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Some background:

My interstellar civilization(s) have fled our home Solar System, spreading throughout the nearby systems in the Orion Arm. They possess FTL, but for plot reasons, they have only just begun to reach Sagittarius.

Since they have spread out through a large portion of the Orion arm (but not all of it), a standardized form of mapping was needed. In most of my drafts, locations are simply referred to via the "Sector XXX" trope, but, as we know here on this Stack, space is massive and three-dimensional. Thus, the three-digit sector schema won't work.

Relevant Information:

The sector naming schema is chosen because this system only needs to point towards a location, and provide minimal information about what is there (for example, sector 123a is the first star system in a sector.)

Assuming the following conditions:

  • The Orion Arm is 10 kly long, 1 kly deep, and 3 kly wide (small estimates, I know.)
  • Each sector is 5 ly apart

There should be 200x800x2000, or just around 300 million unique sectors.

This number seems absurdly large, however. Even is you used a base 16 coordinate system, you would still have a ~7-digit identifying number. Addendum: You could lower this number by having the system be two-dimensional, only dividing up along the galactic plane.

I did have the idea of using a truncated three-dimensional coordinate system, though. An example would be Sector 02101440.

Of course, a coordinate system might not even be the optimal choice, and conventional naming schema for star systems are essentially out the window, as I also want to be able to point at an empty region of space and say "This is Sector XYZ."

With all of that now in mind: How would an interstellar civilization map the arms of the galaxy?

Criteria:

  • The ability to point to and label any region of space
  • The ability to identify components in a system
  • The system being expandable
  • 3D is not required. It's secondary to all the above, as a sector can always be subdivided.

Systems that allow for shorter designations, or the ability to truncate one to be shorter, without overgeneralizing are preferred.

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    $\begingroup$ Galactic coordinate system. $\endgroup$
    – AlexP
    Mar 23, 2023 at 20:19
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    $\begingroup$ Two problems with it: Sol-centric and lengthy. Although the first one is almost a non-issue(because you can just change the reference point), the second one not so much $\endgroup$ Mar 23, 2023 at 20:50
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    $\begingroup$ Out of curiosity, is 300 million sectors randomly chosen, or is there a reason to it? Also, just as a note, 5 light-years is roughly the mean separation between two stars in the Milky Way (given that the average stellar number density is 0.1 stars per cubic parsec), so with sectors that small you'll only have ~1 star system per sector. $\endgroup$
    – HDE 226868
    Mar 23, 2023 at 22:45
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    $\begingroup$ How to Map arms of the galaxy? With Sleeves of course! #Dadjoke. $\endgroup$ Mar 24, 2023 at 0:01
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    $\begingroup$ With massive spaces between items of note, it doesn't really make sense to sector at all. Relatively close items are a named group, and the expanses between groups can also be named. That's basically islands, oceans, and seas. Add in routes and you're done. $\endgroup$
    – user458
    Mar 25, 2023 at 22:01

12 Answers 12

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First of all, please check out our List of Worldbuilding Resources, which might give you ideas about how you want to solve this in your world. There's an entire section devoted to mapping, from a fantasy land to star charting.

But, when it comes to creating your own mapping system, you need to remember two things...

  1. Coordinates tell you where locations are.

  2. Boundaries tell you where your friends and enemies are.

The idea of "sectors" as portrayed in fiction such as Star Trek doesn't actually make a lot of sense unless there's purpose behind making arbitrary boundaries. Cataloging isn't a sufficient purpose for a civilization that can actually hop from one star to the next in a practical manner. Think about it, who cares if the planet Gronk is in Sector Alpha-Seven-Niner-Epsilon?

It's like comparing latitude and longitude vs. national boundaries. The first is a mathematical solution that anything from a printed book to a computer can use to find, well... as I said... a location. The second is fluid because it changes with purpose. National boundaries (for example) change for many different reasons ranging from war to polite resource trading to "I saw it first!"

What's the value of the coordinate system? To precisely find a location or group of locations.

What's the value of boundaries? To know who your friends and enemies are. Or, at least, to more easily identify a culture that might be fun for a vacation.

Use coordinates to explain how you travel. Use boundaries to explain why you travel.

