We divide the year into months of ~30 days because that’s roughly the period of the moon, adapted to fit a solar year.

However, would a civilization living on a world with no moon but a year of ~365 days with seasons and a day/night cycle still come up with a ~30 day month?

I understand that there are benefits to the creation of months, even in non lunar/lunisolar calendar such as in the Gregorian/Julian calendars. They allow for the division of the year in everyday usage (it’s easier to say June 23, 2020 than 175th day of 2020) and schools/businesses can divide the year into quarters. However, these were created with the Moon’s period in mind as a modification of previous lunar/lunisolar calendars.

So would a civilization living without a Moon still come up with months? Would they still be ~30 days or would they be based off of something else? I was thinking about maybe using solar constellations or other stars to divide the year, but they’re not as noticeable as the moon. So would they?

To confirm: these people have 10 fingers and ten toes and are humanoid

  • 1
    $\begingroup$ We came with a 7 day week that has ho relation to any astronomical event. I am pretty sure that even without moon there will be 1 or 2 month/quarter-like cycles for pacing work and bureaucracy. $\endgroup$
    – Vashu
    May 16, 2020 at 2:34
  • $\begingroup$ @Vashu, we have 7 days because of the 7 "wandering stars" in our sky: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. $\endgroup$
    – L.Dutch
    May 16, 2020 at 3:08
  • $\begingroup$ Yep. I knew that. $\endgroup$
    – Galactic
    May 16, 2020 at 3:11
  • $\begingroup$ @L.Dutch - Reinstate Monica: Support for that? Or indeed, for the fact that the ancients recognized Venus & Mercury as being the same when they were visible in the morning or evening. $\endgroup$
    – jamesqf
    May 16, 2020 at 4:22
  • $\begingroup$ @L.Dutch-Reinstate Monica So? If we had 200 wandering stars then you are sure we would have 200 day week? Or maybe somebody would just look at N(5<N<10) mountains/great kings/world wonders and set it to something more managable? $\endgroup$
    – Vashu
    May 17, 2020 at 6:24

6 Answers 6


You mentioned seasons, so it is most likely that your planet has an axial tilt. That means the lengths of daylight would change throughout the year. This makes four points in the calendar - the equinoxes. I suspect these would become a natural division of the year into quarters.

So instead of 12 months, the year would be divided up into four quarters, and they would be named much like our seasons - winter, spring, summer, fall, based on the typical weather patterns and temperature of each. Or they could be named based on the lengthening and shortening days - daylight increasing, daylight maximum, daylight decreasing, daylight minimum.

That gives 91 days per quarter, with one day left over. Universal holiday for New Years' day turnover, maybe?

I also suspect that, humans being humans, and our bodies tending to a seven-day weekly work cycle naturally, there would be weeks of, say 9 days, 10 weeks, and again one day for holidays each quarter (Hoorah!!! Everybody loves holiday celebrations.)

Like Australia, I would suspect periodic payments (rent, utilities, mortgage) would be divided by these 'weeks', and not by the quarters. (In Australia, rent is paid by the week, so as to not have one rent payment for 29 days, another covering a 31 day period - simplifies things like interest, for instance).

  • $\begingroup$ "our bodies tending to a seven-day weekly work cycle naturally" - source for that? I always thought the 7-day week was a pretty arbitrary calendrical invention, unlike the 30-day month and 365-day year. $\endgroup$ May 16, 2020 at 21:20
  • $\begingroup$ @ Rand al'Thor 'The earth was created in seven days'. Okay, maybe it wasn't, but the idea of the seven-day week, with one day off, goes back at least four thousand years ago. Even the Mayan calendar has a week of 13 days. The Hindi religion, perhaps the oldest in the world, pre-Jewish, has a seven day week. Why seven days? Because it is the average immune cycle in humans. en.wikipedia.org/wiki/Immune_cycle 'Day off' is the down time of the cycle. Every person's cycle would not coincide, with a different 'day off', but eventually it became codified and standardized into the calendar. $\endgroup$ May 17, 2020 at 1:02
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    $\begingroup$ I doubt ancients noticed the immune cycle. I mean, it was subtle enough that doctors noticed it only in 19th century. There were quite a few 8-day, 9-day and 10-day weeks. Mayans and Aztecs had 13-day weeks, as you noted. And for all of the ancient 7-day weeks where the reason for 7 is stated in historical sources, that reason is being quarter of a moon cycle. It's just convenient to have a smaller unit of time than a lunar month. Where the "month" itself was e.g. 20 days, there were no weeks. $\endgroup$
    – Alice
    May 17, 2020 at 5:48
  • $\begingroup$ @Alice But they would NOT have missed the 'down time' cycle, the fact that at the low part of the immune cycle, they would not have a lot of energy, they would feel listless, they would 'want to take a day off'. It is not the 'week' per say, it is the tendency to universally want to rest every seven to 14 days. That is universal. Every earth culture has imposed a period of rest, lest the body just 'burn out', even if it is just for half a day every seven days or so. Any employer who pushes employees to work every day continuously without a break has discovered the loss of productivity. $\endgroup$ May 17, 2020 at 14:15
  • $\begingroup$ @Alice 'It's just convenient to have a smaller unit of time than a lunar month.' No, it's not. The fact that it is universal suggests that it is something beyond 'just convenience'. Humans simply have a very difficult time working non-stop full-out for 28 days. It is not 'convenience' that they take a break, it is necessity - the immune cycle. Just because we have only identified the reason in just the last century or so, does not mean the effect and result was always there. $\endgroup$ May 17, 2020 at 14:23

Probably not exactly 30 days, but there's another constant that might help you out here. The number 10, which is a staple throughout cultures and periods. The reason for this, from my understanding, is because we have 10 fingers.

