How might Martian colonists arrange their calendar and keep time?

Let’s assume in this thread that one day humans will settle on Mars. What should their calendar look like and how might they measure the time of day?

Mars has a 687 Earth day year with seasons that are much more variable than earth. A Martian day lasts around 24 hours and (roughly) 39 minutes.

How to measure the day is most interesting to me. Do you extend the definition of a second or do you just end the day at 39 minutes past midnight? Both Philip K. Dick and Kim Stanley Robinson suggested a 39 minute length of time at the end of each day where the clock would effectively stop and would act like a kind of witching hour.

There is a good wiki page which details various suggestions:

Although Mars has seasonal variations of climate they really aren’t much like Earth, nor would anyone be venturing unprotected onto the surface to experience Martian weather. As for time, extending the day by adding a fraction of an hour to correspond to the rotation of the planet while keeping the current divisions of time is trivial with computerized timekeeping (and indeed, entirely workable even with a purely mechanical timepiece) although the effect that would have upon circadian rhythms is unknown, and since no one will be living on the surface or in large transparent domes due to the need for shielding against high energy cosmic radiation that is not blocked by the tenuous atmosphere, hypothetical Martian colonists could just maintain a 24 hour cycle that does not correspond to day and night, as people would do on Earth’s Moon or any other body with a radically different rotation rate. Permanent inhabitants of space are far more likely to inhabit solar-orbiting habitats where Earth-like conditions could be simulated rather than efforts to somehow transform and maintain the surface of Mars to some kind habitable state, and the terrestrial day-night cycle could be simulated with any desired variation.

Altering the length of the second, which is a fundamental unit in the SI ‘Metric’ system of units, would have wide-ranging impacts on virtually every measurement and calculation (except for other fundamental measurements such as distance, mass, charge, and absolute temperature, and potentially even those depending on method of measurement) and would require constant conversions to communicate or interface with any terrestrial system. Since the terrestrial day varies and is also not exactly 86,400 seconds, making such adjustments is already an accepted method of dealing with variation.


Once everything is happening underground or inside enclosed machines and under artificial light, simply standardize that Earthbound UTC is the one and only “local” time everywhere on Mars.

if somebody on Earth or Mars needs to know the Martian season or Martian day/night status at any particular UTC moment at any particular spot on Mars, their local computer can perform their conversion. Just like the app on the front screen of my Android knows sunrise, sunset, day duration, solar altitude, etc., for wherever and whenever I happen to be. Or anywhere / anywhen else I’m interested in.

Now once Mars is fully terraformed so everyone lives on the surface, breathes the atmosphere, and grows crops while Earth has become an uninhabitable hellscape, they can re-create a calendar & time system that best matches their agrarian life on their planet. But they have plenty of time forward from now to figure that all out.

As mentioned, people will be living underground, and not much beholden to the day/night cycles of the surface.

That said, there will likely be some activities that do depend on the surface conditions. Whether it be exploration or resource exploitation, having the sun up may make it easier.

I’m not sure how agriculture would work, but plants can probably deal with more radiation than humans, and you would likely have some sort of greenhouses on the surface, so that would be another thing that would require people to take surface conditions into account.

Right now, people who work on Mars missions often keep a schedule that lines up with the Martian day, as certain things can only be done at certain times. If we had a community on Mars, I don’t know if it would be better to keep people working in such areas to keep a modified schedule, or to simply have people working around the clock, with the time for each activity rotating around.

If we have stuff going on all around the planet, then it’s always noon somewhere.

Because of the distance from the Sun, the surface of Mars gets less than half of the solar insolation but much higher ultraviolet content because there it has no ozone layer in the atmosphere. Mars also has a wide degree of eccentricity in its orbit (e=0.0934) which means there is considerable variation in the intensity of sunlight notwithstanding the global dust storms that can obscure sunlight for many weeks at a time, making any dependence upon sunlight for power or agriculture problematic to say the least. Any agriculture on Mars would very likely need to be underground where it will be accessible and protected, using biological nutrients brought from Earth and using artificial light.


While I agree with Stranger that, absent a revolution in the science of terraforming, there is unlikely to be a Mars colony anytime soon, I think that there will eventually be a scientific outpost on Mars, similar to the outposts we have in Antarctica. How those people handle the Martian “day” will be an interesting question.

The necessarily underground, shielded, nature of any Martian outpost should lead to changes in circadian rhythms similar to the changes found in cave and other isolation experiments. From here:

A significant insight from Siffre’s work was that the circadian cycle was longer than 24-hours. In subsequent experiments, Siffre observed that extended periods in temporal isolation led to a significant lengthening of the sleep-wake cycle. His research showed that a 48-hour cycle was common for cave dwellers, consisting of thirty-six hours of continuous activity and twelve to fourteen hours of sleep.