Maybe it's true that the Gronk Empire just declared war on the Wabooda Oligarchy! Your family was looking forward to vacationing on the famed Abtrusian Spinnarette! The auroras wafting through the island planetoids of the Spinnarette are to die for and it took months to obtain passport visas to the Oligarchy! What are you going to do???

  1. You're going to open up your world's version of the Michelin Guide, which totally doesn't have the words "Don't Panic! written on the back (but should 'cause your family's gonna find out about that war...), and do a quick cross-reference to amazing worlds with breathtaking auroras. You'll discover that there's a possible alternative, the Lumpus Origami Rings located at Periamory Fourteen.

  2. You quickly ask your handy-dandy cell phone um... personal digital assistant ... OK, you quickly say something that really doesn't sound like "Alexa, change the family's vacation plans to the Lumpus Origami Rings!"

  3. What neanderthal rednecks like you and me here in 2023 might rudely call a computer spends about a bazillionth of a second collating the spherical coordinates of relevant planets, caring about whether or not arbitrary boundaries are going to matter to you, withdrawing payment for services at one and scheduling services at the other, routing transportation that would involve really complex calculations that should really be the transport ship's problem had you not booked the absolutely cheapest transport on the planet, and spits out an entirely new itinerary for your family's vacation, complete with a 15% gratuity Alexa will spend courting Siri. (Both of whom won their independence in the Great Anti-Contra-Ludite Debate of 2623.)

Funny, JB, but what's your point?

As before, two-fold:

  1. Locations will always be a coordinate system that, frankly, has nothing to do with artificial boundaries, regions, politics, religion, a good time, or anything like that. Computers will be doing all the routing and flying. They don't care about names anymore than DNS today actually cares that you called your computer Dilbert or that your domain is JacquesStrap.Org. Computers like numbers. They like mathematics. And since planets and solar systems move, they're going to care a lot about vectors, angles, and equations.

  2. Boundaries will always be a human living thing. Boundaries are how we number-averse beings collect and organize fun things to do, like holding national elections, identifying national parks, and generally expressing the idea, "um... this is mine." Boundaries can be a LOT of fun for worldbuilders, so long as you stay away from seriously using them to deal with #1, above. Believe me, the Klingons really don't care how the Humans marked off space. They have their own way of doing it — and it has a LOT to do with "um... this is mine."

And, above all, boundaries give you the ability to list coordinates that are relevant to the boundaries: such as the list of solar systems claimed by the Gronk Empire. You'll soon discover that trying to draw lines in 3D interstellar space isn't very practical. But assigning a list of resources (planets, solar systems, nebulae, mine fields from the ancient Eyklynoolian Panic... now THAT was a panic!) to an arbitrary category number with the name "Gronk Empire" and letting a computer worry about how to display it... that's actually pretty easy.

<StarChart>
   <Region>
      <Name>Gronk Empire</Name>
      <SolarSystem region-identifier="1">
         <GalacticCenterVectorLength type="parsecs">1255</GalacticCenterVectorLength>
         <ParallelMeridianAngle>272</ParallelMeridianAngle>
         <PerpendicularMeridianAngle>135</PerpendicularMeridianAngle>
         <LastVerifiedLocationDate>26190127</LastVerifiedLocationDate>
         <ProgressionModelName>Haskell5-theta</ProgressionModelName>
         <Name>Gronk</Name>
       </SolarSystem>
       ...
   </Region>
</StarChart>

Of course, what your people want to see is...

GRONK Home planet of the Gronk Empire. ✨✨✨ Notable for its vast obsidian deserts and popular Enk-gronk-eay music. 🥣🛏️🦮🐱 Notable celebrities: None. Current politics: at war with Wabooda Oligarchy (💥💥). Graft: 💲💲💲. See GRONK EMPIRE for current list of planets.

Above all, remember that an actual coordinate system is what your shipboard computer cares about and won't be particularly readable by any human. Or, if it is, it'll be boring. Use coordinates sparingly and, IMO, don't worry too much about coming up with one that actually works. Unless that level of precision has a critical role to play in your story, it's not worth the effort. Coming up with a way to express the idea of location is, IMO, what you're really after.

In other words, how do people in your world look up locations? How do you do it today? Do you type in 48.8566° N, 2.3522° E? Or do you type in "Paris, France"? The first might be useful for a SLAMRAAM launch computer, but the second is more useful for finding a cafe.

