

Supposed that a ship with that acceleration rate travels 1,000 light years. It will reach a speed of 200 C after half a year, and 400 C after 1 year. If the ship then decelerates for one year. it will travel for 2 years at an average speed of 200 C and thus travel 400 light years.

So suppose that it accelerates for two years and decelerates for too years. Thus it will travel four years at an average speed of 400 c, and a total distance of 1,600 light years.

Suppose that it accelerates for 1.5 years and decelerates for 1.5 years. It will travel for 3 years at an average rate of 300 c and travel 900 light years.

Suppose that it accelerates for 1.75 years and decelerates for 1.75. It will travel for 3.5 years at an average speed of 350 c, travelling 1,225 light years.

If it accelerates for 1.6 years and decelerates for 1.6 years it will travel for 3.2 years at an average speed of 320 c. for a total distance of 1,024 light years, which is close enough for now.

Suppose that this spaceship accelerates for 10 years and then decelerates for 10 years. It will travel for 20 years at an average speed of 2,000 c, and thus travel 40,000 light years.

Suppose that this spaceship accelerates for 20 years and then decelerates for 20 years. It will travel for 40 years at an average speed of 4,000 c, and thus travel 160,000 light years.

Part Four: Constant Speed.

Suppose that space ships don't accelerate to reach FTL speeds. Instead they instantly reach their travel speeds and instantly stop at the end of the voyage. In our region of the galaxy a star might be only 2 or 3 light years from its nearest neighbor, or as many as seven or 8 light years, but the average distance between a star and its nearest neighbor is about 5 light years.

So in your story a voyage between two stars should be at least about 5 light years long, even if one of those stars is the closest neighbor to the other. The stars in our galaxy are thinly scattered in a sphere about 100,000 light years in diameter, but the vast majority of stars are concentrated in the galactic bulge area a few thousand light years in diameter, and the galactic disc extending our from the bulge, which is approximately 1,000 light years "thick" from "top" to "bottom" and approximately 100,000 light years in diameter.

So a starship travelling through the thickness of the galactic disc in our region of the galaxy would pass close by about 200 stars or so, while a starship travelling through the diameter of the galactic disc would pass close to about 20,000 stars. That assumes that the stars are equally spaced in all regions of the galaxy, even though they are closer together at the core of the galaxy and farther apart in the outer parts of the galactic disc.

Unless your characters happen to make a voyage from one star to its closest neighbor that happens to be unusually close to it, the shortest interstellar voyage they can make in this region of the galaxy is about 5 light years. Since the galactic disc is about 100,000 light years in diameter, the longest straight line interstellar voyage your characters can make within the galactic disc is about 100,000 lightyears long, or about 20,000 times as long as the shortest interstellar voyage they can make in this region of the galaxy.

Since the Earth is about 26,000 light years the center of the galaxy, the farthest your characters can get from Earth within the galactic disc is about 76,000 light years or about 15,200 times as far as the shortest interstellar voyage.

Thus if your space ship travels at a steady speed of 10 times the speed of light, or 10 c, it will take 0.5 years to travel from one star to its nearest neighbor, and about 7,600 years to go from Earth to the far edge of the galactic disc, and about 10,000 years to travel across the diameter of the galactic disc.

If your space ship travels at a steady speed of 1,000 c, it will take 0.005 years or 1.82625 days to travel from one star to its nearest neighbor, and about 7,600 years to go from Earth to the far edge of the galactic disc, and about 10 years to travel across the diameter of the galactic disc.

If your space ship travels at a steady speed of 100,000 c, it will take 0.00005 years or 0.0182625 days, or 0.4383 hours, or 26.298 minutes to travel from one star to its nearest neighbor, and about 0.76 years to go from Earth to the far edge of the galactic disc, and about 1 year to travel across the diameter of the galactic disc.

Part Five: The Scale of Your Story.

The Milky Way Galaxy contains about one to four hundred billion stars (100,000,000,000 to 400,000,000,000 stars) so any government of even one percent of all the stars in the galaxy would be an incredibly complex government. Thus you might want to make your interstellar war fought between between governments ruling tens, hundreds, or thousands of stars, so their governments might be no more complex than that of the USA, or the Holy Roman Empire, or the United Nations, instead of incredibly complex galactic scale governments.

Thus you might want to make the governments fighting your space war rule only tiny fractions of the galaxy, and make the greatest distances traveled in your story only a few tens, or hundreds, or thousands of light years. One effect of that would be to reduce the difference between the shortest and the longest interstellar voyages in your story, and thus reduce the difference in travel times.

Part Six: Faster than Light (FTL) Space Radio?

Will your story have a FTL space radio form of communication, or will space ships be the fastest methods of communication?

