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Assume Mars has developed an indigenous civilization of its own, perhaps as seen in Edgar Rice Burroughs' A Princess of Mars.

Much like Earth, Mars' years and days would be significant for any indigenous peoples. These would drive the seasons and daylight periods, such as they are.

But on Earth, there is an intermediate time period that was important from time immemorial: the phases of the Moon. With a period 28 times that of a day, but 1/13 that of a year, the motion of the moon was a good intermediate length unit of time from which, through long evolution, came our modern concepts of month and week.

But on Mars, the moon(s) are nowhere near as prominent as on Earth, and their periods are much shorter. As seen from Mars and measured in Martian days, Phobos will appear and disappear twice a day, Deimos roughly every 2.5 days.

What astronomical phenomenon, visible from the surface of Mars, would replace the Moon's motion as an intermediate measure of time?

Note: Not a duplicate of this, because this question presupposes indigenous Martians, not Earth colonists; and because this question is not about an 'ideal' calendar, but astronomical phenomena.

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    $\begingroup$ Why would they have to have a measure of months? They could simply have seasons, quarters of the year. Or they could group every tenth, sixteenth, twenty-second, or any arbitrary group of sols. There's really no reason they can't have a completely arbitrary grouping of sols. After all, weeks are arbitrary. $\endgroup$ – Phiteros Dec 20 '17 at 1:25
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    $\begingroup$ @Phiteros Sounds like an answer. Why not write it up? Remember, I'm asking for an astronomical phenomenon, not an arbitrary measure of time. $\endgroup$ – kingledion Dec 20 '17 at 2:25
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    $\begingroup$ @Phiteros Weeks are based on phases of the moon. They are not arbitrary. $\endgroup$ – Deepak Dec 20 '17 at 7:28
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    $\begingroup$ @Deepak The month is based on phases of the moon. Days of the week are not. Most believe the reason we have 7 days in a week are because of biblical or cultural reasons. $\endgroup$ – Phiteros Dec 20 '17 at 7:30
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    $\begingroup$ @Deepak: The native Roman week had eight days, and quite obviously no relationship with the monthly lunar cycles. The seven day week was introduced from the east in the first centuries of the common era, and never ever had a relationship with the months of the Julian calendar, or with the lunar months of the Hebrew calendar. $\endgroup$ – AlexP Dec 20 '17 at 16:20
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weeks and weekweeks would be my guess.

As you stated, Mars doesn't have anything in the night sky that is prominent enough and has a long enough cycle to serve as intermediate division of the year.

Mars does have seasons but they vary quite significantly from equal quarters (142 to 193 sols for spring and autumn). While an indigenous society would probably used season in their calendar, I doubt they would simply gloss over these differences and split the year into equal quarters. So a convenient intermediate division probably won't be derived from seasons either.

As Monica points out in her answer. Weeks are already arbitrary because they seemed like a decent rhythm. It's conceivable that since Mars sols are about 2.7% longer the compounded effect would have caused the Martians to adopt a 6 sol week instead. Six is also more convenient to calculate with than seven.

A weekweek would be 6 weeks or 36 sols, dividing the year into 18 weekweeks, 3 weeks and 3 sols.

You could even go so far and introduce a weekweekweek (6*6*6 = 216 sols) dividing the year into 3 WWW, 0 WW, 3 W and 3 sols.

A base 6 system would also have the neat effect that a five fingered species could easily use each hand as a full digit. Right hand counts sols, left hand counts weeks. If you go up to weekweekweeks you could seamlessly shift the representation of time units through the hands. A four handed species like in that abysmal John Carter movie could actually count through the whole year with their fingers!

Edit: I was wondering how martians would deal with leap sols since their year is significantly off from a whole sol number... The most accurate number I could find was 668.5991 sols per martian year.

Some calculation gives a base year of 668 with the following leap sol rules:

  • Every year has a leap sol (0.4009 sols too many / year)
  • Except every second year doesn't (0.0991 sols too few / year)
  • Except every 10th year does (0.0009 sols too many / year)
  • Except every 1'000th year doesn't (0.0001 sols too few / year)
  • Except every 10'000th year does

Realistically speaking most people won't care for the last two rules (IF they are accurate in the first place) which makes this comparable in complexity to our leap day rules. Of course if they end up with a different base for their number system then they might find different fractions more convenient...

