My moon is the basis for a fantasy setting, but I just can't bring myself to abandon science and say "a wizard did it." I want to make the months and seasons realistic, and I want to find a way to make this, or something like this, work. I'm hopeless in math, so I'd like to enlist some help here.

The gas giant probably needs to be larger than Jupiter because its planet is approximately the size of earth. It orbits twin suns in a binary system (the suns circle a mutual center of gravity). I want my moon's day/night cycle on the habitable moon to be six months, meaning a "day and a night" are the same thing as a "year." It's kind of important for the cultures I'm creating.

I'd prefer a way for both sides of the moon to get some sun. I'm also aware that eclipses are inevitable.

So here are my questions:

  • How long should the orbital period of the moon be, how far away, and how much would it dominate the sky?
  • Axial tilt?
  • What would the sky look like throughout the seasons, and what might the seasons be?
  • Can I use atmospheric pressure and high winds to equalize the (obviously extreme) temperature differences?
  • How might my habitable moon be affected by the rings and other (much, much smaller) moons of this gas giant?
  • Is there any way to limit (though not stop entirely) the vulcanism that would doubtlessly plague a moon like this?
  • Can I build a remotely earthlike planet – in terms of climate – in a scenario like this, even taking into account the above mentioned temperature differences?
  • $\begingroup$ worldbuilding.stackexchange.com/questions/25318/… features some helpful maths for considering seasons in binary star systems. That question only has a planet orbiting one sun, and the day/night cycle is already pretty complex. If you add a gas giant and also say that it orbits both (presumably in a figure 8) then this question becomes pretty gnarly. $\endgroup$
    – Joe Bloggs
    Commented Oct 2, 2015 at 11:31

2 Answers 2


I don't think that this setup will allow for a binary day/night cycle, no matter what rotations you use. I'd love for someone to prove me wrong but I think that having two stars just makes the day/night cycle too complex.

Consider the following: If the gas giant orbits both stars, then at some point it will lie directly between the two stars.

If the moon makes one orbit around the gas giant in this time it's going to have two major states: one where it is illuminated by both stars and only a tiny sliver of the moon is in 'night' and one where one half of the moon is illuminated and the other half is in night, having been occluded by the body of the planet. No matter what rotation you give the moon this leads to there being a distinct difference between the length of the day in one state and the length of the day in the other.

Then you can assume that at some point the planet is going to be on the 'outside' of the two stars, and will only be able to 'see' one star.

At this point the moon has two major states: Total darkness, or half of the moon illuminated. It alters between these states with a period equal to its orbit around the planet, and no matter the rotation of the moon for at least a portion of it's orbit it's going to be night all over.

All of the above assumes that everything is orbiting in the same plane and in a non-eccentric orbit, because it gets even worse if you throw in inclination and eccentricity as you then have to worry about day/night cycles at different latitudes of the moon (and the planet).

All things considered: This is a situation where it's acceptable to not even try justify how your world works, and just say that it happens. Maybe throw in a tactical dust cloud that obscures the stars at a critical point?

  • $\begingroup$ Thank you. Until yesterday my "moon" was going to be a planet with a long day, set up similarly to Venus but a little farther from its sun. Reading here yesterday I wondered if I could make a system like this work. Long days are more important than being a moon, so I'll take the long days and stop worrying about trying to make it more "exotic." $\endgroup$ Commented Oct 2, 2015 at 14:24
  • $\begingroup$ [quote]Consider the following: If the gas giant orbits both stars, then at some point it will lie directly between the two stars.[/quote] Not necessarily. The orbit of the planet can circumscribe both stars. If the stars are red-dwarfs, the gas giant can be very close and orbit around the stars in a matter of days.If the moon's rotation around it's axis is slow enough, you'll get a long day/night cycle. Take Earth for example, move it within Mercury's orbit - now the year is less than 88 days. The Lunar day is about 30 Earth solar days so pretty close... $\endgroup$
    – ventsyv
    Commented Nov 20, 2018 at 21:51

your question maybe would take better place at the astronomy-section of stackexchange... or maybe not. Anyway, while math is my enemy too, I came across such questions while programming a tool for astronomy purposes.

The System

First of... design a stable binary system, before you place your living beeings. There are many out there in space in reality, so its plausible to assume these can hold planets with lifeforms too. But one point creates headache at my side: you want your moons planet orbit both suns. He would orbit the... uh, what was this name... barycenter (?) of these suns in that case, getting pushed around by one of the two suns on a regular base... hell, I don't want to this planet, when it and his two central bodys form a conjunction. Here the math kicks in, and I have to fall back to speculate, that it might rip apart a big gas giant if he needs to be close enough to be inside the habitable zone of this system.

But! Most binary and trinary systems don't have their suns do a close dance around each other. Prove me wrong, but I think in most cases the second sun acts as a remote planet, that happens to do stellar nucleosynthesis. So the second sun orbits the central sun faaar away - think about the oorthian cloud, or what is name was - and have to use what was left of material when the first sun collected itself. So it might be a red dwarf.

