Now that we have established design goals for this calendar, it is time to start mapping an outline. Before that, a quick reminder on the design goals:

  1. The calendar must track both moons and the sun in a cycle that closely repeats itself multiple times within a person’s life time.
  2. The calendar must feel plausible, even if it is not physically possible.
  3. The seasons should be long enough that race, in the traditional sense, is tracked more by the seasons than by the local continent or country.
  4. The calendar is consistent and relatively easy to read, despite it’s complexity.

The first thing I would like to do is to define some terms. Since I have yet to name this planet or it’s moons, all of the term names are placeholder.

  • Year – The amount of time it takes for this planet to revolve once around its sun.
  • Season – One quarter of the year, relative to the planet’s angle facing its sun.
  • Lunar Season – 2/3rds of a season, divided to commemorate the larger primary moon of this planet.
  • Dilunar Season – 1/3rd of a season, divided to commemorate the smaller secondary moon of this planet.
  • Lunar Months – Small divisions of time which track the relationship between the larger moon and the planet.
  • Dilunar Months – Smaller divisions of time which track the relationship between the smaller moon and the planet.
  • Earth Year – The time it takes for Earth to revolve around the Sun. ~325.25 days.
  • Earth Month – The approximate time it takes for a lunar rotation. ~30.5 days.
  • Days – 24 hours, similar on Earth and this planet.
  • Week – Divided into 9 days, each to honor a different god – two of which are for the moons.

I will be using donjon: Fantasy Calendar Generator to assist in my creation of this Calendar. While nothing will be randomly generated, it is much nicer to have a visual aid than to do it entirely in excel or on paper.

The first decision in creating this calendar is a rough outline of what I expect from it. I know the seasons need to be long, and subsequently the year must be long. I also know that I would like to alternate months and seasons depending on the moon. In choosing highly composite numbers, I chose 508 and 270 as my inspiration. Both are excellent numbers with lots of easily divided portions. Seeing that 508 was close enough to double 270, I decided that a moon which is roughly 60% the mass of another might appear as half to a developing civilization. So I moved from 508 to 540. Then I decided to separate these numbers into seasons. Each season consists of a Lunar Season (540) and a Dilunar Season (270) for a total of 810 days per season or 3240 days per year. This roughly comes out to 8.87 Earth years to one year of this world. This seemed reasonable. With leap years and variance days, I could bring that up closer to 10 Earth years each thus reducing reader complexity. Just add a zero to any year counts and you’ll be approximately there. It seems reasonable that anyone who lives ten solar cycles in this world would be considered very old and younger people might measure their age in seasons.

With 540 days in the first Lunar Season, I set each month alternating between the Lunar months and the Dilunar months. At 54 days on average, per ten months, with the Dilunar months existing has only half at 27, this left an excess of 135 days to be spread around between these ten months and any potential leap years. After some tweaking, I arrived at a perfect balance of 72/36 days per month from 54/27. But it is too perfect. Since 270 is exactly half of 540, I know that a similar set up can be made in a Dilunar month at 36/18 months – although I now began to question if I needed the symmetry. After all, shouldn’t a Dilunar month place more emphasis on the Dilunar moon?

At this point, my long calendar breaks down into the following pattern.

Spring | 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 | | 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 |
Summer | 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 | | 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 |
Autumn | 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 | | 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 |
Winter | 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 / 72 / 36 | | 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 / 36 / 18 |

While this technically fits my specifications, it is incredibly repetitive and dull. I now know that the lunar patterns can easily fit a 72/36 pattern with them alternating visibility in Dilunar seasons. Since the smaller, more distant moon, is likely to have a longer orbit, suddenly I noticed there is some merit to having month lengths act inversely to season lengths. However it is too early to determine this without considering any cultural influence just yet.

On Earth, the average lunar cycle takes around 29.53 days out of the 30.44 average days of a Gregorian month. This is a margin of error of less than a single day per month. Furthermore, 11 of the 12 months exist within one day range of each other and the 12th is only 2.25 days below the lower range. Additionally, there is also relative consistency in the way that months alternate between 30 and 31 days with only two inconsistencies.

My first instinct at refreshing this calendar was to try something fun by alternating the lunar months by having the moons lerp vertically from hemisphere to hemisphere each between seasons. Although I know this isn’t technically how moons work, I thought it could be a fun twist. However, with the slower movement on the horizontal axis to ensure the lengthier seasons, this would create a severe imbalance between vertical and horizontal rotation. I do not have the physics aptitude to make a decision on this, but I would imagine that this might cause some degree of wobbling on the polar axis of the planet itself. This is a whole bag of unforeseen consequences that I do not want to open – which narrowly toes the line between impossible and implausible for me.

On the other hand, there are more subtle variations of just that idea that might be function. More importantly, they would allow for other patterns to emerge between the months. The base concept of 72 / 36 / 36 / 18 days per month with 10 months per season and each season divided into two uneven parts is the only constant. Everything else can be adjusted. Lunar & Dilunar festivals can cause certain months to be moved nearer to each other. Alternating months might double up and leap years can consist of multiple days if not months as has already been done on Earth. The ideal times for leap year might be tied between seasons or or added intermittently in each of the smaller months. There’s a lot of room for experimentation here. In fact, there are 412.5 days of room between this current calendar and one that might be exactly equal to ten Earth years. That’s an average of over 5 days available to be added to each month in the name of flavor. The next step is finding a reason for each of them.

Creating a Dilunisolar Calendar
Part 1: Design Goals
Part 2: Days, Months, Seasons, & Years
Part 3: Cultural Significance
The Calendar