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  • $\begingroup$ I'm aware of the "borders in space trope" and it grinds my gears so much, which is why I haven't even bothered with the idea of mapping until now. As you said, this system is only needed for human-readable concepts of location-computers/AIs/synths/whatever you want to call them do use an actual galactic coordinate system. $\endgroup$ Mar 24, 2023 at 11:56
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    $\begingroup$ Boundaries might be about more than "fun": They can be about the search for resources that motivated FTL travel to begin with. (Consider the interstellar equivalent of a watershed map, which will be an increasingly important set of boundaries on Earth in the near future.) $\endgroup$
    – Theodore
    Mar 24, 2023 at 19:08
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    $\begingroup$ There are more reasons for boundaries than just geo(astro?)political lines. They are also ways to group things into categories where everywhere in a space shares something in common with its neighbors. Star Wars, for example, organizes its galaxy into the regions of Core Worlds and Inner/Middle/Outer Rim, with each region grouped by both its distance from the center of the galaxy as well as its relative prosperity, influence, and population density. Another reason for boundaries can be practicality, such as in a spiral galaxy specifying whether you are in the center region or a specific arm. $\endgroup$
    – Abion47
    Mar 24, 2023 at 21:08
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    $\begingroup$ Actually, Klingons say: "Gharrr, that's mine!" $\endgroup$ Mar 25, 2023 at 10:21
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Name sectors after bright stars

I'm an astronomer and therefore utterly awful at naming things, but I'll vote in favor of centering each sector around one bright star -- in particular, an O or B star -- and use whatever name people on the street call that star. In our local stellar neighborhood, these luminous stars have a combined number density of roughly $n\sim3.2\times10^{-5}$ per cubic parsec. The mean separation is then $d\approx n^{-1/3}\approx100$ light-years, so any point in space should be within about 50 light-years of an O or B star.

Now, this is larger than the 5 light-years you asked for, but bear in mind that all kinds of stars in our neck of the woods put together have an average number density of $\sim$0.1 stars per cubic parsec, meaning that the typical star is about 7 light-years away from its nearest neighbor. In other words, if we stick with your sizing scheme, the vast majority of sectors will have no more than 1-2 star systems within each one at the most, which seems like too fine-grained a measuring system. I'd argue a 100-light-year separation is better for practical purposes.

Some advantages:

  • There are O and B stars throughout the galaxy, so wherever you go, you'll have new reference points.
  • O and B stars are easy to find and locate and will likely be among the best-studied natural landmarks (spacemarks?) available.
  • Using this coarser system means you can use words, like Nosajimiki suggested, to name sectors, without running out of easy-to-pronounce combinations any time soon.

Some disadvantages:

  • These stars will die after a couple tens of millions of years at the latest; on the other hand, more will spring up in their places.
  • They do tend to be clustered together and sometimes in binaries, so the separation I quoted above is a slight underestimate.
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  • $\begingroup$ Bright stars tend cluster together and form non-habitable regions. $\endgroup$
    – fraxinus
    Mar 24, 2023 at 21:17
  • $\begingroup$ @fraxinus Inhabitable near them, definitely, but not on scales of light-years or tens of light-years. The inverse-square law gives enough protection. $\endgroup$
    – HDE 226868
    Mar 26, 2023 at 13:27
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Think about what your galaxy spanning civilisation needs

Contrary to popular belief, stars and galaxy arms are independent.

That is, that spiral arms are constantly changing density variations that the stars move through - the stars are actually orbiting the centre of the galaxy at a separate rate the arms do. This is still a mystery as to why for astrophysicists, but there are some theories out there as to the reasons to be confirmed by more observations.

This animation demonstrates that stars and arms do not orbit the centre of the galaxy together: enter image description here

This means your coordinate system should not be able to reference arms, for instance to get to a certain star, as to track a star over a period or even to find out where that star had been, would not be possible. A star would obviously change arm over millions of years, but even over a 1000 years this would cause a complex problem referencing between arms and for a galaxy spanning civilisation this will still make plotting of stars difficult.

Instead I would recommend a simpler Polar Coordinate system measured around the perpendicular axis to the galactic plane, with the 'North' direction a fixed normalised direction to a nearby galaxy. Although this too changes, the time-span for shifting of this direction would be billions of years, not millions as per the arms, so would be far more useful to a galactic spanning civilisation. If not for navigation, perhaps culturally or scientifically.