If there is a space radio type of communication, you will have to decide how fast it is.

Do you want to have characters sometimes have conversations with people in other stars systems with no detectable lag in the conversation? Conversations with astronauts at the distance of the Moon had a noticeable time lag due to the three second round trip time for radio waves. Thus I guess that the round trip travel time of hypothetical interstellar FYL radio can be no more than about 2 seconds to avoid noticeable time lag in conversations.

A light year is defined as the distance light travels in a Julian calendar year of exactly 365.25 Earth days. Thus a round trip conversation between two star systems exactly 5 light years apart would have a round trip distance of 10 light years or 3,652.5 light days, or 87,660 light hours, or 5,259,600 light minutes, or 315,576,000 light seconds.

So if the round trip travel time has to be two seconds or less to avoid noticeable time lag, the speed of interstellar FTL radio has to be at least 157,788,000 c. At that speed communication over 76,000 lightyears will take no more than 0.000481658 years, or 0.175925925 days, or 4.22222 hours, and communication over 100,000 light years will take no more than 0.0063376 years, or 0.231481481 days, or 5.55555 hours.

And maybe a writer will want round trip communications between two stars to take one Earth year for story reasons. In that case the speed of FTL radio will be 152,000 c if the two stars are 76,000 lightyears apart and 200,000 c if the stars are 100,000 light years apart. At those speeds two way communication over 5 light years will take 0.000065789 years, or 0.024029605 days, or 0.576710526 hours, or 0.00005 years, or 0.0182625 days, or 0.4383 hours.

Of course if the two stars that two communication between takes one Earth are not 76,000 or 100,000 light years apart, but much closer, the speed of FTL radio will be much slower, and communications over about 5 light years, the average distance between a star and its nearest neighbor, will take longer.

If your space war happens within a relatively small section of the galaxy, the difference between the longest and shortest communication times will be much smaller, but still significant.

And of course, a writer might decide that the speed of FTL interstellar radio is not constant, but can vary according to scientific factors that the main characters might not understand, not being FTL radio operators, and thus have more leeway in how long interstellar messages might take.

Part Seven: Jumping Between Stars.

And another possibility is that a space ship might make a jump through hyperspace or subspace and disappear from one star system and and reappear in another star system without travelling the distance between the two star systems.

In such a system of interstellar travel every voyage should take exactly the same travel time, which a writer might or might not desire.

If the writer wants starships to teleport between stars they can make interstellar voyages take longer or shorter times by limiting the distances of jumps between stars. If it takes time to recharge the jump engines between jumps, or time to calculate how to jump to the desired destination, the number of jumps the starship has to make to reach the destination will determine the travel time.

Or maybe a starship can travel any distance in a single jump, but various scientific factors change how much time it takes to charge the jump engines, or how long it takes to calculate the jump factors before a jump. Depending on the roles of the main characters, they may understand those factors or be completely ignorant of them.


I agree that in many cases the amount of time until an event can be more important than the velocity, but velocity is easily standardized to the speed of light (a fundamental constant so far as we know). If you're telling stories about humans from Earth or descended from Earth, then this is less of a consideration. You can just say that when humans began exploring the stars they continued using seconds, hours, and years that were in use on Earth. If it's a story about a different civilization, you can use the speed of light to bring it into the reader's frame of reference. For example, if a ship is capable of 100,000 $c$ and it crosses the Milky Way in 20 kreptons, then we know 20 kreptons are about equivalent to our year. Well, assuming they're using a base-10 number system; the numbers could be adjust to other systems, but you'll probably just lose your readers.

I think there are two main options: invent your own units (e.g. warp factor or impulse speed in Star Trek) or to compare to the speed of light.

Invent Your Own Units

Personally, I'm not a fan of the first one. While it's clear that warp 9 is faster than warp 8 (and warp 10 just turns you into a lizard-thing), I don't think it's good for much beyond that. In particular, ST warp is on some kind of weird exponential scale so warp 1 is basically the speed of light and warp 9 is somewhere around 2000 $c$. Since warp 10 is infinite velocity, they have to start tacking on more and more decimals to represent faster speeds like warp 9.975. I think the only benefit to this system is when you have a sprawling franchise and you just want to be able to say "warp drives run at the speed of plot.".

When the USS Voyager got transported 70,000 light years across the galaxy and it would take them 70 years to get home, I pretty much equated that to averaging about 1000x the speed of light and that became my reference for the cruising speed of an ST ship that has to stop regularly to find supplies.