For the uneducated, which meant the majority of the population for the majority of human history, the limits to which they can count is given by their digits, plus two for each hand or foot or whatever they used for the last two. This is why we write numbers in the decimal system today. This is (probably) why numbers, at least in English, have individual names up to twelve, then follow distinct mathematical patterns. Going further, a common unit was "the dozen" because that's what a normal person could verify. Then you could have "two dozen" because you were able to verify one, then the next and so on. Apparently, even the reason we have 24 hour days is somewhat related to the number 10. https://www.abc.net.au/science/articles/2011/11/15/3364432.htm

So, looping back to the concept of months, your civilization could just decide to divide the year by 10, leaving you with 5 36-day months and 5 37-day months. Or, you go back to the number 12, which is conveniently how many months a normal year has, and divide it thereby, because everyone can count to twelve.

  • $\begingroup$ 10 is "special" only because we have 10 fingers. Pretty anthropocentric $\endgroup$
    – L.Dutch
    May 16, 2020 at 6:04
  • $\begingroup$ Indeed, as I wrote. I assumed the people in question are 10-fingered. $\endgroup$
    – Mookuh
    May 16, 2020 at 7:20

The short answer: it depends. The long answer: Our moon has a very intresting rotation and syncronization with the earth and the moon.Our moon just happens to have some very intresting connections to 12(that may have something to do with our existence as a whole), too many that i would need to go into enough detail that i would be bored. Basically, if there is no moon, there is little to no rotation of the planet in question, so all potential systems of time measurement would have to be based upon the star it orbits. From there, it all just depends on how close the planet is to the star, how much it rotates and tilts(seasons would be very complex), the movement of bodies in the system, and so many things that it would be impractical to talk about in one post. Basically, their is little to no chance that the system of time would be close to or even resemble our system of time-days, months, and years would have to be changed and suited to the planet. Note: a intresting question to ask would be would there be eclipses?


it actually was the Romans who invented the concept of month. (two of the twelve months are directly named after Roman emperors: august and July. Julius and Augustus Ceaser)

so most planets would have no concept of months, and from what we know of exoplanets, the length of a world's year varies dramatically, as does the day length, which in some case is the year.

so in most cases the concept of months and weeks would all likelihood only exist here. Of course they would have their own measure of time, but i can guarantee it'll be nothing like what is seen here.

like Trappist 1e, that planet has a year length of six days, and would never have seasons.

but I'm playing a very big guessing game, and we humans have the bad habit of applying our own qualities upon something that has none of them, nor would care. We see this a lot in movies, which is why I don't watch them. most movies, albeit watchable, tends to be very misleading in a big way to those watching them.

  • $\begingroup$ The Romans did "invent" the concept of a month, but they were definitely not the only ones. At least he Greeks and the Hebrews also had months. True, they were not (and are not) the same as the Roman months, but they are indeed divisions of the year, which comprises 12 of them. (Fun factoid: July used to be named Quinctilis, i.e., Fifth, and August used to be named Sextilis, Sixth. Quinctilis was renamed July in honor of C. Julius Caesar. Later, emperor Octavianus Augustus had Sextilis renamed after him; and, in order to make it of equal length with July, a day was taken from February.) $\endgroup$
    – AlexP
    May 16, 2020 at 7:49
  • $\begingroup$ interesting. very good comment $\endgroup$ May 16, 2020 at 17:51
  • $\begingroup$ Welllll .... the Romans invented the names of the months that we use today, but they were certainly not the first people to notice that the moon went through cycles or to use these cycles in their calendars. The Hebrew, Greek, and Babylonian calendars all have months and predate the Roman calendar. Probably many others that I don't know about. $\endgroup$
    – Jay
    May 16, 2020 at 18:58

They certainly would not invent the "month" per se, as it would be a meaningless concept to them. But that doesn't mean that they couldn't invent any unit of time shorter than a year. As @JustinthymeTheSecond points out, another obvious unit of time for them would be the season. And they might well invent some other arbitrary but convenient unit of time. There's nothing particularly natural and obvious about dividing the day into 24 hours. But people nevertheless invented the hour because day was too long a period for many purposes. Likewise there's nothing in nature that corresponds to a decade or a century, but people invented those units because counting in just years got awkward. Etc.


At minimum a division of the calendar at the soltices and equinoxes would seem inevitable for their utility, whether those count as 'months' though is debatable. But some cultures divide the year into five or six seasons, incorporating monsoon cycles and other phenomena, so even what constitutes a season is somewhat subjective.

It also ought to be mentioned that the 12 month calendar's connection to the moon is somewhat tenuous. The moon does 13, not 12, complete orbits every 364 days, only aligning with the solar calendar once every 33 years.

As with our calendars, your moonless Earth would likely subdivide the year for practical and cultural purposes, and like us the calendar would evolve to incorporate changes for political reasons. The rationale for those divisions and the final shape they take would be as arbitrary as our own.


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