How will we tell time on Mars?

I have had Mars on my mind quite a bit lately. Right now, NASA has two robotic rovers driving across the red planet’s surface simultaneously, as well as the first human-made helicopter drone that has ever flown on another world:

I have been enthusiastically following the latest developments and discoveries from these robotic explorers. I have also been watching The Expanse, a science fiction series that is set in a hypothetical near-future where humanity has been colonizing the solar system. On the show, Mars is the most populous and powerful of the space colonies and a major player in the show’s events. Back in the real world, Elon Musk has big plans to help push for the exploration and colonization of Mars by human beings, and his SpaceX company is developing technology to help achieve just that.

However, any would-be Martian would have to face quite a few challenges. The red planet’s atmosphere is only 1% of the density of our own, and what little air Mars has is 95% carbon dioxide. This, combined with being much further from the sun, means Mars regularly experiences temperatures as cold as Antarctica. On top of that, unlike Earth, our neighbor has no magnetosphere to protect against harmful solar radiation. The Martian soil is full of toxic perchlorates that would have to be removed to grow any crops, and while we have now definitively proven there is water on Mars, finding a way to extract enough of it to support a space colony is still a major engineering hurdle. I’m also pretty sure that NASA won’t be willing to spend the money and resources to ship thousands of rolls of toilet paper to another planet.

Now, I’m no scientist or engineer. I don’t pretend to know how to address these issues. However, there is one far simpler challenge I feel confident enough to weigh in on: how will we tell time on Mars?

If a Martian space colony is ever actually created, its residents will need to adopt a timekeeping system that is adapted to the actual conditions in their new home. It makes no sense to keep using Earth’s clocks and calendars, as things like days and years are completely different lengths on Mars than on Earth. Could you imagine looking at the sky, seeing the sun at high noon, and then looking down at your watch telling you it’s 9:43 pm? Or celebrating your birthday from sometime in the mid-afternoon one day until about a half-hour earlier the next day? Or having Christmas twice a year?

No, Mars will need its own timekeeping system. Luckily, some people have already thought of this, and there are some proposals for a Martian clock and calendar.

To figure out how our possible Martian descendants will tell time, we first need to examine how we came to measure the passage of time on Earth. Our ancient ancestors obviously could tell the passage of time in a few ways. The sun rises every morning and sets every evening, neatly dividing time into day and night. In addition, depending on one’s latitude and climate, there are up to four seasons that rotate in a regular, repeating cycle – spring, summer, fall, and winter. Lastly, the moon appears in the sky at night and it appears to change shape in another regular, repeating cycle.

The day-night cycle itself came to be called a “day”, and we eventually figured out this was caused by the Earth’s rotation. Our ancestors named the cycle of seasons a “year”, and eventually discovered that it was the result of Earth orbiting the sun, each orbit taking about 365.256 days. The lunar cycle was dubbed a “month”, and it is the result of Earth’s moon orbiting our planet every 27.32 days, passing into and out of our shadow.

From here, our ancestors developed a number of artificial, arbitrary time measurements that were tailored to meet our needs. A day was divided into 24 hours, each hour was divided into 60 minutes, and each minute was further subdivided into 60 seconds. Later, when scientific research reached the point that more precise definitions were required, an agreement was reached to define one second as “9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”.

As for months and years, there were a number of solutions devised for how to measure these, but for a variety of historical reasons, the global community eventually settled on the Gregorian calendar as the standard measurement system for the passage of months and years. We count Gregorian years from the year 1, the year that a Christian monk named Dionysius Exiguus calculated to be the year of Jesus’ birth. Though this calculation was later proven to be incorrect, the numbering system stuck and is still used today.

The Gregorian year is incredibly accurate, with most years lasting 365 days but a system of adding a 366th day on designated “leap years” keeping the calendar synchronized, but at the cost of a month that is now completely divorced from the lunar cycle. Gregorian months are now essentially an arbitrary unit of time. Lastly, for convenience, it was decided to measure a set of seven days as a “week”, with each day of the week given a name associated with an astrological body in the sky.

Now, let’s apply these basic timekeeping principles to Mars.

Jezero Crater on Mars as seen from the cameras of NASA’s Perseverance rover

When we use Earth-based time measurements, we find that a day on Mars lasts 24 hours, 39 minutes, and 35 seconds. This makes a Martian day and an Earth day very similar, but not identical. NASA’s scientists have taken to calling a day on Mars a “sol” to keep these two time measurements straight.

The good news, though, is that a sol is so close to a day that humans can actually make the adjustment to the new length of time fairly easily. We know this because that’s what the NASA teams who operate Mars missions do. In order to operate the two solar-powered Mars rovers most effectively, they have to adjust their working days to match when the sun is up on Mars. As such, they have taken to adjusting their entire daily routines and lifestyles to Mars time, and have specialized watches that allow them to tell Martian time.

Unsurprisingly, they have decided for simplicity and convenience to divide sols into 24 Martian hours made up of 60 Martian minutes made up of 60 Martian seconds. Each of these Martian time units is just proportionally longer than their Earth counterpart. Simple!

Unfortunately, devising a Martian calendar is not as simple as devising a Martian clock.