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  • $\begingroup$ When you are measuring Mars' seasons, (142 to 193 sols) are you using Martian days at the time measure? Where did you get that info? $\endgroup$ – kingledion Dec 20 '17 at 13:24
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    $\begingroup$ @kingledion Added link to the information about the seasons. Sols are always local units. A sol on Earth is called a day and takes 24 hours. A sol on Mars has no special name and takes about 24 hours and 40 minutes. $\endgroup$ – Kempeth Dec 20 '17 at 13:33
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Please remember that time is an arbitrary concept

Without any outside influence, the very first basis of time is the "day" consisting of a period of "light" and a period of "no light" that we Earthers more commonly know as "night."

  • Day = one Light + one No Light

According to Space.com, the moons of Mars are so close to Mars that they cannot be seen from some latitudes.

...the bulge of Mars' own curvature gets in the way!

Deimos would appear to rise in the east and set in the west.... very.... slowly... while Phobos would appear to rise in the west and set in the east several times each "day". Worst of all, because of their closeness to Mars and the speeds of orbit, they have phases, but the phases occur much faster than we Earthers see with our own Moon.

Not that they're very visible. Deimos would appear to be only 1/19th the size of Luna in the Martian sky and Phobos would look like a slightly oversized Venus. Somewhat unimpressive.

Which means Martians might not develop the concept of a "month" at all.

Due to the orbit around the Sun and the Martian's ability to watch the stars, the concept of a "year" would develop. With the discovery of Mars' orbit, you would eventually discover the two solstices and two equinoxs, which means Mars is (perhaps) more likely to develop "quarters" as a replacement for "months". Kinda...since the orbit is elliptical the periods between solstice and equinox aren't equal.

As for periods shorter than the "light" and "no light" periods, that's REALLY ARBITRARY. Considering Earthers have ten toes and ten fingers it's a wonder they came up with two twelve-hour periods. But that might have been because the ancient Egyptians had two 10-hour periods plus two one-hour periods for twilight.

How many toes do your Martians have?

EDIT: Apparently there are some people who think the details are really, really important. They're not thinking from the Martians' point of view.

  • 1 Martian Year = 1 orbit around the sun = "Year"
  • 1 "light period" + 1 "dark period" = 1 "day"
  • 667.99 "days" or just 668 days = 1 "year"1
  • My Martians' have five fingers and one thumb and four bones per-finger, for 20 bones per "period" of a day for a 40 hour day. (Since hours weren't developed for astronomical reasons on Earth, I see no reason why they must be astronomically-based on Mars.)
  • My Martian's have a religious requirement to rest on the 11th day so their week is 11 days long. Luckily, their pantheon has 15 gods, so each day of the week is named for one of them as are the two equinox and two solstice days.
  • Since Phobos would only appear like a fast-moving planet in the sky, the Martians really don't pay any mind to it (other than for historical purposes. They really did think it was a planet for a long time! Their astrolabes describing its "planetary" motion were very complex!).

  • Deimos, on the other hand, orbits Mars every 107 "hours"2. It's small and the phases are really hard for almost everyone to see, but it's motion is quite obvious. This caused the Obduracien Religious Wars in 1037 as they'd forgotten the number of hours came from counting bones in their hands and some people wanted to favor a lunar calendar over the traditional and now entirely religously-based Oburacien calendar. After the deaths of hundreds of thousands the scientists and religionists finally agreed that there would be a new division of time, the "month," which consisted of 200 days measured from the coincidence of moon-rise with sun-rise on the same day. To the scientists' great frustration, this lead to the new-age belief that the "lunar morning" of the 167th year brought about an alignment of day, month, and year that was truely harmonious! No one has tried to add the first day of the week to that mess due to a rumor that the scientists were going to use anyone who tried in some kind of acid experiment.


1I was too lazy to care about the ratio of Mars' orbital speed to its rotational speed, so as an Earther, I cheated and worked out the ratio comparison from the Earth numbers.

2Martian hours....

