# What would a lunar calendar look like?

First see this great answer on considerations when making a calendar.

Consider a human colony on our own moon. Let's assume that the colony is influenced by the Gregorian calendar. Let's suppose the colony has no reason to keep that calendar so has decided to, or evolved into, a different calendar that means more to what life would be like on the moon. I'm mostly interested in what would mark "days", "weeks", "months" and "years", if they would have a need for any. I'm not really interested in epochs, associated mythology, etc.

I'm not sure if things are significantly different on the Moon such that different hemispheres would warrant different calendars, so I've asked What is the best (NASA preferred) location for a lunar colony? on Space Exploration.

Designing a Lunar calendar from scratch:

The most significant cyclical change on the Moon is the light/dark cycle. Any surface activities need to be scheduled around this, either during the light period (eg. construction benefits from being able to see what you're doing), or during the dark period (astronomy benefits from not having to block out the Sun). The primary divisions of the calendar would be the "month" of 2,551,443 Earth seconds (a full light/dark cycle), and the "half-month" (light or dark) of 1,275,721.5 Earth seconds.

The other cyclical change is the human sleep cycle, which is hardwired to be close to 24 hours (most people can adapt to a day of 22 to 26 hours). Dividing the "month" into an even number of days permits 28, 30, or 32 days, of which a 30-day month gives the closest to a 24-hour day (85,048.1 Earth seconds).

The year isn't a significant unit of time. Unlike on Earth, there is no cycle of seasons to tie the measurement of time to. A multiple of the "month" may exist as a unit of timekeeping, but there won't be any great drive to keep it in sync with the movement of the stars (so, no leap days).

A "half-month" of 15 days strongly suggests a 5-day subdivision, but, like the year, this is a convenience measure rather than a natural cycle. For daytime surface work, "first week" and "third week" have better lighting conditions than "second week" (when the Sun is directly overhead, leading to reduced surface constrast).

Now, the Earth second is rather strongly tied into all sorts of things, and people tend to like their timekeeping to involve integer multiples. An 85,048-second day ties nicely into the light-based month (requiring the addition of 3 "leap seconds" to keep it in sync), but it doesn't have much in the way of prime factors: you can divide it into eight 10,631-second hours, but that's it.

A better value would be an 85,000-second day: with an abundance of 2s and 5s in the prime factorization (plus a pesky 17), it could be divided into 25 hours of 50 minutes of 68 seconds, or any of a number of other ways. This in turn becomes a half-month of 1,275,000 seconds and a month of 2,550,000, requiring the addition of 1443 seconds somewhere to keep the light-based month and the timekeeping month in sync.

• One factor that encourages "years" is the human life span. Adding months for age is a little much. I think the parts about the seconds and hours is a bit too complicated to evolve naturally and be constructed intentionally. That also seems more related to time keeping rather than day keeping, if that makes sense. +1, best answer so far. – fredsbend Jan 17 '15 at 4:47
• If the colony decides to import religion, then the 5-day week may become an issue. – Greg Wochlik Jan 17 '15 at 10:38
• @GregWochlik, if a seven-day week is important, then the colony can go with a 28-day month with days 91122.96 seconds long. – Mark Jan 17 '15 at 11:48

My guess is that, since they would live in a closed environment, they would not have a lot of incentive to change from the Earth calendar (no need to remember that the weed fields must be seeded in February).

Their only "variation" would be "day and night" (with a day and night of 14 Earth days, but that would only be noticeable if they had glass coupulas, or for the people working outside.

OTOH, keeping the Earth calendar would ease communications with Earth. And, to add to that, the Moon does indeed circle the Sun in about 365.25 days $^1$

So, I think they would not have the incentives to change the calendar. If any at all, they would add additional info to it to mark when Sun goes up and down (in a manner similar that many current calendars show the phases of the Moon).

If you want the Moon to set its own calendar, a possible reason would be a political agenda (to emphasize difference with the Earth government and population). In that case, the natural measure would be "months" (28 days, between to sunsets) and/or "half-months" (with "dark half-months" with no sunlight, and "bright half-months" with sunlight).

$^1$ Although, given that the Moon is not exactly at the same point of its orbit around the Earth each end of year, getting the "full circle" around the Sun may differ each year by a few days. But taking that into account would add a lot of complexity that, as expressed before, is useless.

Our calendar is grouped by 1) days 2) market intervals and 3) months , which are the times between full moons (increasing roughly) and 4) solar years.

Humans are too tied to day/night to abandon #1, and #2 and #4 isn't changed. The only cycle that isn't represented that might be important is the lunar day. This is basically the month again.

If you want to lock in on lunar days/month precisely, you would need to abandon the year itself, as there are 12.3 lunar months in a year.

It probably would be more rational to just keep the current calendar and shade the days to show when the site is in daylight or night.

I like the 30-day month with 5-day weeks. However, certain problems would have to be worked out, in addition to the leap-seconds:

The work week would have to be adjusted. One way would be to have 4 days on, 1 day off. This means working 80% of the days, as compared to ~71.43% for a "5 of 7" schedule. So it might be fairer to alternate between "4 on 1 off" and "3 on 2 off". In any given month, you would then work 21 out of 30 days, which is 70%. Depending on when you start that cycle, you would get 4 days off during lunar daylight and 5 during lunar dark, or vice versa. Workers would favor more days off during the daytime, so would want all the "3 on 2 off" weeks scheduled for daylight. Management would want fewer days off in daylight so as to maximize productivity (they would really prefer a 15 on 15 off schedule, with longer shifts (shifts would have to be 40% longer for 15 shifts to amount to 21 days of "regular" shifts.)

Any way that the above is worked out, the calendar still puts businesses and workers on one side of the moon out of synch with those on the other side.

In fact, no matter how you divide a 30-day month into weeks, there would be 30 "international date lines" to contend with; on any given day, it would be 30 different days on various parts of the moon. As the circumference is just under 11,000 km, traveling 375 km east or west at the equator puts you in a different day (and of course much less than 375km near the poles). This could be particularly awkward for traveling salesmen, for example: "I'll see you next Friday." "You mean last Wednesday?" "Yeah." "Sorry, I'm off then".