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I wanted a habitable planet around a binary star system, so I conjured up a system that let me have one.

The problem that I'm facing is that the goldilocks orbital parameters for this planed ended up being 6.5 au from the stars barycenter, with an orbital period of 6.78 years (the second planet on this diagram):

enter image description here

The problem is that for storytelling-related purposes I'm not entirely comfortable with years being almost seven times longer than on Earth.

While the planet itself doesn't have pronounced season changes so having 3-year long "winter" isn't an issue (Temperatures do not drop below zero pretty much everywhere except the polar regions), the problem comes from the timekeeping and description of things like the age of the protagonists. Describing somebody as "old with grey hairs, managed to live up to respectably ancient 17 years" is gotta be confusing for the readers even if I'll state prior to that that a year is seven times longer (Noth to mention that describing a "5-year-old" character having snu-snu might attract serious problems in the real life, up to criminal charges, because it will be out of context or outrages readers that don't bother to read carefully).

There also might be some undesired psychological side-effects on the whole culture of the species living on such a mellow and slow to change planet that I'd also prefer to avoid if possible.

So since on Earth year counting is tied closely to changing seasons I thought about decoupling the year and the revolution time around the planet. What if seasons on the planet would cycle faster than it orbits around the star? Can this be possible? If the seasons cycle three times faster, then it brings the year's length to about 2 years long, which is much easier to work with.

My initial try at the justification was to make the planet have a much higher rate of axial tilt precession, but I have no idea if it is actually possible or doesn't come with some nasty side-effects like "your planet is no longer capable of supporting life now".

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Nov 28 '20 at 7:18
  • $\begingroup$ @Darth Biomech I have thought of a possible solution to makig a habitable planet have such a long year, and added it at the bottom of my answer from Nov. 25. $\endgroup$ – M. A. Golding Dec 17 '20 at 7:13

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He was an old man of 90 harvests

The seasons on this planet are mild, so ancient people wouldn't care much about how often they went around their suns. Instead, they'd care about when to plant crops and when to sow them.

Clearly, no civilization could live off of harvesting crops only every 6 years (starvation was rampant in early human history even when people only had to wait a single year), so presumably they found some crop that has a biological cycle closer to a single Earth year. This cycle became the rhythm that governed their lives, and it naturally made its way into their language (even if the civilization in your story is post-agrarian).

He was a young man of 18 tenmoons

Again, since your planet has mild seasons, perhaps it makes more sense to base the calendar off a different cycle. I like the sound of "tenmoon" as a substitute for year. You don't even need to directly explain it - it's clear from context that your planet has a moon that orbits roughly ten times per Earth year. As long as characters aren't doing anything that makes their age in tenmoons seem different than a person's age in years, the reader will instantly get what you're doing.

Though of course if you don't like making up a word like "tenmoon," you could just say they're 180 moons old. With good subtle writing (or a helpful, less-subtle character alluding to how many days a moon is), it'll become clear and the reader will mentally know to divide by 10. Ages that seem too large don't have the same issues you were worried about with ages that seem too small.

EDIT: I can't believe I didn't think of this earlier. There's a periodic celestial event that it would be very natural for the people of your planet to use: the orbit of the binary suns.

How long does it take the two suns to make a full rotation in the sky? About a year? (they look about 1 AU apart on your chart). This is a good alternative to using moon cycles, especially if it happens to be about a year long, and it's much more fitting for your solar system in particular. The only issue I'm having is that I can't find a good name for a "year" analogue. "Cycles" is a little too vague, given that the planet and stars are all making cycles.

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    $\begingroup$ Ooh, I really like the harvest idea. Especially considering that the length of the sow-grow-harvest-rest cycle can be pretty much of arbitrary length (On our planet some places grow one harvest per year while some southern places manage to get three or four)! $\endgroup$ – Darth Biomech Nov 25 '20 at 13:51
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    $\begingroup$ Made an edit about the orbits of the suns themselves. I think if the period of that orbit is about a year, or even if it's within a factor of 2 either way, it's an excellent basis for a calendar. It's also a cool way to make your setting stand out from other systems that have just one star. $\endgroup$ – Gilad M Nov 25 '20 at 17:13
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    $\begingroup$ A metaphorical word like "dance" might be good for the binary suns orbital period, since the suns exchanging places might remind one of a dance. "It had been four Dances since her husband died" $\endgroup$ – Ross Presser Nov 25 '20 at 18:44
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    $\begingroup$ I'll just leave this here: xkcd.com/483 $\endgroup$ – Michael Seifert Nov 25 '20 at 21:00
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    $\begingroup$ @GiladM, given that the planet doesn't have seasons of its own (no eccentricity or axial tilt), there'll be a weak seasonal cycle caused by the secondary star. This gives you the harvest cycle. And since it's virtually certain the planet's orbital period will be resonant with that of the secondary star, the harvest cycle will be an exact fraction of the astronomical year (say, 1/7th). $\endgroup$ – Mark Nov 25 '20 at 21:44
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One star blocks the heat from the other.