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    $\begingroup$ This is the answer I would have written, except better written and with more orbital mechanics. $\endgroup$ – sphennings Dec 20 '17 at 2:10
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    $\begingroup$ The Babylonians used base 60 and you can count to 12 on one hand (use your thumb to count bones in the fingers). 12s and 60s make more sense from that side. $\endgroup$ – Separatrix Dec 20 '17 at 9:15
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    $\begingroup$ Isn't the main reason for 12 that it is divisible by 2, 3 and 4 ? So if 2, 3 or 4 people want to divide something, it's much easier than using the decimal system? Similar with 60, and 2, 3, 4, 5, 6. $\endgroup$ – vsz Dec 20 '17 at 11:07
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    $\begingroup$ @kingledion, were you feeling persnickity yesterday? What makes you think planetary rotation isn't an "astronomical phenomenon" or that the astronomical phenomenon of the moons wouldn't justify the creation of a month? $\endgroup$ – JBH Dec 20 '17 at 14:21
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    $\begingroup$ @kingledion What JBH hinted at is right on. "Time" is a man made concept. WE MADE IT UP. Get creative--the martians in whatever you are doing can have crazy reasons for whatever measurements of time they want. $\endgroup$ – Jeff.Clark Dec 20 '17 at 23:40
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As you point out, Mars has so much stuff going on in its skies that its moons aren't going to provide meaningful calendrical support. Nothing else is both visible and consistent enough to act as a replacement.

I suggest, therefore, that your indigenous Martians won't look to the sky for divisions of the year. Days yes -- that's regular and obvious, so unless your Martians have a very short or very long "biological day" cycle, they can make use of that. (I'm taking it as implied in your question that your Martians don't, say, hibernate for an Earth month every other month, or sleep 15 minutes every hour, or something like that.)

Seasons are provided by the sun (and axial tilt), so those still work. Subdivisions of seasons could be provided by either computation or function. By computation, I mean that your Martians might find that the concept of "tenth of a season" or "quarter of a season" is a meaningful currency; if so, they can designate it without regard to visible markers. Our weeks are like that; they aren't quarters of the monthly lunar cycle. We humans have just found a unit of seven days to be useful. Your Martians can do that too.

Alternatively, they might have sub-season units of time based on things they do -- plowing time, planting time, livestock birthing time, and so on. These chunks of time will not be uniform in length. They also will not be uniform across the whole planet, so once your Martians develop global communication, functional designations won't work as functional designations. They could remain as historical designations -- they don't plow during this time now, but this stretch of time is still called "plowing time" because they once did. Planet-wide standardization of timekeeping was a late development here on Earth, when synchronization of (e.g.) train schedules became necessary. Your Martians won't need to all agree on time designations until they're interacting (and scheduling events) with other groups with different calendars.

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Deimos would be a good week-keeper.

Now consider the situation with Mars' moons. Deimos takes 30 hours and 18 minutes to make one swing around Mars, and the Red Planet makes one full turn on its axis every 24 hours and 37 minutes. So an observer on the Martian surface would see Deimos rise in the east, but the moon would then move across the Martian sky at a very slow pace. In fact, it would take about 33 hours to get directly overhead (or very nearly so), and then another 33 hours to descend and set in the west.

And then, the Martian explorer would have to wait another 66 hours before Deimos again reappeared above the eastern horizon!

So one full circuit of Deimos would take 132 hours or 5.5 days. To me that seems like a pretty decent week equivalent. And moon-based, no less!


A little more esoteric: sunspots.

In smoky conditions on earth, one can see sunspots with the naked eye. I was in the So.Cal fires in 2003 and people were calling into radio stations asking what was wrong with the sun. You could see the spots easily with the naked eye. Here is a more recent image from this year's fires. sunspots thru smoke http://www.newslincolncounty.com/archives/187061

Curiosity could see sunspots from Mars. https://www.jpl.nasa.gov/news/news.php?feature=4657 sunspots as seen from Mars

A solar rotation is about a month. If you could view the sun steadily thru the Martian haze could you track the sun's rotation using sunspots? Maybe.

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Earth would be clearly visible in their sky, as would Earth's moon. They would see a bright blue dot with a smaller white dot following it around, sometimes on the sun side and sometimes away from the sun. They would quickly note that the small dot flips back and forth across the big dot about every 28 days. At maximum separation, they'd appear about 0.25 degrees apart; for comparison, the full moon is about 0.5 degrees across.

So, it's possible that the answer to your question is "Earth months, or something very close to them."

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  • $\begingroup$ Pictures from orbit are stunning, but the pictures I've seen from the ground don't show the moon clearly. Jupiter might also be reasonable, but each gets lost in the sun for a reasonable amount of time each orbit. $\endgroup$ – user25818 Dec 20 '17 at 20:57
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    $\begingroup$ It's noteworthy that Earth cannot be observed all night on Mars (very much like Venus on Earth). I am afraid that a small dot circling another small dot (and a separation of two small dots is not as interesting as a full disk of roughly the same size) is not that interesting to base your calendar on it. $\endgroup$ – Ghanima Dec 20 '17 at 21:11

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