Imagine a world where the seasons are long and harsh. Where the two moons wax and wane as they tell the story of two lovers dancing across the sky.

This is the world which I am creating for use in all of my games. One steeped in medieval fantasy and where magic integrated into all ecosystems as a constant of reality. What kind of calendar would the cultures in this world have? Perhaps one that tracks lunar cycles as well as solar. A lunisolar calendar, as people all around Earth have created. Except here there are two moons and magic is in abundance.

To create this dilunisolar calendar, I started with my 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 design goal is purely technical and fortunately, there are already tools to assist with this. From my end, this was a matter of choosing a number that divides nicely into many other smaller numbers. Highly composite numbers are often used in calendars with some variance. Our own Gregorian Calendar features just over 360 days which divide nicely into several factors. Two moons also have significant impact on the the geography of this world. Increased tectonic activity and a fierce seas are the first among many potential changes to the world.

The second goal is much more complex. Using Universe Sandbox and Artifexian’s calendar spreadsheet (download link), I was able to create a physically possible calendar which synchronized two moons and a solar rotation in a longer than average year cycle. The two lunar bodies were physically capable of following a nearly consistent pattern functionally indefinitely with one moon at 110% of our Lunar mass and the other at a much further orbit and only about 63% of our Lunar mass. Unfortunately, getting these numbers to coordinate meant that one dilunisolar cycle would take hundreds of thousands of years before repeating even once. This breaking the first rule, and with the abundance of magic in this world, I had decided to move from possible to plausible. After all, in magical worlds, gods are explicitly real and these two moons are the known embodiment of two major gods. If the world needs changing to enhance the fantasy, it can be done so long as it is not beyond the realm of the mind’s immersion.

The third goal is a personal preference. While humans do exist on this world, as do many other fantasy races, they are attributed small racial alterations based on the season of their gestation. For this to occur, the seasons must be long and consistent enough for the bodies to adapt to this necessity. Anthropologists might differentiate racial differences by the season of one’s birth, rather than continent of their ancestry. You might meet a Summer elf and a Autumn dwarf rather than an Asian man or African woman. And similar to Game of Thrones, they’ll be saying Winter is Coming for a number of years on Earth before Winter finally arrives.

The fourth goal is all about consistency and clarity. While a dilunisolar calendar is inherently more complex than other types of calendars, it does not need to be difficult to read as other artificial constructs often are. David J. Peterson, developer of famous conlanguages like Dothraki (Game of Thrones HBO series), Trigedasleng (The 100 TV series), and Orcish (Warcraft movie), talks in his book, The Art of Language Invention, about not forcing the reader to have to do math to recognize variables of time. If your world only has 23 hours in a day’s rotation, rather than cut 5 minutes from each hour and have 55 minute-long hours, it is better to write as if an hour is comparable to that of Earth and to know as an author that it isn’t. Unless the time difference is essential to the plot, it is easier to hold immersion if the reader isn’t having to calculate difference. And while not all readers might calculate these differences, enough will do so put your book or game down. I know I certainly would. Furthermore, small changes to gravity scale, atmospheric composition, and so on are just difficult to imagine. It might make sense for Nal Hutta to have a noxious surface due to pollution or for the Moon to have low enough gravity for extended hops but anything less extreme is unfamiliar. Then I would be doing a lot of telling and very little showing. For these reasons, it is safe to assume that this world shares similar qualities as Earth. It is within the Goldilocks Zone of its sun where water can be found in liquid form on a terrestrial planet. The gravity is similar and life generally follows similar patterns. I might pull inspiration from Ediacaran life or from magic, but the core physics of this world match that of our own.

 

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