If both stars and your planet are all collinear then your planet is getting hit with one stars worth of heat, the other star is occluded by the closer star. When the stars rotate 90 degrees in their orbits, and both stars are visible to the planet, then your planet gets the power of both of them at once.

So by making your binary system spin faster or slower (by adjusting the distance between stars) you can speed up or slow down the apparent seasons.


Of course you won't get even length seasons from this, I don't know the exact size of your suns and their exact orbits, and haven't calculated charts, but extrapolating from our sun I think over a 365 day period it's plausible you'll probably get ~15 days of colder temperatures as one sun partially occluded the other. 2 weeks of snowfall in an otherwise temperate zone would be worthy of being the basis of a calendar.

If the suns have different colours you could have 2 winters per year. The yellow winter and the red winter, for example. This allows the suns to be closer together than if you limit yourself to one winter per year.

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    $\begingroup$ I don't think this would work well ... 1) the occlusion would be at most a few days long, 2) the distant star wouldn't be providing that much insolation anyway when it's at the distant point. $\endgroup$ – Ross Presser Nov 25 '20 at 13:38
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    $\begingroup$ Yeah, for the reasons Ross pointed out, I would be highly skeptical of this explanation. In addition, it assumes that the plane of the suns' rotation is the same as that of the planet's, which isn't always true (in the diagram, the suns appear to be at a significant angle relative to the planets). If I'm interpreting the picture correctly, the two suns and the planet will never align exactly like that. $\endgroup$ – Gilad M Nov 25 '20 at 14:04
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    $\begingroup$ @RossPresser Even if the distant star is twice as far as the near one, it'll still provide 20% of the total insolation, which seems like a plenty big enough loss to have an effect. Completely agree that the effect will be too short-lived and transient to be considered a "season", though. One could play around with the period of the binary star - longer periods would lead to longer occlusions, but they would become less frequent, so I'm not sure there's a workable solution. $\endgroup$ – Nuclear Hoagie Nov 25 '20 at 14:29
  • $\begingroup$ You don't need occlusion, the changing distances to the two stars is more than enough. The amount of energy you get from the sun falls of with the square of the distance. We stay basically the same distance from our start cause it's near the center of our orbit. But in a binary system the two stars are constantly getting closer and further away by significant amounts. Depending on the relative orbits and brightnesses of the stars you get some interesting effects, but it's basically 1 or 2 summers per binary orbit. Two when they give similar light at their closest approach. $\endgroup$ – Gordon Wrigley Nov 28 '20 at 12:18
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Why do humans measure time in years? The reason is that the yearly cycle of seasons is very important for our lifestyle. Depending on the time of the year, humans dress differently, work differently, spend their recreational time differently and eat differently. This was even more extreme when we were still an agricultural society.

The seasons on Earth are caused by axial tilt. So when your planet would have barely any tilt, then it would not experience a seasonal cycle synchronized with its orbit. That would give you space to come up with another naturally occurring periodic phenomenon to serve as a unit of timekeeping.

So when you want a similar unit of timekeeping, then you have to find a natural phenomenon which occurs periodically and has a very large effect on the lifestyle of your species.

Some options could be:

  • Strong tides. The tides caused by the planet's moon(s) affect the life of the inhabitants so much that they use it as an increment of timekeeping.
  • Biological effects. Perhaps there is some species of insects or perfectly normal beasts which have a lifecycle which is synchronized throughout their species (like cicadas). The lifecycle of these animals (or plants?) might also affect the life of the inhabitants in a significant way.
  • Periodic volcanic activity causing regular volcanic winters.
  • Periodic weather phenomena like dust storms on Mars.
  • The star itself is highly irregular and causes seasons due to varying light output.
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You could leave the seasons out of it and somewhat decouple the time counting from astronomical events.