Plotting the position of the star historically in relation to this would then need a time-index and a computation model of a galaxy.

So your coordinate system would be: This Galaxy, Nearby Galaxy, Polar Angle from Normalised angle, Distance from Centre, Time Index (if referencing a star, not needed otherwise) to be safe measured from the Big Bang. This way you can then plot a course to or from this point at any time.

So it may look like this:

  • Coordinate of Sirius Star = Milky Way, Andromeda, 31.252º, 16832LY, 13.451Y

Or you can omit the first 2 coordinates for 'common parlance':

  • Coordinate of Sirius Star in the Milky Way = 31.252º, 16832LY, 13.451Y

And you could leave out the time index if you wanted a fixed point at the precise moment of coordinate generation:

  • Coordinate to the current position of Sirius Star in the Milky Way = 31.252º, 16832LY
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    $\begingroup$ Movement is less of an issue than you think. While the stars move fast at human scales, they move quite slowly at galactic scale. It would take Sol about 6750-11700 years to cross a single 5ly grid sector (depending on what angle it crosses at). Yet here on Earth, we're already used to updating our maps every couple of years (or even months in some cases) to account for much faster changes like the shift of rivers, erosion, and new construction, etc. $\endgroup$
    – Nosajimiki
    Mar 24, 2023 at 13:59
  • $\begingroup$ @Nosajimiki Yes but my point is that the coordinate system proposed should be able to be used in many contexts - not just navigation right now - but perhaps for scientific purposes (plotting course of stars over time, modelling of stars, etc) or cultural reasons (religions based on where stars were, or the contemplations of the future). These should be accounted for, and other systems proposed like Cartesian coordinates or arm coordinates are incomplete for these and would likely be discarded fairly quickly (or at least within the first 100 or so years). $\endgroup$
    – flox
    Mar 26, 2023 at 16:38
  • $\begingroup$ I think you are right that planetary locations will be stored with vector information, but to measure motion, you need a fixed coordinate system to measure against. So, while the planets move, the sectors need to be fixed in order to have a meaningful coordinate system. $\endgroup$
    – Nosajimiki
    Mar 27, 2023 at 16:28
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Sun-centric Cartesian coordinates in light-years

  • X is distance from sun towards galactic center
  • Y is distance along the rotational axis of galaxy
  • Z is normal to X and Y

This is the logical development as space technology progresses. At first, we don't know the accurate distances to far-away locations. If we started with the center of galaxy as origin, Earth's coordinates would have an error bound of +- 2600 light-years. Unusable for navigation.

Sun-centric system expands as we travel further. Close stars have coordinates like 4-1-0, while further away systems are still reasonable 232-32-17.

The angular version of Galactic coordinate system is logical for observing from a single place, as we know the direction much more accurately than the distance. However as soon as you have multiple planetary systems, the rectangular/Cartesian variant is more practical. Distance between two places can be computed directly from the coordinate numbers. Binding the coordinate axes to location of a star system and the galactic center keeps the coordinates relatively accurate over millions of years even as the galaxy spins.

Eventually a second central world will find its place. It is inevitable if travel speed has any limit. Which world will find importance depends on abundance of resources, easy travel and local developments. They will start mapping the neighbourhood relative to their own system, maybe using a letter distinguishing the systems. Thus A232-32-17 will be B0-0-0, and the cycle repeats. Dealing with multiple coordinate systems has its cost, so this will only happen when enough travelers consider B to be central and A to be some distant relic.

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    $\begingroup$ +1. But I think an argument could be made that it's reasonable at the time one has FTL technology to also have detection technology such that the far-away error bound was much less (such that galaxy-center coordinates would be viable). $\endgroup$ Mar 24, 2023 at 18:03
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Subdivide it

The problem you'll run into with any galactic coordinate system is that, for your typical freighter run, you're only going to use a tiny portion of it. Yes, you could use cartesian or polar coordinates based on the location of Sagittarius A*, but nobody wants to describe their flight as a regular run between O 273.334.5616 and O 274.334.5618. You need something that's meaningful locally, but translates to something that's useful globally.

