Saturday 18 March 2023

What is the Moon Good For?

So, I recently discovered that the world upon which The City State of the Invincible Overlord (CSIO) is situated has two moons, according to the Necromancer edition of the setting. There’s nothing I know of in the original release to say that this is the case, but it’s not unlikely enough for me to reject it.

Partly this is because it never mattered.

Never mattered that there was a second moon in the sky? In decades (on and off) of play? If that’s the case, then does it matter if there’s even a single moon? What, in short, is the moon doing for us?

The reason is that at least one moon has an important game purpose - modulating lycanthropy. This is, of course, vitally important if any PC is afflicted with this curse, but also if the PCs know of an NPC with it.

Simulation Vs Gamism in Lunar Orbits

I looked up the information and found that the “main” moon, Howla, had a period of 27 days and that the small one, Vannis, just 13.

Clearly exactly 27 days and exactly 13 days would be improbable to say the least. Since I was working on a calendar program (more of this in a later post) my first instinct was to add a small fraction of a day to each number. But on reflection I stuck with the unrealistic round numbers.

Adding fractions of a day to the lunar cycle may add realism but any such addition is subject to the hassle/reward ratio — is the hassle of adding some feature to the game world outweighed by the reward of how that feature improves the experience of playing the game. A hassle/reward ratio greater than one means you shouldn’t do it. Remember that the reward part includes both the players and the DM. Just because you personally love complicated calendars or weird languages doesn’t mean your players do too.

So, unless your group of players has more than a couple of astronomers in it, making lunar orbital periods non-integer is a bad idea. Leaving the numbers as exact values means that predicting the lunar phase is easy - easy on you, easy on the players. That makes it easy for you all to think about the implications of the passage of time. And in the case of a werewolf scenario, for example, the focus should be on the adventure and the werewolf, not working out how many hours 0.13 of a day is and trying to calculate backwards to previous reports of attacks over the last year. To hell with that!

Werewolves aside, there are plenty of functions related to the moon. Illumination is a big one - whether trying to travel into the night or trying to get into places unseen. Another one is religious, with many of the deities in D&DG having holy days linked to the phase of the moon. Which leads to quasi-religious things like demonic rituals which must be performed on a night with no or a full moon.

Ambiguous Druids, Batman!

The “official” relationship between druids and moons is a bit unclear. They are stated as worshipping the sun and moon - the combination of which creates the lunar phases, of course - and the sickles they use represent the moon. However, there is no reference to moons in the definitions of their material components. I would suggest that there should be some requirement in connection to lesser mistletoe.

Sailor Moon

A lesser usage for the moon is the fact that it controls the tides. This assumes, as I do, that at least one moon is as large as our own moon and has a similar effective gravitational field down on the surface of the parent planet.

Stable Diffusion displaying a weak grasp
 of lunar phases

Luney Tunes

If one moon is good, useful, and pretty easy to use, then two is probably better, yes?

Well, yes! The two moons given for CSIO are pretty easy to track even with two of them and with two moons you can easily plan for special but rare effects like two full moons or nights with no moons at all. In the case of CSIO these two events happen once every 350 days (each) and so are likely to be once-in-a-year possibilities for all sorts of rituals and shenanigans. Again, the simple nature of integer lunar periods makes it fairly easy to predict these events and the DM can plan NPC actions ahead of time and players can attempt to predict them too.

With an annual event like this, there is potentially a lot at stake for PCs and NPCs alike - if the deadline is missed or a ritual interrupted there is a long time to wait for another chance.

I would suggest that in a world with two moons that druids’ Greater Mistletoe be linked to the night of the double full moon instead of the night of Midsummer.

Priest of the Rising Moon

With two moons you could have a priesthood of clerics who’s spells are modified by the combined value of the moons. For example, as I write this the two moons of CSIO are at 77% and 35%, for a total of 112%. Spells could have their various parameters modified to reflect this (probably in bands - hassle/reward again), so that when the two moons are full their spells are twice as effective but when both are new they are powerless.

While this might be too much variation for a single-classed PC, it could be fun for an NPC cult or for multi-classed characters who are not completely dependant on their clerical abilities.

The Dark Side of the Moon

A final thing about multiple moons is that they can eclipse each other and the sun. This is much harder to model than just the phases but with events this rare the DM could just handwave it by introducing an upcoming, or unanticpated, event as desired.

These sorts of events fit nicely with “The Stars are Right” scenarios where the stakes are even higher than they are for annual events. If someone stops the ritual tonight it might be centuries before it can be tried again!

That’s some thoughts on moons for now. Next time, or the time after, I’ll look at how this fits in with Gygax’s notion of why time is so important and how I know what phases the moons are over the City State of the Invincible Overlord.

Yes - I’ll be looking at 1:1 time.


  1. Can't wait for your take on 1:1 time. Looking forward to it!

  2. I was literally sitting here thinking "I regret mentioning 1:1 time being the next post". I will get around to it, but the post was going to have lots of "clever" Javascript to do with time and I had forgotten how crappy JS is even in its own domain so it's been a pain in the arse and then work blew up.

    But, soon. Yes. Sooooon.