Most people I know are pretty comfortable working with units like weeks and hours without them being directly observable in nature. We use units like this because sticking to days, years and lunar months would give us too large or too small numbers to be comfortable.

Six is a good number with some interesting properties, to the point a mathematician might even say it's perfect (precisely what this means isn't so important, but priests in your world might like this fact to justify the divine order of things). With such long years it makes perfect sense to divide it into sub-units and why not make them six? It would be like when economists often care more about quarters than years. Now, as we translate into English the word the people there use for such a unit, "year" seems like at least a reasonable translation.

You now get 3 years of each season, summer and winter, dividing them naturally into pre-, mid- and post- stages. Telling someone your age, especially if you're not that old, using solar years seems too coarse. Just like Earth parents might talk about their 18 month old baby, a parent in your world might worry about their 14 sixths old daughter who is way too young to be looking at boys the way she does (some as old as 18!).

Solar years could still be important; 3 solar years is not an unreasonable age of being considered an adult, but you can work around that. Make sure to always refer to them as "solar years" or maybe even abbreviated to "solars". Characters telling their age might just use the number, like we often do in English.

This could have some interesting effects, such as solar birthdays becoming much more important.

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Binary star:

Your answer is right there in your star system configuration, you just need to fiddle a bit with the suns.

You currently have two suns, one heavy and bright, the other much smaller?

Change this so that one star is massive, but not very luminous. This one mainly determines your orbits for the system. As stellar mass usually is a strong determinant for luminosity, you might need to make it an old black hole or something.

Let the other star be smaller, but brighter. This is your prime light source "sun" for the system. Have this one orbit in the same plane as the planets, with a suitable orbital time of slightly less than 1 year. When it is nearer the planet, the season is summer. When it is on the other side of the system, the planet experiences winter.

enter image description here

Unfortunately, while this does answer your question, I suspect it defeats its own purpose, as it forces you to redesign your solar system, which specific design is what prompted this question in the first place, sorry.

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  • $\begingroup$ Any answer to this question requires some redesign of the system. If the OP thinks a particular idea changes things too much, they don't have to use it, but the answer itself is perfectly valid. $\endgroup$ – WillRoss1 Nov 25 '20 at 16:47
  • $\begingroup$ The answer is good (+1), but 1 year orbit binary star probably makes the planet's 6 year orbit unstable. One orbit should be very larger of shorter than the other one. $\endgroup$ – Pere Nov 25 '20 at 19:07
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    $\begingroup$ @Pere It could be stable, if the planet's orbit is a resonant one with the secondary star. But yes, with the star at a full 0.248 of the planet's semimajor orbit, it is pushing the odds a bit. $\endgroup$ – PcMan Nov 25 '20 at 20:08
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"Hysterical Raisins"

"Historical reasons" are the only reason that people on another planet would measure time using Earth terms.

Already on the ISS, on the moon, and when we eventually get to Mars, we use (or will initially use) UTC, with Julian days and the Gregorian calendar.

But it stretches reader credulity to have your planet be spinning with a period of 24 hours, and your seasonal cycles be 365.25 days long, and so on.

Sure, it COULD happen, but it's just not plausible, and that strains the suspension of disbelief, or far worse, gives the reader a feeling of contempt for the author, either "they didn't even consider the orbital period would be different!" or "they couldn't even handle writing for a different orbit, and had to fudge things!"

So if you can state that they're recent Human immigrants, then use UTC and have fun playing on the way they manage their lives to handle the weird local times, then go for it! Circadian cycles won't change much, so a 34-hour day could mess people right up! :) MAKE it hard for them, and call it out. "Damn, I hate these every-third-day night shifts", type stuff. If they have a local and a UTC day, what do they call each one? If they just say "day", which one do people assume they mean? Do they instead call the local one a "cycle" or what?


But if you can't make them use UTC, then arguably the better answer is Just Don't.

For me, then, @RossPresser gave the best answer: DO NOT USE EARTH TIME for measuring time on a different planet. It's always a grating point in sci-fi when you see that. "Days" are the only reasonable thing you can steal from Earth time, and in a binary star system, even that becomes a bit vague.

If you're trying to hide that it's not Earth, then either go the "historical reasons" path and don't call out the pain points of making UTC work locally until after the "reveal", or for an alien race, just don't mention years or ages.

