RogerBW's Blog

Faking the Moon 09 November 2014

I run a lot of role-playing games set in the real world. (As Ken Hite points out, the background material is much richer and has had more people working on it, even if it's sometimes less plausible.) One surprisingly easy technique to add consistency and a sense of realism is being able to say what the moon is doing.

PCs tend to be the sort of people who do stuff at night, and the level of lighting is often important. Of course one can just arbitrarily decide what the moon's up to, or generate it randomly; but getting a close approximation actually isn't very hard.

Obviously you can do this more accurately if you're willing to have a computer at the game table, as I mentioned recently in the context of Harpoon, but I find that this distracts me from GMing, so I don't. The US Naval Observatory will generate a sun- or moon-rise and -set table for a year for any location, and if the exact details mattered (e.g. if I were running a campaign involving vampires that was based in a single city) I'd probably print that out. And then lose it among my notes. (I like to have just one sheet of paper with all the notes I need for a session.)

In case you don't want to go to quite that level of detail, here's the trick I use. First, I already have a campaign calendar (useful so that I can tick off days and keep track of the overall date during long investigations and other such operations). I generally use the output of gcal, but any way you can get a simple textual calendar will do; after all, the idea is that it shouldn't take up much room on the note sheet. I might fit two or three months into the same vertical space, thus:

     October                   November                  December
Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
       1  2  3  4  5                      1  2       1  2  3  4  5  6  7
 6  7  8  9 10 11 12       3  4  5  6  7  8  9       8  9 10 11 12 13 14
13 14 15 16 17 18 19      10 11 12 13 14 15 16      15 16 17 18 19 20 21
20 21 22 23 24 25 26      17 18 19 20 21 22 23      22 23 24 25 26 27 28
27 28 29 30 31            24 25 26 27 28 29 30      29 30 31

Then, since gcal doesn't integrate its moon phase calculations, I use it or check a moon phase table (from the US Naval Observatory again) to find out new and full moon dates, and mark them on (* for full moon, X for new):

     October                   November                  December
Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
       1  2  3  4  5                      1  2       1  2  3  4  5  6* 7
 6  7  8* 9 10 11 12       3  4  5  6* 7  8  9       8  9 10 11 12 13 14
13 14 15 16 17 18 19      10 11 12 13 14 15 16      15 16 17 18 19 20 21
20 21 22 23X24 25 26      17 18 19 20 21 22X23      22X23 24 25 26 27 28
27 28 29 30 31            24 25 26 27 28 29 30      29 30 31

From this I can work out an approximate moon phase for any date. 4th of October? Four days before full. 14th of November? Waning half moon. (New moons can be solar eclipses, and full moons can be lunar ones, but I shan't deal with that here.)

Moonrise and moonset approximations come from a mental model that the moon slowly lags further and further behind the sun. At the moment of new moon, when they're in nearly the same position in the sky, they'll rise and set at about the same time, but the moon rises a little under an hour later each day (though this varies quite a bit). At full moon, they'll be pretty much opposite (i.e. there will be full moonlight all night). A waxing half moon has moonrise about noon, and moonset about midnight, meaning that the first half of the night is lit and the second half isn't; a waning half moon is the other way round, with lunar illumination from midnight until dawn.

So when a PCs goes sneaking about on the night of the 19th of November, I can say "that's slightly after the waning half moon, so the moon will rise about 2am". This strikes me as a convenient middle way between simply making stuff up and using a full-on ephemeris.


  1. Posted by Dr Bob at 10:21am on 09 November 2014

    When playing in one of your games I really HOPE what the moon is doing is orbiting the Earth! Otherwise things could get messy. :-)

    Love the Ken Hite quote.

    When I ran Werewolf I had a diary with the phases of the moon in it, that I or the players could consult. Annoyingly, many modern diaries don't have that info in them, though they are delighted to tell me when Children's Day is in Japan, or when Groundhog Day is in North America.

  2. Posted by RogerBW at 10:24am on 09 November 2014

    Hmm. Space: 1999 the RPG. Maybe I should.

  3. Posted by Ashley Pollard at 04:52pm on 09 November 2014

    Among those writers who will talk to me in my writers group, and by that I mean hang and chat about stuff, I've acquired a reputation for being technically savvy, but really this is only down to my research background from being a nurse and having to check that outcomes from trials were worth implementing.

  4. Posted by RogerBW at 05:00pm on 09 November 2014

    This does rather cross over with part of your post today, doesn't it? For RPG purposes the vital thing is that I should be able to answer questions fast - I don't want to break the mood of the game while I'm looking something up - but accurately enough to be consistent, because when you've got a bunch of people (who aren't the primary author) exploring a world and working out what they can and can't do they need consistency in order to play the game at all. I don't mind taking a bit longer to get the absolutely precise answer for PBEM Harpoon, because if my email comes out five minutes later than it might have it's not going to make a difference to the game.

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