Moon calendar with a permanent full Moon

The Situation

This can be considered a continuation of the following question:

Permanent Full moon via "Magic"

In this question I asked how a permanent full moon could be possible. While I got my information, I now have another problem, which is literally having no moon phases to indicate the length of a month.

Which brings me to my question. What sort of changes a permanent full moon could present to indicate the beginning of a new month?

Important Information

My world's Time Measurements:

1 Year = 360 days
1 Year = 12 months
1 Month = 30 days


My moon's properties:

Heliocentric Orbit
1 Orbit = 360 days
There is only 1. Moon
Permanently on Earth's night side


My magic system:

Can't remove physical laws
Can unnaturally change properties of objects
Requires magical energy


Other Information:

Focus only on the moon
Sun = Magical Energy source
Earth = Can redirect Sun's Energy


Note: English is not my primary language so please excuse any grammar or spelling errors.

• If there are any questions about my magic system or any other problems just let me know and I'll try to fix them. Mar 28 at 0:13
• In the Roman calendar tradition, which by now covers all the world except those countries which still use the Muslim calendar, the months have had nothing to do with the phases of the Moon since at least 2,500 years ago... (And why would your civilization choose a lunar calendar over a solar calendar when the Moon is always full?) Mar 28 at 0:37
• I'll admit that i did not know about the that. Though a moon based calendar is generally something i would expect of my dominant moon worshiping Religions. Mar 28 at 0:45
• Are you limited to one moon?
– Mark
Mar 28 at 10:40
• Yes @Mark i am a second moon just has a lot of consequences i don't want really to get into. Mar 28 at 11:49

Here's one suggestion: have the moon spin.

If the moon is rotating about any axis, then while it's held stationary landmark craters or divots on the Moon will visibly move around. If you line the Moon up right with the planet and select an adequate rotation axis, you could have striations running the length of the Moon be visible from the surface of the planet: the number of striations visible, or how far they are in one direction, determines which day of the month it is.

As the month progresses, the visible features of the moon will change, and that tells you how far into the month you are. We don't observe this on Earth because our Moon is finally locked to Earth: it's rotation period is exactly equal to its orbital period, so the same side is always facing us. In your case, your moon isn't really "orbiting" so much as being suspended at Lagrange-2 by angels (or something like that, depending on which answer to your last question you like best), so it's allowed to rotate as much as it wants, probably with a period of 30 days given that's how long your month is.

• Yes thank you that is indeed an interesting concept. I could imagine something like a big structure on the moon indicating the end of month or something. Also the moon is magically locked in its position by the sun's energy as the sun is essentially a big ball of magic energy in my world. The moon is also slightly bigger in the sky then in our world. So it functions similar to the solution i marked in my other question. Mar 28 at 1:03
• Awesome! If it's a big ball of energy rather than a celestial, physical object, this also works; have there exist a "Lunar minimum and maximum" like there are solar minima and maxima, so that by observing the brightness and slight color changes in the moon through a telescope over that 30-day period you can also tell what time of the month it is. Mar 28 at 1:19
• i had a similar idea though i am struggling with how this effect could be activated. Mar 28 at 8:20
• Probably something like tides on the surface of the Moon; the Sun is actually a giant swirling mess of tides and currents, and when they interact in particular ways they get more and more chaotic until the magnetic dynamo reverses polarity and we return to solar minimum. No reason something similar couldn't happen to the Moon. Mar 28 at 12:23
• While something like that might be possible for my moon consider that it still has a solid surface. Mar 28 at 12:26

Annual Moon Reckoning

Terrestrial year calendars often make use of the Sun to determine when a year ends and a new one begins. Such as watching for sunrise on the solstice kind of thing. This, of course, will work just fine on your world! But your focus people are Moon worshippers and they don't really care what the Sun gets up to on a warm solstice morning!

The nice thing about having a permanently full Moon that is always fixed in space opposite the Sun is that your people can use the Moon the same way humans used the Sun!

As their Earth orbits the Sun, day by day each moonrise will tick one degree along the sky. Their priest-astrologers will eventually hit on the realisation: "Hey! We've all been watching this big round purple thing in the sky, and like, like, every 360 nights, right, it comes back to this same curious orange and blue hazy bit in the sky! What do you think? Call it a year?"

It won't take long for the priest-mathologers to listen in on the conversation and start seeing what properties 360 has. They'll soon enough realise that 360 wossnames make for a nice circle and that it can be nicely divided into almost two dozen different kinds of subdivisions, making calculation easy! As a byproduct of maths, the concept of the 15 month year will be a natural, as even priest-astrologers like to take a month's vacation down on the coast!

As we can see, lunar phases are not required for deriving months; and your people's lunar calendar will be the foundation of the year!