  • "She just started High School!"
  • "He was counting the days until he graduated from college... not long now"
  • "I'll be there in a bit"
  • "remember that real cold winter when we were kids?" (do you need to explicitly state there was only ONE winter when they were kids?)
  • "He was clearly elderly, but despite her heavy makeup the woman on his arm looked barely legal"
  • "sure, he thought, his middle-aged spread was becoming a problem, but he still had it where it counted"
  • ... and so on.

You pretty much never need to tell someone's precise age. How many real-life people do you know the precise age of, other than close family and classmates who are obviously going to be your age? For me, I can think of exactly none.

It only adds a very slight burden to doublecheck our work in one of the last edit passes to search for all date/time reference terms (year, teenager, hour, minute, second, month, twenties, ...) and replace them with sensible alternatives, but it's a very rare story which requires more time-precision than "today"/"tomorrow"/"when I get back from work".

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Seasons aren't purely dependent on (planetary) Orbital Periods

In your image, it appears you have a binary pair of stars, with one star significantly larger than the other. This implies to me that the larger of the two will be emitting the most energy and thus contributing the most to planetary warmth. If that difference is sufficiently large, we might be able to ignore the effect of the smaller star for any planet that is orbiting the pair (as Enor does) rather than being coorbital with the smaller star (as Hirr appears to be).

Except: any planet that orbits the barycenter of two stars will be closer at any given time to one star than the other, and even if we ignore the emissions of the smaller star, its mass will impart a "wobble" to the larger star. This wobble can be the source of seasonal variation.

This gives you some potentially interesting cultural effects. If we measure by "seasons" or "harvests", the time between harvests is dependent on the mutual orbit of the two stars, and could be much shorter than 6.7 years. On the other hand, cultures that depend on the stars (e.g. oceanfaring ones) would care more about the long-cycle swing of the constellations across the sky.

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Just don't use years at all

There are plenty of other units to use:

Count time in months

Precedent: the Bible
It's thought by some scholars that the insanely long lives of some Biblical characters were actually measured in months -- i.e. Methuselah died not at 969 years but 969 months (about 78.5 years).

Caveat: you'll have to infodump exactly how long a month is (orbital period of the planet's moon).

Count time in (work) shifts

Precedent: I can't find it right now but I remember a story, taking place on a tidally locked planet. They distinguished between Days (a full rotation / revolution) and years (old style Earth years), with a line something like this:

48 years to a Day .... 365 days to a Year ... so how long was a day? three shifts!

Count time in seconds

Precedent: Heart of the Comet, David Brin & Gregory Benford

“There’s rules fellow. Capture without harm or blood spilled isn’t vendetta, it’s fair coup. You work for us in Hydro for ten megaseconds -- that’s about four months, old style -- with maybe time off for good behavior.”

Or just use the longer year anyway

Precedent: Podkayne of Mars, Robert A. Heinlein

Perhaps you have seen a book titled: Eleven Years Old: The Pre-Adolescent Adjustment Crisis in the Male. I read it, hoping that it would help me to cope with my brother. Clark is just six, but the "Eleven Years" referred to in that title are Terran years because it was written on Earth. If you will apply the conversion factor of 1.8808 to attain real years, you will see that my brother is exactly eleven of those undersized Earth years old.

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    $\begingroup$ I don't understand the shifts part of your answer. $\endgroup$ – theonlygusti Nov 25 '20 at 17:06
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    $\begingroup$ @theonlygusti Companies where operations are open around the clock generally have three shifts, with a different team of workers for each shift. For example 8AM to 4PM, 4PM to 12 midnight, 12 midnight to 8 AM. $\endgroup$ – Ross Presser Nov 25 '20 at 18:37
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    $\begingroup$ The Outcasts of Heaven Belt by Joan D. Vinge may be the better example for counting time in seconds, as seconds and SI prefixes are used exclusively and pervasively throughout the story. $\endgroup$ – straycat Nov 25 '20 at 21:20
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Two different possibilities:

  1. Treat it as a translation issue. The natives speak a language that definitely is not English, and you are presenting a translation. The character might be 5 smeerpyear old, but when translated into Engish, he will be 30something.