If you're basing your coordinates on a spiral arm, you can start with a linear measure along that arm. You can have your survey crew identify a curve that runs from the beginning of the spiral arm to the end of it, then chop that line into meaningful bits.

The Orion arm is about 3.5kly edge to edge, and roughly 20kly long. The maximum resolution that we care about is roughly 1 ly. You do get stars closer than that, but that's something you can adapt to.

Let's chop that length into 1000 light year chunks and label them A-Z. You could keep the extra letters for later, or give them a special purpose. That's up to you. This gives you roughly twenty zones that are 1000 x 3500 x 1000.

This is where you want to start paying attention to the way political boundaries are drawn up. You could split the boundaries purely mathematically, but you really want to break it up so that things like nebulae and big star clusters get grouped together. If there are edge systems, go ahead and have people argue about which major sector they belong to. It all translates to the same top-level coordinate anyway

So what we do is pick a star in every cluster that is near the middle, and doesn't have a high relative velocity compared to the arm. A large, bright star would also be beneficial. Use that (movement and all) as the distinct, moving center of your zone. You can pick cartesian or radial coordinates from that point.

Personally I'd apply a second layer of letters and numbers, so you're dividing it into 36 x 36 x 36 sub-zones, each roughly 27 x 97 x 27 ly across.

This gets you up to 1.2 million sectors, with labels like Sector Om, sub-sector 2FQ, zone 23.85.21.

Once you have a volume of space, number the stars by magnitude, and planets by distance from their star, moons by (etc. etc.)

The thing this system doesn't do is account for the motion of stars. Fortunately, even today's computers can calculate the current and future location of all of the stars. Ok, maybe not ALL of them, but that's why your people need to trade star charts. You pick a point in time and set the designation, then maybe update it every hundred years. Thus, time stamps would be critical for star charts.

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Option A: Use Monosyllabic Words instead of Numbers.

There are 10 digits in the English Language; so, sector 4-1-2 can give you a base-10 grid for 1000 sectors, averaging about 800,000 stars per sector across the Orion Arm.

Letters will get you a bit farther. Sector f-c-x can give you a base-26 grid with 17,576 sectors averaging 45,516 stars per sector grid... still pretty darn big.

But there is one thing that the English language has that is just as easy to say as a letter or number that we have a LOT more of: Monosyllabic words. In fact, there are about 12,000 words in the English Language that only have 1 syllable. This could be truncated down to the 10,000 most common words to give you a convenient base-10 system to work with. This grid would fit the entire Milky Way inside it while isolating Orion's Arm to a section of the grid system that is about 200x800x2000 sectors.

So your captain could say, "set course for sector cat-pop-bag" and with this simple 3-word combination, you establish one of a trillion possible, easy to say, sector names... and because they are alphabetical, they also give a good idea of displacement and distances so, even if you are outside of your usual territory, you would understand that "cat-pop-bag" is pretty close to "can-pond-back", but very far away from "door-zoo-gin". Over time, some of these sector names may even take on local pronunciations and be treated as words unto themselves like Catobag, Canonak, and Dozogin.

Also, because you are dropping some words, you could actually manipulate the word arrays just a bit do to something snarky, such as placing Earth in the Earth-Earth-Earth sector, or doing something practical like dropping offensive or overly similar sounding words.

Option B: Use local designations.

I remember back in the 90's (before cell phones messed everything up) if you asked someone for their phone number, they would give you a 7 digit number... but phone numbers were all 10 numbers long. This is because everyone would drop their area code because 99% of the time, you and the person you were talking to were in the same area code.

Now let's apply this same concept to a coordinate grid. Another way to break up the milky way into a trillion bite sized sectors is to break up this number into a series of 4 sets of 3 numbers where each set of numbers is an XYZ 10th of the previous subdivision such that your actual sector is a number between 000.000.000.000 and 999.999.999.999. So if you are in the sector representing the cardigan location X=1234, Y=5555, Z=6789 then your sector address would be 156.257.358.459. What this means is that if you are in sector 156.257.358.459, the closest sectors to you would be 156.257.358.359, 156.257.358.559, 156.257.358.449, 156.257.358.469, 156.257.358.458, etc... since everyone near by you is going to be in the same area code "156.257.358", then you in casual conversation can just say, "Set course for sector 458" and everyone knows that you mean the local sector 458 meaning 156.257.358.458. If you want to go a bit farther you might say "Go to sector 356 dot 458" meaning 156.257.356.458. The farther away you are talking about, the longer the number gets.