• Sounds good though what could make them think that they want to divide the year into 12 separate pieces? Mar 28 at 8:24
• Without the Moon moving at all, months can be a cultural convention - not a natural observation. Do you want 10 months with 36 days? 15 months with 24 days each? Each kingdom its own calendar? Why not, there is nothing saying they have to be twelve. Mar 28 at 14:24
• I would prefer it to be linked to the way the moon acts @Martin Grey. A month should have a significant reason to exist for mt moon worshipers. Mar 28 at 19:46
• @Fallenspacerock, develop a sky with twelve equatorial constellations, and basic astrology says that a "month" is the period when the Moon is hanging out in one of those constellations.
– Mark
Mar 29 at 0:02
• @Fallenspacerock --- For cultural reasons, you might look into ancient festivals or earlier gods that have been assumed or accreted into the current Moon god. Months could be as pragmatic as the ritual corn / rice / grain distribution from state-temple granaries. Mar 31 at 5:26

Somewhere there's a world named Firth that has two moons, called Makoon and Maloon, with a beautiful calendar that was originally inspired by the interplay of the phases of these two moons. On that world there's an aspiring writer named Meteorite who has developed an idea for a fictitious world named Earth that mysteriously has only one moon.

Meteorite has just asked the stackexchange worldbuilders of Firth how to make Earth's calendar have proper makonths and malonths when there is only one moon that people on Earth could observe.

I'd give the same answer to Meteorite that I give to you: people will parcel out time into whatever units make sense to them based on a combination of natural phenomena and convenience. There's no astronomical observation that gave us the week. We divide time into weeks anyway.

If the population of your world decide that a 360-day year needs to be broken up into smaller intervals, they'll do so. If 30 days is what they think is a good unit of time, that will be their month (or whatever they choose to call it).

It doesn't need to be based on astronomy.

One advantage to not having obvious phases of the moon to base a month on is that if you decide you need to break up the year into something like a series of months, you're much less likely to develop calendars where the calendar year contains a variable number of months (like the Hebrew calendar) or is way out of synch with the seasons of the solar year (like the Islamic calendar). If you want to write dates without having to count days literally from the start of each year (day 253 of the year 3025), you'll just divide the year into whatever subdivisions you find convenient and use those in your numbering. If you let the Babylonians do this, and they observed a 360-day year, I think you might just end up with six 60-day months, since they liked numbering things that way (which is often cited as part of the chain of events that led to the 60-minute hour).

• You make some good points though i should clarify that there is only 1 moon on my world. Mar 28 at 17:02
• That being said, it would be much easier to use a zodiacal calendar if you can just look at the night sky any time you want and the moon is always in the constellation appropriate to the day of the year, instead of having to keep time and measure angles or check right after sunset.
– g s
Mar 28 at 18:26
• The point isn't that your fictional world has more moons than ours; the point is that you see the second moon as irrelevant to the question, because having two astronomical sources of a month is outside your experience. The people on your world would likewise see our phases of the moon as irrelevant, because that's completely outside their experience; why would there be an astronomical reason for any subdivision of time longer than a day and shorter than a year? It's like asking how they celebrate Christmas in Tehran. Mar 28 at 18:37
• @gs Good point, that's something we don't have and that could be just as obvious to common people on that planet as the phases of the moon are to us. That would be a great element to incorporate in this worldbuilding exercise. Mar 28 at 18:39
• Riffing on the zodiacal calendar, suppose the ancient people on this world picked out the ten brightest stars along the path of the Moon through the background of stars and decided that each month started when the Moon was in conjunction with one of those stars. You could end up with ten months of very unequal lengths inherited by those people's descendants thousands of years later, all arising from very simple and obvious observations of nature. Mar 28 at 18:55

Executive summary: Use the motion of the moon against the stars, with a healthy dose of divine intervention to make everything come out nice and neat.

So you have, first of all, angels who keep the moon in its unnatural orbit around the sun, always nearly (but not quite) in the shadow of the planet.

The same angels could also be responsible for the fact that this planet's year is 360 days long, not 365.2425-something days (and changing) like ours. And not just close to 360 days, like 360.15 days, but exactly 360 days on average, due to imperceptible tweaks they perform on the planet's rate of rotation to maintain that exact number of days per year.

The angels also put some rather large, luminous stars in space a few light-years from this planet, on or very near the plane of this planet's rotation around its sun, spaced at exact 30-degree angles as viewed from the planet's sun. These stars are so close and so luminous that they are the brightest things in the sky after the sun and the moon.

Every 30 days, like clockwork (did I say "like" clockwork? it is clockwork!) the moon comes into conjunction with one of those twelve very bright stars. And the people on the planet have decided that this event is noteworthy enough to call it the start of a new "month".

I like divine intervention in this story because without some kind of deliberate design by sentient beings, the 360-day year is already just too convenient for my suspension of disbelief. (They don't have to be angels; a Ringworld Engineers scenario would work too. But you get to pick your designer.)

Update: I see that the angels were in an answer to the earlier question, not something you planned. I suppose a sentient moon is not going to place stars light-years away.

The people on your planet can still identify stars spaced at whatever intervals you want around the sky in order to divide the year into months as the moon comes into conjunction with each star. But without some kind of supernatural design of the constellations, we would expect a relatively random distribution of bright and dim stars, so the people would have to decide how long they want their months to be and then choose the stars that will mark the start of each month.

You might want the planet to be sentient too in order to self-regulate its spin for that perfect 360-day year.

• Definitely an idea @David K though i don't really have litteral angels in my world it's more like the moon is kept magically in place by its own sentience. The angels would probably just exist in my moon cults mythologie. So while the idea of the 12 stars is pretty neat itvl would generally be highly impossible to place them in such a convenient way. But the idea itself is great just have you an idea how these way points more subtle. Or any other signs that the moon could give of. Don't get me wrong there is still a lot of magic involved just not that much. Mar 28 at 21:48