No human has ever walked on Mars, and yet people have been designing Martian calendars for more than a century now! In fact, there have been at least 70 different calendars proposed, and none of these is official.
It's a challenging and fun problem, and it's never too early to think about these things. In that spirit, let's look at a new variation, the Martian Common Era calendar (MCE).
But the MCE calendar is more than just an idea to use "when we are on Mars someday." It's also for fun today.
Do you want to celebrate the Martian New Year? We can, if we can agree on when it is. Want to celebrate your Martian birthday? Those are special because they are twice as rare as regular birthdays.
By the way, this article isn't meant to be very technical. If you want real technical details, a web search will give you a wealth of information about timekeeping on Mars.
One more thing: This article talks about "Earth's calendar," but that is a little misleading. We've had many different calendars on Earth, and many cultures use different ones even today. But the calendar in most common use is the Gregorian, probably the nearest thing to a universal calendar that humans have ever had.
So let's look at a little background information. Mars is farther out from the sun than the earth is, and so it travels around the sun more slowly. In fact, a Martian "year" is nearly twice as long as one of ours — twenty-three of our months, give or take a little.
But the Martian day is almost the same as an earthly day; this is sort of a cosmic coincidence. A day on Mars is roughly 24 hours and 39 minutes; we call it a sol (as in solar) to distinguish it from an earthly day. This usage dates back to the 1970s when NASA placed the Viking lander on Mars; the scientists on the that team naturally needed to talk about the "day and night" patterns of the world they were studying.
Some people prefer to use alternate terms for a "Martian year." These terms include mir and orb (as in orbit). Most people seem content to say "year" both for Earth and for Mars (and they may clarify by saying "Martian year," "Mars year," "M-year," or something else).
One thing that makes calendars hard (even on Earth) is that the year is not made up of an exact number of days. When the Earth orbits for a full 365 days, it is still almost 6 hours short of going all the way around the sun. That, of course, is why we add a leap year roughly every four years.
Mars, on the other hand, orbits the sun in about 687 earth days or 668 sols. But again, when the 668 sols have passed, there is still a fraction of a sol left before it goes all the way around the sun. In this case, the fraction is nearer to a half, so it makes sense for a Martian calendar to have a leap year almost every other year.
So these facts are a matter of astronomy. They're totally beyond our control. Things that are more within our control are:
On Mars, we could pick any day to start the year. A logical choice might be the first day of spring. After all, we think of spring as a time of "new beginnings." In fact, this choice is widespread among planetary scientists, for Mars and even for other planets; and most Martian calendars have made that same decision. So let's stick with that.
The picky reader may ask: Which first day of spring? After all, the northern hemisphere's vernal (spring) equinox is the same as the southern hemisphere's autumnal (fall) equinox. But here, as in many things, there is an arbitrary convention; we assume that the northern hemisphere is the standard.
When we draw a typical diagram of the solar system, we are looking downward onto the Sun's north pole (and that of the Earth and Mars as well). So the first day of the Martian year will be the spring equinox in the northern hemisphere, but the fall equinox in the southern hemisphere.
Next let's look at the question of months. We divide time on Earth into months because that's roughly the length of the lunar cycle. Of course, once again nature doesn't cooperate — the number of lunar months in a year is not exactly twelve, and a lunar month isn't an exact number of days.
On Mars, Earth's lunar cycle is pretty meaningless. But we've gotten used to dividing a year into these convenient periods — a period much larger than a day but much smaller than a year.
It's not necessary to divide a year into months at all, of course. One possible Martian calendar would just number the days from 1 to 668 (or 669 in a leap year). This has the virtue of simplicity, but it does sound rather boring. To date a letter "Day 226" makes it sound like an entry from a prisoner's diary. Birthdays would also sound boring — "My birthday is 348, and hers is 529." Very practical, maybe, but not much fun.
We might say: "Well, Mars has two moons. Can we use them for our calendar, as we do with Earth's moon?" But these tiny, fast-moving satellites are no help here. The outer moon Deimos goes around Mars in less than a day and a half. The inner moon Phobos actually orbits in less than 8 hours. So they're completely unsuitable for this purpose.
The usual approach for a Mars calendar is to make the months more or less arbitrary. Some have divided the year intuitively into 12 months; the disadvantage is that the months then have more than 50 days each, far more than we're used to.
Another common choice is 24 months that are very near in length to our earthly months. Some calendars "juggle" the lengths of the months so that each season occupies the same set of months each year. In these calendars, a season would then be 3 months or 6 months (or even 4 — as there is one calendar that divides the year into 16 months of 40-something days each).
To make a month a "comfortable" length, the MCE calendar uses 24 months of 28 days each (except the final "short" month). The next task is to give names to these months.
Some 24-month calendars have suggested a scheme where there is a "First January" and a "Second January" (and so on for all the months). Others have suggested elaborate culturally-neutral naming schemes.