Another thing - two real close dancer suns will eat each others material - the bigger one can steal stuff from the smaller one, if close enough. That might result in a pretty spontaneous combustion of the small one. But to make this work, the big one need to be pretty big (H or O class? Hmmmm... no, any class super or hyper giant may be sufficient). At least the "bright" ones tend to have pretty short lives... short enough, to go supernovae and destroy their small follow star before any life could develop.

Well, I would recommend a decent stable system, where star number two is a far orbiter. Feel free to update my knowledge about this if I recalled this wrong.

The Planet

Big gas giant, even bigger than Jupiter? Don't go to far... let me kick my brain... but I think over 1*10^28kg of mass this thing will fuse some low level nuclearsynthesis. Other words: you might get a brown dwarf. They consume a special element I can't remember (something containing a y?), don't do what real stars do but would be much warmer than a common gas giant.

Even a Jupiter size gas giant will be a bad place to orbit around. Ever heard of jupiters music? Feel free to google this :) But anyway, jupiter size gas giants may be pretty ray bursters. This may encourage evolution, but wouldn't be a pleasureful place to life for human like creatures. Okay, I may overestimate the power of the emitted rays, but that something you might think about.

A planet this big will catch most stuff in its orbit. I think astronomers call this "clean its orbit". Due to many millenias every piece that came across the gas giant will be slung away. It can go away, it can become a new satellite, or it may impact your moon. When life there did grow naturally, most of these planet killing stones should have gone... but maybe not all. Oh, same count for your moon. It will clean its path, so if its an earth-like stone planet, you probably won't have any rings at this gas giant.

The moon

Ehm... well... you know... to get a orbital period of one earth year for a planet (moon) about the same size (and mass!) as our beloved earth, you need a earth-like orbit.
So... about 150.000.000 Kilometers. If you have a sun-like star AND your gas giant inside the habitable zone, this would get... warm in summer. Very warm. Well, you moon-earth would slam inside the central sun or get catched by it in a mercury like orbit. Hm... one point I'm not sure about right now is... how much closer to its central body a satellite may come when taking the central bodys mass into account. I think you won't need the whole 150.000.000km, but... hm... 15 Million maybe. Argh, I have no access to my program here, but I think someone who can calculate this will pop up in this thread pretty soon :) Or use one of the free calculators in the internet... but to make a bet: to have an orbital period of one year for a planet of a earth like mass in 15 million kilometers semimajor axis, you would need a central body of 1/100 sun masses... that is... a brown dwarf? sigh there is a reason I need a program to guess stuff like this. whats 1.9 x 10^30 / 100 ? 1.9 x 10^28? well, that would be pretty brown dwarf, wouldn't it?

But! but, but but... You want a bound rotation of your moon. That means your moon needs to be close to you gas giant. close enough, that the gravity of the gas giant can friction break your moons rotation using waves, air and even continental plates. So... this close, you will receive lovely amounts of hard em rays and get a forced orbital period of... some weeks in best.

But you say "lol its magnet field would neutralize this noob"? But we need to take care of the seismic activity and vulcanism, which requires a moon with death core. Correction: a earth size planet is likely to have a liquid core very long. Even Mars and Venus should have one, Mercury... don't know. Hell, didn't the biggest of Jupiters moons claim a magnet field? It looks like all your creatures can do is embrace the natural disasters. But anyway, once rotation got bound, things will calm don I think.

How to neutralize the temperature? Okay, I'm off here. If you don't want to get your moon scorched by the central sun, it should be at the outer range of the habitable zone. But... here a brown dwarf would be useful, because it does emit heat I think. So the side bound to the gas giant / brown dwarf may receive enough energy to allow liquid water, while the other side... you may install a ice princess over there.

To expect what the seasons be like, I would like to wait for what moon - planet - binary sun constellation you vote at the end. Because it will matter a lot. To answer you last question... maybe. If you can accumulate enough energy from what sources available to get about the same level as we do have on earth, it may work. Get this energy from a sun, or two, a brown dwarf, an active gas giant... but don't get too much, or you get a Venus, and too less will create you a Mars. Then avoid heavy hard rays, so no blue or white central star and a peaceful gas giant. To avoid heavy changes of temperature and climate, you need to stick in a narrow piece of your habitable zone, so no wide orbits around that gas giant (needed for bound rotation too), which means you have to forfeit the six month day/night I'm afraid of. But you can give your gas giant a slight elliptical orbit, so it will bring its moon closer to the sun half of its year, which may be... well, I didn't talk about this, but if your gas giant wants to be stable, you may need orbit him in a distance that take many years for a single circuit. That would make a maya-civilization happy, because their calendar would work pretty well telling when the next "summer" will arrive.

So. I'm out of words for now. And interested in your decision about this system at the end.

Have a nice day

Edit: Oh well, that answer in that first comment to your questions is 10^28 times better than my stuff... :( keep with that

  • $\begingroup$ Ultimately my "moon," now a planet again, will either be in the habitable zone around a single star or in an orbit similar to Kepler-47c (a gas giant orbiting the suns of Kepler 47, the only known circumbinary planet with a stable orbit in the habitable zone). Thank you for talking me out of a crazy idea. :) $\endgroup$ Commented Oct 2, 2015 at 14:35

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