  2. Even on Earth, there were/are some important calendars based on lunar cycles, not solar seasons (Islamic calendar being perhaps the most prominent). Your planet has a moon with an orbital period somewhat similar to our month, and the "year" has 12 months because it is such a nice round number (and/or they count in duodecimal). Or the period is 18 days, and the year has 20 months (because they have 20 fingers+toes, like the Mayans). Or any other combination. WIthout a moon, a "year" might be the epicycle of a prominent gas giant (though orbital stability probably forbids such a close gas giant). Or perhaps the "month" could be based on the length of a menstrual period.

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Touching on the same concepts as some others, if the orbital period of the stars themselves is about one year, and they occlude each other you may have a another "clock" to go by. Depending on the relative size and color of the stars, the color of sunlight itself may change significantly about once a year for a few days, as the larger star occludes the smaller star. ("The red/blue days") If it was noticeable, this would be a pretty celestially significant event, and would probably have a lot of cultural impact.

If the civilization on this planet "grew up" here, this kind of regular event would probably feature heavily in their history, like leaders scheduling great battles on the "red days" for luck, and there would almost certainly be worked into their calendars. (Our own ancient civilizations made a pretty big deal out of eclipses and regular comets.)

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This would be a commment but it is too long for that.

Is it even possible for a habitable planet to have a year 6.78 times as long as an Earth year?

At twice the distance from a light source the light will be one quarter as bright. If the two stars are equal in luminosity, the combined light from both of them will equal the light level from one of them when a planet is 1.4142 times as far away as it would be from only one of the stars.

Thus if the two stars have equal luminosity, the inner and outer edges of their combined circumstellar habitable zone will be only 1.4142 times as large as the inner and outer edges of the circumstellar habitable zone around only one of those stars.

In our solar system Mars has a year 1.88 Earth years long and Jupiter has a year 11.86 Earth years long. The asteroid Ceres has a year 4.61 Earth years long.

So a planet orbiting our Sun with a year 6.78 Earth years long would orbit somewhere between the orbits of Ceres and Jupiter. And it would be far too cold to have liquid water on its surface with an atmosphere breathable for beings similar to humans. If it had a more exotic type of atmosphere with a lot more greenhouse gases, it might be warm enough for Earth type lifeforms, but then the atmosphere would almost certainly be unbreathable for large multicelled land animals such as the natives are probably supposed to be.

If each of the two stars in your system are much more luminous than the Sun, a planet at that distance could be warm enough for Earth type life. But each of the two stars should be more massive than than the sun of they are more luminous, so their combined gravitational force on the planet should be much stronger than that of the Sun on a planet at that distance. Thus the planet would have a faster orbital speed and it would take it much less than 6.78 Earth years to complet one orbit.

And a writer can keep adjusting the masses and luminosities of the two stars until he finds a realistic, naturally occuring mass/luminosity that produces an orbital period of 6.78 Earth years somewhere in the systems combined circumstellar habitable zone.

Known exoplanets have orbital periods ranging from a few hours to hundreds of thousands of years. Since the habitable zones of stars are much narrower than the range between the closest and farthest possible planetary orbits, the range in oribital periods of planets in the habitable zones of stars is much smaller, but could still include orbital periods of tens or maybe hundreds of years.

Except that stars luminous enough to have their habitable zones wide enough to include orbits that wide and long probably cannot shine with a steady luminosity long enough for their planets to become habitable for beings similar to humans.

The best scientific discussion of the requirements for planets habitable for human beings, and thus for aliens with similar environmental requirements, is Habitable Planets for Man, Stephen H. Dole, 1964, 2007.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[1]

Dole explains that it took billions of years for Earth to acquire an oxygen rich atmosphere and become habitable for beings who breath oxygen. Dole also explains that the most massive and luminous stars remain on the main sequence and shine with reasonably steady luminosity for periods shorter than the billions of years necessary for a planet orbiting them to become habitable.

According to Dole's calculations, it should be impossible for a star more massive and luminous than an spectral class F2 star to have a habitable planet. It would also have to be luminosity class V instead of a more luminous, giant type of star.

https://www.centauri-dreams.org/2014/03/27/habitability-the-case-for-f-class-stars/[2]

So you need to find the mass and luminosity of spectral type F2V stars and calculate the inner and outer edges of their habital zones. Then mulitply by 1.4142 since your system is a binary. Then calculate the orbital periods of planets orbiting at the inner and outer edges of the combined circumstellar habitable zones from the distances and from the combined masses of the stars. If a planet oriting at the outer edge of the combined circumstellar habitable zone of a double F2V system would have an orbital period of less than 6.78 Earth years, then it would be almost totally impossible for any habitable planet anywhere to have a year as long as 6.78 Earth years.

