So, this world has three moons. Blue, Red, Yellow.

There are eight months in the year. Each month has a unique "full moon" combination resulting in a different colored sky. That is the only requirement.

In no particular order, they are:

  • Blue
  • Red
  • Yellow
  • Green (Blue and Yellow together)
  • Purple (Blue and Red together)
  • Orange (Red and Yellow together)
  • White (All three together)
  • Black (All new moons)

I came across this amazing calendar tool which makes it easy to try various setups.

I had been trying to use three tenday weeks, so a month is 30 days and a year is 240 days, but I haven't been able to figure out right cycle for the moons. You're able to specific the length of each moons cycle, as well as give it an offset.

My world is based on eight elements, each tied to one of the colors, so it's important to keep the calendar to eight months, but how the months are structured doesn't matter to me at all, nor does how long the full moons last. It also doesn't matter which order they happen in.

I've included the "restore code" needed to generate my calendar so far, if you'd like to take a shot at using the tool. Just paste it into the box on the Save/Restore tab! You can ignore the month names, I'll move those around based on the resulting colors :)

Thank you for your time!

{"year_len":240,"events":1,"n_months":8,"months":["Shashti (Earth)","Hansa (Air)","Docar (Fire)","Okraz (Water)","Mitne (Light)","Whedabra (Darkness)","Cygat (Chaos)","Gavir (Order)"],"month_len":{"Shashti (Earth)":30,"Hansa (Air)":30,"Docar (Fire)":30,"Okraz (Water)":30,"Mitne (Light)":30,"Whedabra (Darkness)":30,"Cygat (Chaos)":30,"Gavir (Order)":30},"week_len":10,"weekdays":["Dask (Mother)","Deevdru (Son)","Yavsis (Aunt)","Autgabin (Grandson)","Nafldask (Grandmother)","Naflopsola (Grandfather)","Autnaril (Granddaughter)","Iosta (Uncle)","Hianag (Daughter)","Opsola (Father)"],"n_moons":3,"moons":["Blue","Yellow","Red"],"lunar_cyc":{"Blue":60,"Yellow":60,"Red":60},"lunar_shf":{"Blue":10,"Yellow":30,"Red":50},"year":1476,"first_day":0,"notes":{}}

2 Answers 2


This looks like a case for Grey Codes!

Grey Codes were always one of my favourite parts of computer science. In short, it's a way of ordering binary values so that every step changes only one bit/digit at a time, but it loops smoothly through all possible values. In this instance, that's exactly what we want: we can represent each month as either being "lit" by a moon or not. To demonstrate:

One moon has a cycle of 240 days, one moon has a cycle of 120, and one moon has a cycle of 60. This results in the following:

Blue Red Yellow Outcome
1 X X X Black
2 O X X Blue
3 O O X Purple
4 X O X Red
5 X O O Orange
6 O O O White
7 O X O Green
8 X X O Yellow

Now, there are a number of ways you can tweak this. For instance:

  • What determines if a moon is "lit"? It might be anything past 50% brightness, or it might be while it's waxing (or waning). Offsetting the cycles should fix this, depending on what you want. Just set the "start" of the cycle to be either the day of/after the new moon, or the day of waning/waxing gibbous (depending on preference).
  • Are you using RGB rules, pigment rules, or a mix? This is up to you, but it seems like you're using additive light rules, with all three combining to white and using RGB. In that case, you'd have a different combination set (RG= yellow, RB = pink/magenta, GB = cyan). But that's entirely unrelated to the question, and might screw with the rest of your building.
  • What order do you want? I went with the aforementioned, but there are two cyclical grey code patterns (note White Month is on either the 4th or 6th):
Op. 1 Op. 2
000 000
100 100
110 110
010 111
011 101
111 001
101 011
001 010
  • $\begingroup$ Okay, this is interesting, thank you. I've tried some variations with this. and I can't quite seem to duplicate the chart lol. Assuming we consider a full or waxing/waning gibbous moon as lit sort of gives me enough of an color variation that I can just handwave the stuff for my games, Ideally I'd like them to only be considered "lit" when properly full. I realize that's a much more difficult ask, and am wondering if maybe I should have asked this question under a more astronomical/math based exchange? I really appreciate this chart however, thank you. I'm gonna keep trying :) $\endgroup$
    – Orion
    Jan 21, 2023 at 0:01
  • $\begingroup$ So, after crunching the numbers as hard as I can, I'm fairly certain what I'm looking for isn't actually possible, which means I have to do some handwaving to make it work. I'm okay with that, I just wanted to try and make things as accurate as possible lol. Thanks for the charts! Oh, I'm also going with Yellow, Magenta, and Cyan as the colors. Thanks for pointing that out too! $\endgroup$
    – Orion
    Jan 23, 2023 at 11:55
  • 1
    $\begingroup$ @Orion Glad to help! Ultimately, if you're trying to use only phases, you can't really get away with it since there'd necessarily be a range of light. That said, the sky/moon shifting hues through the year sounds cool. Alternatively, you could have the moons rise and fall periodically. Make them always "full" (or at least having their effect) but in an orbit. The above chart would still work, just represent if they're "in the sky" or not. That would give you a distinct binary (and could make the ~day it takes for them to go from there to gone and vice versa it a cool, significant event). $\endgroup$
    – Aos Sidhe
    Jan 23, 2023 at 16:41

Your moons are going to change phase all together.

Sorry. That is how phases work. Think about why our moon has phases. It is because of the direction the sunlight is coming from. From the perspective of your moons, the sun is coming from the same direction for all of them so all will be synchronized as regards phase.

Examples: Phobos and Deimos .

p and d


Very cool image of the Earth and the moon as seen from Galileo spacecraft heading out.

earth and moon


These bodies which are close to each other are in the same phase because the sun is coming from the same angle for both. You will need to dream up a different method to have your moons looking different. Maybe that can be a new question?

More moons! Here are Io and Europa with Jupiter. They are in the same phase.

io and europa


  • 1
    $\begingroup$ Depending on the distance of the moons from the planet, they'll have different orbit lengths. Just imagine it's midnight, and you have a moon about to set, one just rising, and one on the other side of the planet. While they're all lit from the same side, their distance and angle you view them from changes how full they look. science.howstuffworks.com/moon4.htm Check the image here and just imagine two extra moons, further away and different sizes, and put them in different orbits and I think you'll see what I mean. $\endgroup$
    – Orion
    Jan 22, 2023 at 15:11
  • $\begingroup$ Hm. Maybe you are right. If I am right it seems light moon phase should be a function of the planetary year. We will see what the other folks say when they wake up. $\endgroup$
    – Willk
    Jan 22, 2023 at 15:41

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