The choice for MCE was to use the conventional twelve month names and intersperse the names of the signs of the zodiac. This solution is not "culturally neutral" as such; but virtually every language in the world has its own names for the twelve months and the twelve astrological signs (so that they are easily translated). Surely for most people, that is close enough.
So MCE starts with the usual "January" and then follow with Gemini. Why Gemini? Well, if you view the solar system as a clock with the sun at the center, the constellations are arranged around it so that Mars is "in" Gemini at the time of its vernal equinox.
This, by the way, is a heliocentric or sun-centered viewpoint. So it's 180 degrees opposite to the approach followed by ancient astronomers, who thought according to an Earth-centric standard, calculating in which constellation the sun fell. (This is interesting because the sun blinds you to whatever stars or planets are behind it; they went by their records and their math).
By the old kind of thinking, Martians would find the sun to be "in Sagittarius" at their vernal equinox, so we could name that month Sagittarius. However, now that we know that the planets orbit the sun, not the Earth, let's think and act accordingly.
By now, you may be thinking of the old rhyme, "Thirty days hath September..." How many days should each Martian month have? There is more than one way to allocate them. We could adjust them so that they fit the Martian seasons; these are "lopsided" or irregular because of the highly elliptical orbit Mars has.
Let's take a simple approach and give each month 28 days. The exception, of course, will be the final month, Taurus; it will have only 24 (giving a year of 668 sols). When there is a leap year, let's add the day naturally to the end of the year, giving Taurus a 25th day (rather than adding to the end of the second month as we do every fourth year).
Wait a minute! Leap years? Yes, those are an issue on Mars — more so than on Earth, in fact. You may think leap years in our Earthly calendar are "easy." In a sense, they are. But there are more rules than most people realize!
The Earth orbits the sun in roughly 365 days and 6 hours — the leftover portion is why we have leap years. In four years, we accumulate almost an entire day, so we add a day to "re-synchronize."
But that's a little too much correction. We've over-corrected by about 42 minutes. So the next rule is: When the year is a multiple of 100 (like 1800 or 1900), we will skip a leap year. We then add yet another rule: If a century year (a multiple of 100) is also a multiple of 400, then it will be a leap year. This is why 1800 and 1900 were not leap years, but 2000 was.
The bad news is, Martian leap years will be just as complex. The good news is, we can manage that complexity much the way we do with our Earthly calendar. We won't go into all the math here. The solution used by the MCE calendar is to let each decade have six leap years and four "non-leap" years; the leap years will have 669 sols and the others will have 668.
As a side note, we're used to leap years occurring fairly rarely — but in this Mars calendar, leap years are actually more common than the others. Let's use the more intuitive terms "short year" and "long year." A long year is then the same as a leap year, of course.
The easy way to allocate these leap years is to this. Odd numbered years have an odd number of days (669), and even-numbered years will have an even number of days (668). Since this is only five "long years" per decade, let's add the rule that a multiple of 10 is also a leap year (or long year).
To make the math come out better, we'll add two more rules: A century year (multiple of 100) is a short year, but a millennial year (multiple of 1000) will be a long year.
This raises yet another question: How do we number the years? In calendar jargon, when is our epoch?
The Gregorian calendar numbers the years according to a scheme devised by a medieval monk who tried to estimate the birth year of Jesus, using that for Year 1. (Most authorities believe he was off by at least four years.) This is the Common Era, now commonly abbreviated "C.E." (as in 2019 CE). For centuries, of course, the abbreviation was "A.D." (_[ Anno Domini] or "the year of our Lord").
For Mars, there are several logical choices. One common suggestion has been to use the date of the first robotic landing on Mars (by the Viking lander, in 1976). This has one disadvantage: Many common dates (such as the year of someone's birth or some twentieth century event) will be negative dates. As of 2019, it has been fewer than 30 Martian years since Viking landed. Having the current year of only two digits would feel strange to many people.
Another common choice is to mark the first year that Mars was observed in a telescope, the year we call 1609. In this system, our year 2019 would roughly correspond to the Martian year 218.
But the Martian Common Era calendar is built on yet another option. It anchors the Martian epoch as close to our accustomed one as possible (hence the name "Martian Common Era"). By pure coincidence, Mars had a vernal equinox about three weeks into January of our Year 1. (Our calendar has no Year 0, largely because Roman numerals have no zero. This explains why the 21st century actually began on January 1, 2001.)
So let's call this early equinox the Martian epoch. Choosing the epoch near our accustomed one gives the Martian calendar a moderately interesting property: If you take the ratio of the Earthly and Martian year numbers, you will get a meaningful number. It is 1.88 — the same as the ratio of a Martian year to an Earthly year.