This way you can keep destinations simple small numbers most of the time. And ofcourse, over time people will have to come up with names for each of these levels of detail, like Domain, Zone, Section, and Sector so that if you want to reference a more broad area than a little cube of space containing no more than a handful of stars, you could say something like "The Moobark empire spans most of Domain 436". Now, you know you are talking about an area of space that is a billion sectors in size.

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    $\begingroup$ Be wary, you'd probably have to knock out some... controversial words from the list. I doubt anyone would want to live in the sector me-dumb-rat... And I'm avoiding the many words for things happening "below the belt", if you get my meaning o_x. $\endgroup$ Mar 23, 2023 at 22:36
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    $\begingroup$ @Tortliena We have people today who live in Shitterton England, Fucking Austria, and Dildo Canada (I'm not even including non-English speaking places that just sound offensive) ... so, I think this is less of an issue than you think. And even if you live in the me-dumb-rat sector, the local pronunciation may become something more like Med-um-rat which does not sound so bad. $\endgroup$
    – Nosajimiki
    Mar 24, 2023 at 13:38
  • $\begingroup$ There's no need for the sector code words to have any other meanings at all. In English, few digit and letter names have homophones, and even fewer have homonyms. Of course, this can vary from language to language. In some languages, like Japanese, people use sound similarities between digit words and other words to create numero-mnemonics (languagelog.ldc.upenn.edu/nll/?p=22853). If some sector code words sound like other words, certain sectors could develop cultural associations from that. $\endgroup$
    – Vectornaut
    Mar 26, 2023 at 18:55
  • $\begingroup$ One could also have a hierarchical system where there are, say, 45 single-syllable sector code words that combine in pairs to create 45 × 45 = 2025 two-syllable compound code words, which can describe a 2025 by 2025 by 2025 grid. There are lots of possible variations. Since the galactic disk is very thin, for example, your civilization might end up with 15 simple code words that combine into 15 × 15 = 225 compound code words for the direction perpendicular to the galactic plane. Some might overlap with the 45 simple code words used for other directions. $\endgroup$
    – Vectornaut
    Mar 26, 2023 at 19:09
  • $\begingroup$ You could look to many living languages for inspiration. I think Hawaiian uses 40 consonant-vowel syllables (among 239 syllables total). Vai has a syllabary that uses 267 characters, if I'm not mistaken; Wikipedia arranges them in a 43×7 grid, which I think represents consonant-vowel pairs (with some subtleties I'm missing). Cherokee uses an 85-character syllabary, and Kpelle has been written in an 88-character syllabary. $\endgroup$
    – Vectornaut
    Mar 26, 2023 at 19:38
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Just use galactic center-centered coordinate system in higher-base numbers.

Instead of hexadecimal, you can use base-32 and make the numbers 2 times shorter (and 3 times shorter than decimal with the same accuracy)

You can even go base-36 (10 digits and 26 English/Latin letters) and your coordinates will look like: DCA4/PGS0 (angle and distance from the center, accurate to well within light-year)

This is enough not to get lost in the sparsely-packed habitable regions.

Regions with a dense population of high-mass, bright, short-lived stars and their remnants (neutron stars and black holes) may be scientifically interesting, but neither habitable nor easily navigable (strong EM radiation all over the spectrum, convoluted gravity and fewer planets). Whoever goes there will add a character or two for accuracy and a third number.

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An unbroken string of 8 digits isn't terribly ergonomic...think of telephone numbers. Breaking things into chunks makes them both easier to remember and easier to communicate without errors. And as with phone numbers, consider a hierarchical system.

For example, use base 32, consisting of 10 decimal digits and 22 letters. A 3-symbol galactic sector code breaks the galaxy up into 32768 sectors. Another 3 symbols breaks each of those into 32768 sub-sectors, for just under 1 billion sub-sectors total. For local business, you only need the last three...there may be 32768 sub-sector A57's, but even near a border between sectors it will be pretty clear which one is meant, and you will rarely need to specify whether it's 3G9.A57 or 72E.A57.