One way out that would be to make the two stars more luminous than F2V stars, pushing their combined habitable zone out far enough for a plent in it to have a year 6.78 Earth years long. That would make the stars too short lived for the planet to become habitable naturally, so it would have to have been terraformed and given an oxygen rich atmosphere by an advanced civilization sometime in its plast.

Another way out would be to make the system a double double system. It would have four F2V stars in two pairs. Each pair might obit each other at a distance of about 2 million miles, nearly touching, and the two pairs would orbit each other at about five times that distance, or ten million miles. In such a system the inner and outer edges of the combined circumstellar habitable zone would be twice as wide as the zone around a single F2V star.

Another way out would be be to go to the PlanetPlanet blog and The Ultimate Solar system section, devoted to designing solar systems with the highest possible number of habitable planets.

https://planetplanet.net/the-ultimate-solar-system/[3]

Some of those solar systems are so statistically improbable to occur naturally that they would have to have been constructed by advanced civilizations.

And some of the most far out examples there may have set ups where habitable planets could have years as long as 6.78 Earth years or longer.

Added 12-17-2020

One way to have a habitable planet with a year 6.78 Earth years long is to have it orbit outside the circumstellar habitable of its star so that it would be too cold for life except that it has a major source of heat additional to the heat from the star.

One way to do that would be to use tidal heading from tidal ineteractions with other objects in its solar system.

Such an process is discussed in my answer dated 12-17-2020 to this question:

https://worldbuilding.stackexchange.com/questions/192131/what-are-the-upper-and-lower-year-lengths-for-a-habitable-planet-of-40-eridani-a/192221#192221[4]

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  • $\begingroup$ Well, I'm rather floaty about the math for defining planetary characteristics (Especially that of a binary system), so I just went with what Space Engine told me was the distance at which the planet would be in a goldilocks zone of a binary system with A1 V and G2 V stars $\endgroup$ – Darth Biomech Dec 17 '20 at 7:49
  • $\begingroup$ The whole system started because I wanted a homeworld that would have one blue and one orange sun in the sky. And back when I was designing it i though that the scientific consensus was that it is impossible for close binaries to have stable planetary systems, so I tried to justify both the system's existence and the fact that it has an old blue star by making the entire thing an artificial construct by an advanced precursor race that uses klarketech to distort gravity fields to keep the orbits stable by making the two stars act like one single mass at their barycenter. $\endgroup$ – Darth Biomech Dec 17 '20 at 8:03
  • $\begingroup$ Several years later the science had new discoveries with planets being observed around binaries, but by that time "you're living on a humongous ancient artificial stellar construct" became too important for the history of the species of that planet to edit that justification away, and besides that I still wanted a blue sun in the sky of their world. $\endgroup$ – Darth Biomech Dec 17 '20 at 8:06
  • $\begingroup$ The flaw in using Dole's logic for this discussion is the assumption that life has to start evolving on a given planet to exist there. If a planet in the habitable zone of an F-class star were seeded with life from some other star system, it could develop a proper ecosystem in far less time. Green algae, for example, could rapidly produce an oxygen atmosphere if it had no predators to keep it in check -- and that takes a lot less work to produce than a fully-engineered star system. $\endgroup$ – papidave Dec 19 '20 at 17:18
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Add more planetary " axis wobbles" to the orbit, that way seasons can be entirely independent of year length

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  • $\begingroup$ This really should be the answer, but can you add some more details and maybe supporting evidence for this? $\endgroup$ – Greg Burghardt Nov 25 '20 at 18:10
  • $\begingroup$ This was what originally came to mind for me too, but it doesn't work. "Wobble" is actually axial precession. On Earth it is a torque-induced precession caused by the Earth's axial bulge as it interacts gravitically with the moon and sun, and is about 26000 years long. To have significant precession in a shorter time there would have to be a lot more torque than could be reasonably exerted on a planet. Also, a precession on the order of a year would bleed off rotational energy monstrously and lead to a planet that didn't rotate. $\endgroup$ – Kurt Fitzner Nov 28 '20 at 5:48
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Presumably the inhabitants are aliens, not Terrans? If so, they just live 6.78 times slower, so that one of their years feels, subjectively, just about as long as one of our years does to us.

Think of it like "cat years," but in reverse.