If you think about this for a moment, it becomes obvious. While Mars orbits the sun 100 times, the Earth will orbit about 188 times. This makes it possible to quickly estimate a Martian year from an Earthly one (or vice versa). Since a Martian year is about 1.88 of our years, the Martian year 1000 would be approximately our year 1880. (Interestingly, this is almost the exact year that Italian astronomer Schiaparelli was observing Mars and wrote about the "channels" of water that he thought he saw.)
So we know where our calendar starts and where each year starts. We know the names of the months and how long each year is. We know when the leap years are. One question we haven't asked is: How do we deal with days of the week?
Again, there are multiple possibilities. One possibility is to forget them altogether — throw away the idea of a "week" as a division of time. But in nearly all countries and cultures, we have been used to them for many centuries.
Another possibility is to use a week that is some length other than 7 days (or sols in this case). This is not unheard of even in Earthly calendars; but again, most people worldwide have recognized a seven-day week for a very long time.
Some Martian calendars use weeks of seven sols or other lengths. Most of them use sophisticated or colorful naming systems for the sake of cultural neutrality, novelty, or other reasons.
In keeping with the theme of simplicity and intuitiveness, the MCE calendar keeps the "normal" seven-day cycle. Likewise, it keeps the ordinary names, which after all may be translated into many different languages.
Some Martian calendars have the days of the week fixed with respect to the year; so the year always starts on the same day of the week. For example, the 20th day of the third month will always be the same day of the week from year to year. This is appealing in many ways, but some people might have small issues with it.
First of all, the end of the year introduces a "jump" where, for example, a Wednesday on the last day of the year might be followed immediately by a Sunday on the first day of the next year. Wouldn't this be a little jarring? "Wednesday was two days ago, so today is Fri— oh, wait, New Year's! Today is Sunday."
And secondly, some people won't entirely like the concept of a date occurring on the same weekday every year. Isn't that a little boring? Do you want your birthday to fall on Monday every year? Should New Year's Day (or the first sol of spring) always be on a Sunday? Let's have some variety instead. Let's start the cycle at some known point and let it stretch forward and backward, the seven-day cycle running independently of the rest of the calendar, more or less as it does on Earth.
Of course, the Martian Common Era calendar does offer a few small advantages in that respect. Notice that each month (except the last one) is 28 days long, which is a multiple of 7. This means that the first of each month will be the same day of the week for the entire year. If January 1 starts the year off on a Tuesday, then Gemini 1, February 1, and all the others will also fall on a Tuesday. In fact, each month except Taurus will have the same pattern of weekdays all year long (and even it will follow that pattern until it runs out of days).
Let's look at terminology a little further. Obviously there is some ambiguity if we don't specify which calendar or planet we are talking about. If I say "the month of Gemini," it's clear I mean Mars; but if I say "the month of April," it is unclear. For that reason, I suggest the convention of an optional "M" with a hyphen to be used in front of a name, a unit, or a number as needed.
So I might say "Martian April" or "M-April"; I might refer to "20 M-years" (meaning 37 Earthly years). I might refer to "the M-year 1076" or even "the year M-1076"; however, in the case of years, context will usually be enough to distinguish the two. To be formal about year, we can always say "the year 1076 MCE."
So given what we have so far, we can calculate that New Year's Day in 2010 (Jan 1, 2010) was the Martian date Tuesday, M-February 11, 1069 MCE. For another example: July 4, 2021 will be Wednesday, the 4th of Leo 1075 MCE. Take the following Martian New Year (January 1, 1076 MCE); oddly enough, this will be on our Christmas Day, December 25, 2022.
But of course we're cheating when I say that some Earthly date is "the same" as some Martian date. After all, even on Earth we don't agree on what day it is — people in the USA may say it is one day, but it's already "tomorrow" in other parts of the world such as Japan.
In reality, a Martian date overlaps with an Earthly date. The natural "crude" conversion works this way. We take the beginning point of the date in one calendar and ask: In what day (in the other calendar) does this timepoint fall? But if you take into account time zones on Earth (and on Mars!), you will occasionally disagree by a day. And yes, Mars has time zones (more on that later).
A natural question is: What holidays might be associated with this kind of calendar? Well — future dates on Mars may have religious or civil significance attached, but we can't predict much of that in advance. When there are humans on Mars, they will take care of such matters as they arise, regardless of what calendar they use.
Take Christmas, for example. Conceivably we could pick a date in history and translate it to the Martian reckoning; but why try? and what date would we use? Our December 25 falls on a different Martian day each M-year. For that matter, we've never been certain what year Jesus was born, much less the exact date. (December 25 is purely a convenient fiction, taken from the old Roman holiday of Saturnalia.)
So let's ignore things such as Christmas and Ramadan, as well as the numerous other holidays such as Thanksgiving and Mother's Day. This still leaves two categories of holidays we might celebrate: those based on astronomy and those based on modern events relating to Mars.