This may be especially relevant if the majority of martian surface activity is through telerobotics rather than actual human activity on the surface, providing minimal exposure to the martian day-night cycle.

As far as growing tomatoes on Mars, I cannot really add to what has been said, but as for actual Mars missions (and the human scientists running them), the clock is kept using local mean solar time; see the NASA FAQ
A couple did use true solar time, though.

Note that the mission clock display refers to “sols”, which stands for Martian solar days.

ETA the people working on these missions already have to handle Martian “days”, so you could ask them about the effects… I assume the control room has unlimited coffee.

Even at half insolation, even with dust storms, you are still going to be get quite a bit of free lighting from the sun. It takes a whole lot of power to get 500 watts per square meter lighting hundreds of acres worth of crops. That’s 2 megawatts per acre that you get to save, assuming 100% efficiency lighting. Probably closer to 10 or so megawatts per acre in capacity, though usage would only be about half that, as you turn them off at “night”.

You can generally feed about 2 people from an acre of crops(it’s a complicated issue, to maintain a typical American’s diet requires more like 3 acres per person), let’s say that we are much more efficient, and we feed 5. That’s a megawatt of power required just to power the lights to make the food for one person.

Silica glass is pretty good at screening out UV. Plants may not mind being grown in much lower atmospheric pressure, with higher CO2 content. Fusing together Martian regolith into glass has some challenges, but so would digging caverns beneath the surface large enough to provide the room for agriculture.

It will take a fair amount of trial and error to figure out the best way of producing food on Mars (assuming we ever get that far), but having a much lower cost of the infrastructure and power required for farmland may make up for the difficulties.

They would not be in temporal isolation. They would have clocks, they would have commerce, they would have times that they start and end work.

Having more exposure to the Martian day would tend to cause people’s rhythms to become less attuned to Earth time, not more.

The problem is that after a few of weeks without sunlight, any agricultural grain crops will die. Some food cultivars, and in particular the appropriately-named nightshades, are more tolerant to low light conditions but even they aren’t going to thrive without consistent sunlight. Given that such crops are presumably necessary for the survival of the colonists, not only for food but regeneration of oxygen supplies, it simply isn’t feasible to rely on sunlight on the Mars surface. Since even a highly efficient bioregenerative controlled ecological life support system (CELSS) would require on the order of 10 kW/person just for basic life support (heating, air and waste recycling, et cetera) not including any other communication and mission requirements, the need for a power source independent of solar energy is required anyway.

As for putting a ‘greenhouse’ on the surface, aside from the low and variable insolation there is also the problem that such a structure would have to be able of containing the pressure (even at half of Earth sea level standard pressure of 7 psi) of over 48 kPa or over 1000 lbf per square foot of surface. Containing that pressure in a large structure utilizing transparent glass panels would require a massive amount of structural reinforcement (e.g. aluminum or steel framing) which could not be manufactured from local materials without first establishing the ability to locate, extract, refine, smelt, and roll or extrude structural sections notwithstanding the weight and complexity of delivery glass panels to the surface of Mars. And of course, such a large amount of surface exposed to the frigid outside temperature on the surface of Mars (dipping down below -90 ℃ even in equatorial regions) would radiate away a massive amount of heat, especially at night and during dust storms when there is no incoming solar radiation.

In short, everything that would be required to facilitate normal agriculture on the surface of Mars using the marginal amount of sunlight would be a fantastical logistical and technical challenge. While establishing the infrastructure to perform hydroponic or other underground food cultivation is no picnic, either, it would be be significantly easier to protect and ensure reliability than attempting to grow crops on the surface and relying on highly variable sunlight that at its best is about equivalent to evening solar insolation on Earth.


There was an experiment I heard about where the subject was underground in a cave, couldn’t see the sun etc. Here’s an interview with the experimenter, a Frenchman:

…there was a psychological test. I had to count from 1 to 120, at the rate of one digit per second. With that test we made a great discovery: it took me five minutes to count to 120. In other words, I psychologically experienced five real minutes as though they were two.


Interestingly, during the subsequent experiments I did with other research subjects, all of the people in the caves showed cycles longer than twenty-four hours. In fact, it became common for them to achieve cycles lasting forty-eight hours: They would have thirty-six hours of continuous activity followed by twelve to fourteen hours of sleep.

Very interesting read: CABINET / Caveman: An Interview with Michel Siffre

And as far as altering the length of a second or minute etc., the French strike again:

Each day in the Republican Calendar was divided into ten hours, each hour into 100 decimal minutes, and each decimal minute into 100 decimal seconds. Thus an hour was 144 conventional minutes (more than twice as long as a conventional hour), a minute was 86.4 conventional seconds (44% longer than a conventional minute), and a second was 0.864 conventional seconds (13.6% shorter than a conventional second).