(If they ARE Terrans, somehow, then you're missing an opportunity if the lengthened year isn't a plot point.)

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Your planet has a very strong axial precession, with a period of about one Earth's year. This would cause seasons very easily since the main "source" of seasons here on Earth is the fact that one hemisphere is receiving sunlight in a much more direct manner than the other.

I have no clue whether or not such fast precession can ever occur on a rocky planet, but I assume you can safely handwave that part.

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They do not divide their time into years, but into seasons. Even on Earth, "four" is not set in stone, cultures have more or less. Then they count their ages in seasons because it gives them a more useful measure of time.

(Perhaps they have different crops for each season because they can't prevent spoilage over the course of the year so that one crop a year works.)

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  • $\begingroup$ Example: Japan has a "Rainy" season that overlaps the spring-Summer change $\endgroup$ – Trish Nov 27 '20 at 8:59
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Who says your orbit must be 6.78 years?

I am by no means an expert in celestial mechanics, but it seems to me that orbiting a dual star means some wonky cool gravity effects are in play. Who's to say that they don't slingshot your planet along with a faster orbit? Some Earth-dude's simulator?

Please keep in mind that humanity is just scratching the surface of what mysteries the universe holds — which means the mathematical models we use are already obsolete. Consider this Earth-sized planet that the astronomy class I took in college said couldn't exist. The "old model" said its size should have pushed it away from the star and slowed its orbit.

Except that we found one that didn't... Cool!

Sometimes we get too hung up on the idea that what we humans understand today is "The Truth." It's not. It's just a snapshot. It's a little bit better than what we understood yesterday and a little bit worse than what we'll understand tomorrow.

Alternatively...

They're your stars, and your planet is orbiting around two of them over the course of one, shall we say, celestial year. But during that time you have two summers, one for each star you swing around. "Winter" is when your planet is mid-way (from an energy output perspective) between the two stars. And this is assuming your planet's orbit is a convenient circle. In this instance you have two seasonal years because there are two summers and two winters inside of one celestial year.

OK, now we're down to 3.39 years. Now let's use your axial tilt and assume those two stars are bookin' around each other (this is a lot less fun if their interdependent orbits are really slow). If this is the case, then in a single celestial year your planet will present a tilt-toward both stars and a tilt-away from both stars when closest to each star. (Think of it like this, the planet takes 6.78 years to circle its orbit once. During that time the stars have rotated through their orbits twice.) This halves the length of the seasonal year again. Now we're down to what I hope is a palatable 1.695 Earth years per seasonal year.

However, given that you've tagged this and not , I think it's more than enough to just push the planet around faster. You haven't explained why you're creating this star system. Let's assume you're writing a story. 99.999% of your readers won't know if what you're describing is "correct" or not, which is compounded by humanity re-evaluating what "correct" is with each new astronomical discovery. The 0.001% that would know won't care if you write a good story. If they did, they'd never read anything Tolkien wrote.

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Smaller Stars
The first thing that comes to mind is to simply reduce the combined mass of your stars in order to move the goldilocks zone closer, and subsequently reduce the orbital period of your planet. I haven't done any math here, but I imagine you could shoot for a combined total of 1 solar mass, allowing for 2 stars on the upper end of the red dwarf range (which caps out around .6 solar masses) or perhaps a mid range orange dwarf (~.7-.8 sm) and a small to medium red dwarf (~.2-.3 sm).

No Seasons
A planet with a 0 degree tilt will not experience any seasons at all. You would still have different climate bands at which to grow different crops and establish various cultures, each adapted to their region. Admittedly, this has some serious drawbacks, especially if you are going for harder sci-fi. Seasons play an important role in limiting the spread of pathogens and even allowing certain crops to grow (potatoes, wheat and corn, for example, can only grow in areas where winters are particularly cold, but summers are somewhat warm). Here is an interesting article going into more depth on this

Mild Seasons and Migratory Cultures
Giving your planet a slight tilt could allow for enough temperature variation to mitigate the issues mentioned above, among others. Due to the extreme length of each season (3 years of winter, 3 years of summer), cultures would have ample time to migrate between climate zones as things shift.