One year on Mars lasts about 668.5907 sols (roughly 687 Earth days). This makes a Martian year not-quite-twice as long as an Earth year, so a person born on Mars would probably be acknowledged as a mature adult and no longer a minor on his or her 10th birthday. It also means that any calendar trying to keep the year synchronized with the Martian orbit will need more leap years than non-leap years. Perhaps a Martian calendar would normally be 669 sols, but periodically remove a sol to create a “neap year”.

It’s probably safe to assume years on Mars would be numbered from the date of the first landing of humans on the planet. Already, NASA’s Mars rover teams measure dates relative to the date their rover landed – as I write this, the Perseverance rover is on Sol 71. It’s not too much of a stretch to assume that whatever Martian year humans first set foot on the red soil will be designated “Year 1” of the Martian calendar.

An even more giant leap

It’s when we get to months, however, that the real problems set in. Mars has two moons – Phobos and Deimos. However, Phobos completes one orbit in less than eight Martian hours, and Deimos completes one in about a sol and a quarter. Now, I could potentially see Martians colloquially using “a Phobos” to mean several hours or “a Deimos” to mean a little bit longer than a sol; a Martian might say a sentence like “I’ll have your car fixed in a Phobos.” But, just like with our modern calendar, Martian months are going to have to be some arbitrary time period.

This is the biggest difference between proposed Martian calendars that I have found. How do we divide up the year?

One proposal, the Davidian calendar, takes the same approach as the Martian clock. It divides the year up into 12 months, just like on Earth, but makes each month 55 or 56 sols long. While this seems simple enough at first, it doesn’t take into account Martian seasons.

Remember, on Earth, we devised the year to keep track of the seasons. Seasons on Earth are all about the same length because our planet’s elliptical orbit is centered around the Sun. In contrast, Mars’s orbit swings out further away from the sun for part of the year, resulting in seasons of very different lengths. Given that Martian winters are associated with massive dust storms that can sometimes cover the entire planet, it would be useful to keep track of seasons on Mars. This proposed calendar bases month-lengths on the position of Mars’s orbit around the sun, resulting in months ranging in length from 46 to 67 sols. While our Gregorian months vary in length a bit, this would be a bit of a wild swing in month-lengths.

A compromise proposal would be to divide the Martian year into more months. The proposed Darian calendar would divide the year into 24 months that are 27 or 28 sols long. This allows Martians to have months of relatively consistent length that are not too different from the ones we use on Earth, while still being able to track the seasons easily.

Plus, this creates an opportunity to come up with new names for these Martian months. While the Davidian calendar keeps the same month names as the Gregorian calendar, the creator of the Davidian calendar decided to use the names of the 12 Zodiac constellations in both Latin and Sanskrit to name the 24 Martian months. Not only is this less boring, but it also makes it unlikely to confuse Martian and Earth dates – if you say “March 7th”, nobody will ask, “Earth or Mars?”

Given the various options, I personally prefer the Darian calendar. It is a great compromise between the familiar and the innovative.

This just leaves the week, and both the Darian and Davidian calendars propose a week that is seven sols long, again preserving what we are used to on Earth. However, both have decided to invent new names for the days of the week.

The Davidian calendar leaves six of the seven days alone, but changes Tuesday to “Gaiaday”. This is because Tuesday is named after the Norse god Tiw, who was associated with the ancient Roman god Mars. If all the other days of the week are named for astrological objects in our solar system, the assumption goes, it makes no sense to have a day named after the planet you are on. From a Martian’s point of view, Earth is an astrological object in the sky, so why not name a day after the Greek goddess of the Earth?

NASA took this super-zoomed-in picture of Earth as it looks from Mars

The Darian calendar’s proposal is both more and less radical. It keeps the etymologies of the days of the week the same, but uses Latin names – Monday is Sol Lunae, Tuesday is Sol Martis, etc. This results in the awkward situation that Sunday would become Sol Solis, and that’s just silly.

Personally, if I were to name the days of the Martian week, I would probably call them Phobosol (Sunday), Deimosol (Monday), Gaiasol (Tuesday), Wednesol (Wednesday), Thursol (Thursday), Frisol (Friday), and Satursol (Saturday). I like the idea of replacing “day” with “sol” to be consistent with Martian usage, and I think it makes more sense to name the sols of the Martian week after Mars’s moons than Earth’s moon.

Last, but not least, there’s the question of holidays. I’m sure Martians will invent their own holidays to celebrate, with the anniversary of the first landing on Mars almost certainly being one of them. What about holidays brought from Earth, though? When will Martians celebrate Christmas? One proposal I read would have Mars celebrate Earth holidays at the same time it is celebrated on Earth, leading to Christmas falling twice a year in most Martian years. But I don’t accept this proposal. As I said earlier, if Mars celebrated Christmas when Earth does, they will end up with Christmas falling from 10:24 am on Sagittarius 9 to 9:43 am on Sagittarius 10, and that’s impractical. I also doubt they would be willing to hold a holiday twice a year. No, I think Martians will end up picking an arbitrary sol to serve as “Martian Christmas”, and do the same for other Martian equivalents to Earth holidays.

That’s my opinion, at least. Obviously, at this point, this is all just hypothetical, since any human settlement on Mars is many years off in the future, even by the most optimistic predictions. Still, it’s fun to theorize and predict. Ultimately, what clock or calendar future Martians end up using will be up to them.

Do you have a question about flags? My next Cat Flag blog post will be a Q&A about the various flags of the world, so if you have a question you want answered, feel free to ask in the comments below!

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