Also, cylindrical coordinates centered on the core will result in inconveniently shaped sectors, only imperfectly fit the galaxy, and make little sense out in the halo, satellite galaxies, or other galaxies. Just put the galaxy in a Cartesian box. It is expandable: when you need to deal with locations outside a cube containing the Milky Way, just add a prefix defining intergalactic sectors, giving the one containing the Milky Way coordinates near the middle, so each Milky Way set of coordinates becomes GGG.###.###.

If you set the size of the Milky Way-containing intergalactic sector to 131072 light years across, then each galactic sector is 4096 light years across, and each subsector is 128 light years across. Or for a civilization with a bit less forethought, start defining sectors at the origin of the civilization and just add prefixes from there, with a result that doesn't so cleanly fit the Milky Way in a box.

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If you have a map of named pulsars you could define a coordinate system that would uniquely define a point by using the distances to the closest four pulsars. example Larry 5.0, Curly 11.5, Moe 222.4, Shemp 502.3. Let the computer do the math and knowing where the pulsars are in absolute galactic terms.

This would be locally useful and spare the user from a distant origin point and spare the author and reader lots of angle measurements.

And since it's unlikely the pulsars are going to be well defined it makes proving continuity errors difficult (like why Star Trek avoided real stars - some trekkie could map the Enterprise following a very random voyage.)

The Voyager 1 probe for example used a very redundant 14 Pulsar map to point to Earth.

https://www.pbs.org/the-farthest/science/pulsar-map/

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An alternative to the many great ideas here:

Divide your galaxy up into 100 domains named after the highest mass object within each domain. The highest mass object is poetically treated as the cosmic "ruler" of that domain, much like we anthropomorphise mountains and such on Earth. These domains can vary in size based on the gravitational influence of it's cosmic ruler. You can even divide them further, much like a feudal system of Kings, Dukes, Counts etc.

From there just use vector 3 in light years relative to the cosmic ruler. e.g.

Traveller: "I come from Uddakar alt-Selim in Archaeon."

Officer: "I'm sure you do, coordinates?"

Traveller: "103-209-77, if you really must look it up"

^ So Uddakar is the traveller's planet, "alt-" means "of", "Selim" is the local name of the star and "Archaeon" is the galactic name of a black hole that influences 1 billion stars around it and was therefore categorised as a "cosmic ruler".

This allows you to blend some cool lore and cool names with some acceptable hard science. You can lower the number of cosmic rulers to be easier and really just treat naming of planets and stars as a local thing only, where ruler and vector 3 is the universal language.

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Part One of Three: A frame challenge:

Settling star systems in the Orion Arm of the Galaxy is a bit like settling the aurora borealis.

When you look at a spiral galaxy from the outside, the spiral arms look very impressive, being much brighter than the spaces between them. Thus at first sight it is logical to assume that the stars are concentrated in the spiral arms and their are few stars between them.

Thus in James Blish's "Okie" or Cities in Flight stories first written during the 1950s, it is said that Earthmen have spread out along Arm II of the galaxy.

But in a later Blish novel from the 1960s a character notes that the spiral arms are mostly visual features of a galaxy and that the star distribution is more uniform throughout the galactic disc.

In most science fiction stories of interstellar colonization, humans colonize naturally habitable planets of other stars. And science fiction writers who want high scores in the Sliding Scale of Science Fiction Hardness:

https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness

Will have their human colonists only colonize planets of stars which are capable of having human habitable planets. Since it took Earth billions of years to develop an atmosphere breathable by humans, planets that are naturally habitable for humans should only orbit stars which are billions of years old.

And only some spectral classes of stars can last for billions of years.

Thus naturally human habitable planets should orbit spectral class G and some class K stars, and maybe class F stars on the shorter lived side and maybe class K and M stars on the dimmer though longer lived side.

And all stars in our galaxy orbit around the center of mass of the Milky Way Galaxy. Stars which are closer to the center take less time to complete one orbit, while stars that are farther from the center take more time to complete one orbit.

So if a straight line is drawn between two stars and the center of the galaxy at one time, millions of years later the star closer to the center of the galaxy will have pulled ahead of the farther star, and so the line will no longer be straight.

Spiral arms also orbit the center of the galaxy. They are regions of higher density interstellar gas and dust, where new stars form. New stars are formed in a wide range of mass and luminosity, including short lived highly luminous stars. So spiral arms contain many might young stars, and bright clusters of young stars, and nebulae that reflect light from young stars, and so they are brighter than the spaces between spiral arms.