They also made weeks 10 days long…

As someone who regularly worked day and night shifts all my life, I think that a Martian Sol would be relatively easy to become accustomed to. The fact that Martian days have a few extra minutes could be easily allowed for by sufficiently sophisticated timing devices.

As someone who routinely operates +/- 1 time zone from home, I agree that the ~25 hour Martian “day” would be within normal human variation for at least most people. Witness however the many people who report in our DST changeover whining threads that changing their bedtime and wakeup by a mere 30 minutes takes them months to overcome. Anecdotally, individual variation appears large.

As to the research on humans deprived of external time signals, it’s all over the map. A bunch of 1960s research “proved” 25-26 hours was the natural human state. And lots of confident publications resulted. Much of which was thoroughly disproved by experiments done later delivering different answers. We’ve had more than one thread on this.

What’s the actual factual situation about human’s natural free-running circadian rhythm? Hellifino. But confident assertions that it’s been settled at 25-ish hours are known to be bunk.

Thanks for all the replies. I agree redefining a second would be problematic, however I also think that a new colony of Mars dwellers eventually might not want a system that is basically a retrofit of Earth’s but would rather have a Martian system that they could call their own.

I wonder if the second would remain the same for all the technical reasons that @Stranger_On_A_Train suggested in his excellent response, but that it would be used in a new clock. Perhaps you could have 65 seconds per minute and 65 minutes per hour with a 21 hour day which would be very close to exactly a day on Mars (I may have messed up the maths but you get the idea). I just wonder if some future Mars politician would suggest something like this one day.

I can’t see the point in changing the day from an earth solar day to a day which is neither an earth solar day nor a Mars solar day. “Very close to exactly a day on Mars” is useless, since your day would gradually but inexorably move out of synch with the apparent movements of the Sun. It would take you longer to get to a point where “midnight” occurred when the sun was highest in the sky, but you’d get there just the same.

If you’re keeping the second, you may as well keep the minute and the hour - there is no advantage to changing either. You then have a choice of (a) keeping the 24-hour day, which as others have said might make sense if you are living largely underground, or (b) switching to a Martian solar day, which would be 24 hours, 39 minutes and 35 seconds long.

I don’t see that option (b) presents any huge problems. You can easily have a clock in which 24:39:34 is following by 00:00:00, and then by 00:00:01. The fact that the day is not an integral number of hours is no more a problem than the fact that the month is not an integral number of weeks. It has consequences, but they are all easily manageable.

You’re probably right - however I will point out that Earth’s system is not exactly bang on either. There are regular adjustments to make our time fit with the rotation and orbit of the Earth.

This question reminds me of an old science fiction story: The people who left earth and colonized space kept to their old habits from earth. They even insisted on bringing their children back to earth to familiarize them with the “mother planet”. The children HATED having to adapt to gravity. The end of the story was that the children would develop their own space-centered society, and it was the parents who needed counseling to adapt to this.

Even on Earth not everyone agrees, but the principle of civil timekeeping is that you can look at your watch and tell when the sun is up. For that reason there are things like leap seconds; the rotation of the Earth is not magically synchronised to a uniform scale of time.

If, hypothetically, it is important to know something like that on Mars, and for some applications it obviously is, then it is only natural to use a measure like mean solar time, as is the case (more or less) on Earth. It is a relative non-issue whether you subsequently choose to divide a Martian day into 10 or 24 or 360.

It might not have to be caves per se: humans and agriculture require a non-small amount of water, which could be stored between the thick panes of a glass dome over part of the habitat, affording the residents an opportunity to have a distorted view of the bleak landscape outside. The water in the dome would stop most of the radiation that penetrates the wispy atmosphere, and would be structured so that no one could accidentally step into its solar focus during the day.

The day length would really not be a problem for the Martians. They would simply adapt to it, one way or another. The people who might struggle are the ones that rely on Earth’s cycles to guide their faith. For Abrahamic religions that rely on the moon to mark major intervals, neither Phobos nor Deimos have any significant interval, though one might use exact conjunctions or something of the sort. Of course, if there are devout Muslims among the residents, facing Mecca to pray becomes something of a challenge.

At least during Martian night for most of the Martian year it’s easier to see Mecca from Mars than it is from, say, Medina. Just look for the bright object in the sky; that’s Earth. Mecca’s right there.

Of course if it really mattered, there’d be an app for that. For whatever “it” and “that” we happen to be discussing.

For most of a Martian night, Earth is not visible. It might appear at sunrise or sunset, as Venus does on earth, but well before midnight, it will have set. At least it will be a crescent, though, when you can see it.