As temperatures cool they move closer to the equator, spending 6-12 Earth months at each stop, allowing for plenty of time to get in multiple harvests of the crops that thrive there, and build back up strength and supplies for the next leg of the journey. As temperatures rise, they move further away from the equator, eventually reaching their "summer home" where the ground, after a long, cold winter, is once again ready for a new crop of potatoes and the like. Since they would have so much time at each stop, they could easily establish permanent settlements at each one. Reusing these settlements year after year would make them truly migratory, as opposed to nomadic. (Also, people in the Northern and Southern hemispheres would have opposite migration cycles, which could create an interesting dynamic.)

Of course, these migration patterns would only apply to older cultures. Once systems for trade, transportation and communication are established (as well as reliable artificial heating and cooling sources), societies could remain in their settlements all year, producing crops for part of the year and other goods (textiles, tools, etc.) in the off season, importing and exporting as needed.

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Base your time system off of the relative motion of two objects instead of just one.

Your target planet in question is the second planet in the solar system. What's the orbital period of that first planet? You could define the end of a "year" as the point in time when the inner planet transits the sun.

If you're building the system from scratch, you could fine-tune the planet's moon. Use the phases of the moon to define the months (like we do), and then define the end of a "year" as the point in time when a solar eclipse occurs over a pre-defined location on the planet. You could rig the moon's orbital period to give you whatever time interval you want. Similarly, if your moon is not tidally-locked like ours is, one revolution of the moon could be a year (or season).

You can use the same principle with any number of easily observable astronomical objects to build your space-clock with whatever intervals you desire. You can add things like comets, distant constellations, etc to your system without disturbing your star/planet setup, and you should have the freedom to fine-tune their intervals without any real impact on what you already have. Perhaps the appearance of a highly-visible comet marks the end of each decade, and your definition for "year" is derived from that.

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Perhaps you could consider a more radical modification to your star system, to make an easier solution to your question.

There are thought to be two ways in which a planet could orbit in a binary system in a stable manner:

  1. A close binary, with distant planets orbiting the stellar barycentre. As others have suggested, you would probably have to look to other ways of marking time - moons, seasons, eclipses, or some other regularly occurring astronomical event.

  2. A widely separated binary, with the planets orbiting just one of the stars. To see how this might turn out in a fictional setting, I suggest you take a look at Brian Aldiss' Helliconia trilogy, in which the planet orbits the secondary star in a much more life-friendly year of 480 days. The much brighter primary really messes with the civilization every 2000 years or so.

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  • $\begingroup$ Hello Nick. Thanks for joining Worldbuilding. This is an informative comment, but it's not an answer as it doesn't describe how the OP could measure seasons the way they wish. Please take our tour and read through the help center to better understand how our site works. Thanks! $\endgroup$ – JBH Nov 28 '20 at 5:44
  • $\begingroup$ I am suggesting the OP could consider altering the scenario to provide a more Earthlike year. I think that is just as valid an answer as suggesting measuring time by something other than 7-year long years; and something no other answers covered. The link I provided also goes into how the books' characters measure time in their world - something that might serve to inspire the OP. And being an occasional contributor, I understand how this site works - questions are asked and answers are provided, and that is what I have done here. $\endgroup$ – Nick Nov 28 '20 at 5:57
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Seasons shorter than your solar year could be caused by a significantly lengthened solar day. It is not unheard of to have a very long solar day - Mercury, for example, has a solar day of about 176 Earth days. Making the day on your planet roughly 365-ish earth days long would give you the same basic four seasons we have. It would also give you some interesting talking points since Sunrise = Spring, Mid-Day = Summer, Sunset = Fall, Mid-Night = Winter. You could name your seasons such.

Intuitively one would think that the summers would be much hotter and winters much colder than what we observe on Earth, and they would tend to be at least somewhat, but not catastrophically so. For example, remember the North and South poles on Earth both go through months of "night" here and they are both livable. The atmosphere would carry enough heat to the night side of the planet to keep it from getting too cold. This could be further mitigated by placing the planet on the outer edge of the goldilocks zone to reduce solar radiation and offsetting that with increased greenhouse gasses relative to the Earth. Reduced solar radiation overall and increased greenhouse gasses would tend to mitigate the temperature differential between the day and night. Anything that served to increase the atmospheric density (whether or not greenhouse gasses are involved) would also tend to increase the efficiency that heat is carried from the day to night side.

Some thought into the positioning of land masses could be used to cause a planet with ocean currents that quite efficiently carried heat as well.

A combination of these factors could conceivably make a world with a day the length of an Earth year have little more variation in "day" and "night" temperatures than we currently see between summer and winter today on Earth.

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