The third act of Andy Slack's gaming blog

Something I’ve noticed of late; the number of worlds per subsector in published products doesn’t match the number you’d expect from the rules as written.

Said rules have stated since 1977 that the typical subsector has a 50% chance of a world in each hex, which would give you 40 worlds on average. Some versions explicitly say it’s OK to modify the die rolls to give you a one-third (or two-thirds) chance, which would give you 27 worlds or 53 worlds respectively. Stars Without Number uses similar star maps and different rules, which still give about a one-third chance of a world in each hex; and the old GDW boardgames, Imperium and Dark Nebula, have about one system per three hexes if you adjust the scale to one parsec per hex.

Published materials from Classic Traveller such as The Spinward Marches or The Solomani Rim use about a one-third probability, and more recent materials from Mongoose are using 25% or less, with 20% (16 worlds) becoming common. So I sometimes wonder why the official rules still cite a 50% chance per hex, when the publishers clearly aren’t using that, and as far as I can tell, never have. First world problems, eh?

This line of thought begs the question of how many worlds you actually need. Statistics from WotC and Sly Flourish suggest that the typical group plays every 2-3 weeks and plays a game for about five years before moving on. That’s for D&D of course, but I have no reason to suspect other games are different. That’s somewhere between 87 and 130 sessions, and while I’ve played in several games that hit the 100 session mark, I’ve only played in one that went massively over that, which I’m discarding as a statistical outlier. I have no idea how many sessions a typical group spends on a specific world; but in my games it has ranged from none at all to five or more, so let’s say an average of two to three.

Therefore, a campaign ought to need about 40-50 worlds; two subsectors’ worth at one world per three hexes, maybe just a single subsector at one world every other hex. Your PCs may well range over a sector or more, but they’re unlikely to visit more than a few dozen worlds; probably, the only reason you’re generating – or buying – hundreds of them is that you’re not sure in advance which ones will be interesting.

In short, a single subsector should keep you going for several years, especially if by design or blind luck it’s well-suited to your group’s desires; and it’s not likely you’ll ever need more than two subsectors’ worth of planets. If you’re ranging over a bigger region than that, more power to you, but you’re probably spending time detailing worlds you’ll never use.

Update 19 March 2020: There may even be some scientific evidence to support this approach… In “Evidence for a Conserved Quantity in Human Mobility“, the authors explain: “We reveal that mobility patterns evolve significantly yet smoothly, and that the number of familiar locations an individual visits at any point is a conserved quantity with a typical size of ∼25 locations.” You can start going to a new place, but if you do, one of your old haunts is kicked off the list. With one familiar location per world, that’s about one subsector’s worth with a 1/3 chance of a world in each hex; detail more locations and you should need fewer worlds.

Comments on: "Traveller: Worlds per Subsector" (2)

  1. Christopher Kubasik with the Tales to Astound blog has made a similar reccomendation that one or two sub-sectors is plenty for a long time. He has also suggested focusing on a smaller number of worlds within a few parsecs of the starting world.

    This is what I have done with my Wine Dark Rift setting. I plotted out a couple sub-sectors and had a computer generate the worlds, and I assigned names to all of them (by producing a list of names from the areas surrounding the Wine Dark Sea on Judges Guild Wilderlands of High Fantasy maps). I then used SWN tags to add additional information to the worlds in the small portion of the setting of play that is within the Imperium and then a couple outside that.

    I add more detail as a world becomes relevant (either from the players going there or information the players get).

    I have a few vague ideas outside of that, but otherwise it’s a lot of just in time setting design.

  2. I am using a similar number with my Doldrums setting – 1/3 chance per hex, or even 1 in 6.
    I’ve used 50%, and found: 1. The maps are too crowded; 2. It becomes more difficult to add greater detail to worlds; 3. It is more difficult to funnel adventures (and players) towards desired (ie, developed) sites.
    In these cases, forme anyway, “less is more”.

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