For example, celebrating the new year and celebrating the spring equinox are human traditions that have been observed in countless cultures for thousands of years. In this calendar, we kill two birds with one stone, as they are the same day (in the Martian northern hemisphere).
Earth will be the "real home" of mankind for quite some time in the future. Therefore any notable relationship between the planets in their orbits will be a cause for observance on Mars. I expect a "Conjunction Day" when the planets are across the sun from each other, and an "Opposition Day" when they are aligned on the same side of the sun. These will also be a matter of practical concern, as communication with Earth will be easiest as we approach opposition, and will be difficult or impossible at conjunction. They will also add variety to life, as they will occur fairly rarely and irregularly, not on days we can predict just by counting on our fingers. So they will "float" through the calendar, more like Easter than Christmas.
I can imagine that "Viking Day" might be celebrated on Mars. The Viking lander was the first successful landing by a probe on Mars on July 20, 1976 (which itself was the anniversary of the Apollo 11 moon landing). This corresponds to the date Virgo 12, 1051 MCE (a Martian Tuesday).
It might make sense to celebrate the winter solstice on Mars, as every culture on Earth does. This won't matter quite as much as on ancient Earth; the Martian settlers won't be "outdoor farmers," but they may still care about seasonal variations.
Another big day might be the anniversary of the first human landing on Mars — definitely a day worth celebrating hundreds of years into the future! But of course, we don't know yet when that day will be.
What about Martian versions of Earthly holidays? Will there ever be a Martian Thanksgiving or Halloween or any of those? Only time will tell.
So far, we've discussed the calendar, but not timekeeping in terms of hours and minutes. Let's dig into that, though it isn't "strictly" a calendar topic. There are two or three way of dealing with Martian time (bearing in mind that a Martian sol is a little over 24 hours, 39 minutes).
One school of thought says: Forget tradition; let's institute a totally new method of timekeeping. And there is some appeal to this. Most of our measurements in modern times are "metric" or base-10 systems. Some have suggested dividing the day into 1,000 divisions that are like minutes (or 100,000 that are like seconds). Such units might be called "ticks" or "beats" or other similar names; specifying a time of day might be as simple as mentioning a number— "The time is now 592," for example.
Overall, however, this is too complex for many people's tastes. It introduces totally new terminology and a new set of arbitrary units, and it's completely incompatible with what we're used to.
Another approach was taken by Kim Stanley Robinson in his Mars trilogy. In these novels, the Martian settlers measured time as we do from midnight to midnight -- but the extra 39 minutes were "off the clock." Time, in effect, "stopped" during that interval, which they used for relaxation or celebration or whatever. This is an entertaining solution, and the solution we'll look at shortly is very similar.
The nearest thing we have to an "official" clock for Mars is the one used by many NASA scientists -- what is sometimes called stretched time. Because the Martian day is so near our own, it makes sense to divide it into 24 hours (each of which is about 4% longer than ours). So each "stretched" hour is made up of 60 "stretched minutes" each of which is 60 "stretched seconds." So this clock goes from midnight to midnight just as we're accustomed to, but it doesn't stay in sync with Earthly clocks. (A fun bit of trivia: more than one NASA scientist on a Mars team has had a custom-designed watch or clock that ran on Mars time.)
One small problem with "stretched time" is that our common units now have different values. Every single day we use seconds and minutes to measure duration; it's very undesirable to have two separate standards for those (even though the difference is a little less than 4 percent).
The solution that accompanies the MCE is this: we keep our usual hours, minutes, and seconds, and we simply let the clock run longer. So the day would start at 0:00:00 and run until roughly 24:39:35 (plus a fraction of a second). If you're staring at the clock at the exact end of the day, you will see a slight hesitation (or your clock will fall behind by a fraction of a second). And the "seconds" on the clock will jump from 35 to 0.
This brings us to the issue of time zones. It seems reasonable to set them up similarly to what we have on Earth (or we could dispense with them entirely). Obviously the sun rises and sets at different times in different places. Mars is just like Earth: when the sun rises in one location, then halfway around the planet, the sun is setting.
On Earth, we've agreed that the standard is Universal Time (what we used to call "Greenwich Mean Time"). The Martian equivalent of Greenwich, by the way, is a little crater named Airy Zero, inside Airy Crater. This is naturally located on the prime meridian (zero longitude).
At this point, someone will wonder about Daylight Saving Time. Let's avoid that issue. In the opinion of many, if there is one thing we could leave behind on Earth, this is it. Even discontinuing the practice on Earth might not be a bad idea.
If we settle Mars, we will have countless challenges and opportunities before us. Worrying about clocks and calendars will be only one of thousands of issues we have to face. So we do little projects like this one. It's wise, or at least fun, to take care of what details we can ahead of time.