"Lights" for FTL; "PSL" for Sublight

In the Culture books by Iain M. Banks (and I'm sure there are plenty of other sci-fi sources that use a similar system), he uses "lights" as a measure of velocity. A ship travelling 144,000 times the speed of light would be said to have a velocity of 144 kilolights. Similarly, distances tended to be measured in light-seconds, light-minutes, light-weeks, etc. He established early on in the narration that a second and metre were so similar to Earth ones that it's not going to matter if a day in the Culture is 15% longer than an Earth day.

If your ships are travelling at relativistic velocities, but not superluminal velocities, then you could go with a fraction of c. Gene Roddenberry's Andromeda called it PSL, so 25 PSL was 25% of the speed of light. Though "point-two-five $c$ probably works just as well.

For interstellar travel, I think that "lights" is the way to go. I think that most people think of interstellar distances (if they think about them at all) in light-years; Proxima Centauri is about 4 ly away; the Milky Way is a bit over 100,000 ly in diameter; etc.) so it makes it easy for readers to calculate times in their head (if they care to do so). If a ship is capable of 24 gigalights (that sucker is moving) then I know this is a really advanced civilization that can travel the entire Universe as it pleases.

For velocities less than c, it's a bit trickier. While I know that the moon is about 1.25 light-seconds away, and the sun is about 8 light-minutes, I don't know the distances for other objects in our solar system in those units. I think km or astronomical units (AU) is a more common metric for those distances.


If any of your civilizations are Earth-based (or primarily descended from Earth), I think it makes sense to stick with light-years, light-weeks, light-seconds, etc. If it's a different civilization, you can use the trick of using a fundamental constant $c$ along with a known reference distance (like the diameter of the Milky Way) to give an impression of how times and distances work for that civilization. You can also pop on SI prefixes to keep the numbers from getting too big (kly for kilolight-years). Personally, I would recommend finding a way to establish that the alien versions of distance and time all work out to roughly similar to Earth ones to keep it easier for readers.


The units here make me uncomfortable. It's taking a velocity (something we could turn into m/s), multiplying by 1 second, and then dividing by 3600 seconds.

$c \cdot \frac{1 \textrm{ s}}{3600 \textrm{ s}} = \frac{c}{3600}$

So ultimately, we're just using a scaled down version of the speed of light. Then 3600 light-seconds-per-hour is just the speed of light. If you need numbers much smaller than $c$, maybe this is workable, but 1 light-second-per-hour is only about 83 km/s which seems easier to deal with.


You don't actually need a new measure unit. There are mainly two methods to save you from using units other than km/s or km/h. I list them as the titles below.

New Technology to Achieve Low Speed

All you need is a new technology which can finish the travel in a short time even with a slow (compared to light) speed, so that you can continued to use a not-that-big number with km/h.

You can try the Einstein-Rosen bridge, aka the wormhole, which connects two distant places (in an astronomical sense) so you can travel across billions of miles by something equivalent to a few miles.

This is what many sci-fictions do. It can save you from bothering about a world of problems arising from the high speed of the space craft and the general relativity, say, the accordant time and people's behaviours, if you're rigorous enough about your works.

New Unit to Achieve Small number

(Warning: The numbers mentioned here are way too large for the current observable universe even if you use nanometer, but since it's a fiction, maybe you have an even larger universe in your story.)

Maybe you prefer a single word that represent a large number, then try googol (= $10^{100}$, which is a '1' follows by 100 '0'), which inspired the name of the famous search engine google. (see this)

If that's not large enough for you, there are (from really large to even larger):

  1. centillion = $10^{303}$ or $10^{600}$, depending on number naming system.
  2. millinillion = $10^{3003}$ or $10^{6000}$, depending on number naming system.
  3. googolplex = $10^{{10}^{100}}$ = $10^{googol}$, obviously a larger version of googol, as the name suggests.
  4. Graham's number, it's so large that even if you have a storage disk as large as the observable universe, you still can't storage the number digit by digit. This is also quite famous among hardcore sci-fans, since the famous popular science writer Martin Gardner had once introduced it to the general public back in 1977.

For space-based distances and speeds, you could use "astro-units" or "AU" for astronomical units (1 AU = approximately 93 million miles). You can then measure velocity in terms of AU per hour (AU/h) or AU per day (AU/d).

To make it more useful within a solar system, you could also use "planetary units" (1 PU = the average distance from the planet to its star).

For velocity, you could use "planetary units per hour" (PU/h) or "planetary units per day" (PU/d).

For more practical comparisons to speeds we're familiar with, you could use terms like "sub-light speed" or "super-light speed" for speeds much slower or faster than light respectively.

As for an abbreviation, you could use "PUs" or "AUh" for Planetary Units or Astronomical Units per hour, similar to the abbreviation "knots" for nautical miles.


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