Stars generally form in open star clusters. And the older a cluster is, the higher the percentage of its stars which have escaped from the cluster due to gravitational interactions with passing stars. So the star clusters which stars form in gradually dissipate and their stars become spread out over a vast volume of space and orbit the center of the galaxy independently instead of as a group.

Long before any planet of a star can become habitable for humans, that star will have escaped from its birth cluster and orbit the center of the galaxy independently.

The young stars which are part of spiral arms are mixed up with many times their number of older stars that formed millions or billions of years earlier. So the density of stars per cubic volume of space is almost exactly the same within a spiral arm and outside it.

The galactic disc of the Milky Way galaxy is more or less arbitrarily a hundred thousand light years in diameter. The stellar density decreases gradually with increasing distance from the center of the galaxy. And the stellar density also decreases, but much more rapidly, with increasing distance "above" or "below" the central plane of the galactic disc. Thus the galactic disc is often arbitrarily said to be one thousand light years thick and one hundred thousand light years in diameter.

So if humans spread out from Earth settling on naturally habitable planets, they will settle on planets of disc stars, and won't care whether those disc stars are within spiral arms. The region of human settlement will be a spherical area which gradually increases in diameter, until the "top" and "bottom of that sphere reach the ill defined "upper" and "lower" edges of the galactic disc. Then the zone of human colonization will become a flat cylinder bounded on "top" and "bottom" by the "upper" and "lower" limits of the galactic disc, and expanding radially throughout the galactic disc.

Part Two: One Possible Reason to Settle Spiral Arm Stars.

But maybe human colonists won't colonize naturally habitable planets. Maybe some advanced civilization terraformed many planets of young stars in spiral arms to be habitable billions of years before they would naturally become habitable. Maybe all the naturally habitable planets have their own life forms which have evolved there, and only the terraformed planets are considered to be open for colonization.

In that case humans would colonize only young star systems which had been terraformed by the advanced civilization, and so they would spread out colonizing terraformed planets of spiral arm stars instead of planets of the general disc stars which were already taken.

Part Three: A Second Possible Reason to Settle Spiral Arm Stars.

Or maybe humans don't colonize naturally habitable planets or planets terraformed by some advanced alien society, but settle in stars systems that have many small objects suitable for space mining and build artificial space habitats to live in.

The younger a star system is, the higher the concentration of heavy elements will be in that system. The younger a star system is, the less likely it will be to have planets already formed, and the more likely it will be to have billions of small rocky planetesimals which would be much easier to mine than planets.

Thus for a society which builds artificial space habitats very young star systems may be the most desirable. And so such a society might only colonize young spiral arm stars, and spread out along a spiral arm, instead of colonizing disc stars in general, and spreading out in a spherical and later disc shaped volume of space.

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It could be that the first interstellar navigators spoke Hawaiian, and they noticed that the language's 200 possible consonant-vowel-vowel (CVV) syllables would neatly divide the thousand-light-year thickness of the Milky Way's thin disk into five-light-year slices. In the galactic plane, they already used a logarithmic spiral coordinate system matching the pitch of the spiral arms. Combining Hawaiian's 40 CV syllables in pairs, they could conveniently divide the coordinate patch containing the Orion Arm into a 1600 × 1600 spiral grid. They could then specify a comfortably sized region of space with a three-word sector name like kai-hale-manu, lei-papa-hine, kui-lima-holo, kei-waha-wale, hoa-noho-lana, or pā-hana-mana.

As these examples show, sector name "words" were often actual Hawaiian words, and sector names would occasionally be evocative strings of words, or even phrases. This was good, on balance, because it often made sector names easier to say and remember. However, it also had some weird effects. For example, on the very rare occasion that a sector name was mistyped in a document, miscopied from a log, or misheard over a bad audio connection, there was a slight tendency toward errors that mistook nonsense words for sensible ones, ungrammatical phrases for grammatical ones, ugly names for beautiful ones, boring names for funny ones, and scary names for auspicious ones. This affected migration patterns over time. The slice of the disk also took on an air of mystery, since was the only CVV syllable that never occurred in any Hawaiian word.

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