I would like to create a world that is at a roughly medieval level of scientific understanding and which - as far as it appears to the inhabitants of that world - has a highly erratic day and night cycle, with long periods of 'day' (of many Earth months in duration) and very unpredictable periods of 'night' (that would be less frequent than 'day', sometimes long, and unpredictable in length for the inhabitants of the world).

I know similar questions have been asked before (see links below) but I think my question is slightly different as I'm not looking for an ACTUALLY erratic or unpredictable day or night cycle, just something where perhaps the patterns involved are so complex and take place over such long periods of time that, to observers on the world's surface who have only medieval technology, it would SEEM erratic and random. I would, however, like to understand the science of how it might work myself so I can have a basis for working out how the population would experience things.

Here's what I thought:

The world is actually an atmospheric moon orbiting a very large planet (probably a gas supergiant) which is itself in quite an eccentric orbit around one of a pair of binary stars that are in a very complex orbit with each other, something along the lines of the image below.

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That would already give a very complex pattern to SEASONS as the gas giant the moon orbited would sometimes be near both stars, sometimes near one and sometimes near neither. It would also account for there being more periods of 'day' than of 'night'.

It doesn't help with creating what appears to be a very erratic day / night cycle for the population of the moon though, so then I thought that if the rotational speed of the moon itself, as well as the speed of its rotation around the gas giant, were slow enough then there might be reasonably frequent (yet complex in pattern) periods where both suns are behind the gas giant and then the people on the moon would experience two types of 'night' over an extended period - firstly, a very dark and starless sky as they face the gas giant which is blocking out both suns and secondly a still dark but now starlit sky as they face away from the gas giant, which is still blocking out the suns. The star / no star transition might be fairly rhythmic as it would just depend on the rotation of the moon but how long the overall experience of darkness would last would presumably be too difficult for the population to know.

Such an alignment would presumably come up much less often than one in which there were either periods of perpetual 'day' (when at least one of the suns was visible whatever the rotation of the moon) or a reasonably regular day / night cycle (when at least one of the suns was visible at some point in the rotation of the moon) but - as far as I can see - could account for the PERCEPTION of an unpredictable day / night cycle with occurrences of long, extended 'nights' every now and again.

So, my questions are:

  1. With a modicum of handwavium, does this seem to present at least a plausible (if highly improbable!) explanation for the EXPERIENCE of an erratic day / night cycle for the inhabitants of the moon?
  2. If not (or even if so), any better / simpler proposals?

Many thanks!

PS - I'm aware of the Three Body Problem but don't want the world to be regularly razed by fire, even if it might get uncomfortably hot at some points!

Previous threads read:

How could a planet have erratic days?

How do seasons work in a binary system (planet orbits one star, not both)?

Realistic ways a Planet or Large Moon could have Variable Day/Night periods

Could a body of orbiting debris artificially vary the length of night?

  • $\begingroup$ Close, ish, orbits around Kerr black holes are very chaotic if that is what you mean. en.m.wikipedia.org/wiki/… $\endgroup$
    – ErikHall
    Oct 19 at 16:10
  • 1
    $\begingroup$ A significant problem: It does not take advanced technology to find patterns in the events in the sky. It just needs a society that will make and collate observations over a few lifetimes, and can do some very basic mathematics. For example, Earth's cycle of eclipses was known to the Neo-Babylonian empire's astronomers. $\endgroup$ Oct 19 at 16:38
  • $\begingroup$ @ErikHall - thanks, so the two suns could be in chaotic orbit around such a black hole. Would that (at times) be visible from the moon? $\endgroup$
    – Curious-7
    Oct 19 at 16:51
  • $\begingroup$ @JohnDallman - Absolutely, Kepler and co were doing that way back when! But I’m thinking that our solar system is relatively simple (compared to my proposal) so, if the patterns took 100s or 1000s of years to repeat due to the complexity then such observations wouldnt have been possible for the population of the moon yet… what do you think? $\endgroup$
    – Curious-7
    Oct 19 at 16:53
  • 1
    $\begingroup$ Beware, anything hugely complex is almost certainly unstable. $\endgroup$ Oct 22 at 15:34

2 Answers 2


Not naturally. Erratic orbits are temporary over geological time and rapidly fatal over the course of a single orbit, as you probably discovered from your look at similar questions.

If you don't mind having a technological interaction and replacing "world" with "region", you could have an erratic day/night schedule on the dark side of a tidally locked planet as a result of some prehistoric orbital mirrors with old, malfunctioning station-keeping thrusters.


My offer to you: The planet with two-and-a-half suns, with the best new years eve fireworks one could ever ask for!

It is nothing very spectacular, sadly, but it may be plausible up to a point.

What should be avoided

First off, I think any erratic orbit (no matter if its done by the Gas Giant or its Moon) will doom that world to never ever develop life in the first place. If your Point of Interest (lets refer to it as Planet of Interest, PoI, from now on) is switching between extreme heat and cold on a global level, you will end up with a population of Tardigrade as your most complex life form.

Or it just gets ejected out of your solar system. Than even those would have a hard time.

Also, please refrain from adding Black Holes to your System. Yes, you can have a perfectly stable orbit around one, but if it finds something.... to eat, expect a nice shower of Gamma Rays and other nasty stuff, which isn't the most healthy thing to get exposed to.

What to do?

That being said... there may be a setup that could be able to deliver the result you need.

Disclaimer: I am idling at work (waiting for databases to finish copying), so this is all but a thoughtful constructed scenario. But it could be a base to build upon.


  1. Make PoI a tiny winy bit bigger than earth, and add a very active dynamo core for additional magnetic shielding!
  2. forfeit the erratic orbit an a local scale, but keep your PoI around a gas giant (lets call it GoI). Also, make the Gas Giant a failed sun... a Hot Jupiter, or a Brown Dwarf.
  3. put GoI into an orbit around something a tad smaller than our sun, lets say a K class star (KoI)
  4. put that KoI into a wide circumbinary orbit with... say... a F class, or better, a big G class (otherwise it would go boom before your PoI could get a medieval society maybe). Also, it needs to have a certain level of brightness when observed from PoI. (we still call it FoI, because GoI is already in use)
  5. One thing to note: we are working with very delicate orbits here... GoI needs to stick around the very outer rim of KoI's goldilock zone, but not to far, or it would get pulled off by FoI... I am NOT certain that this is stable in the long term (see the note below Bonus at the very end)
  6. offer the GoI a nice ring system, like the one Saturn does own.
  7. dot the space between KoI and FoI with A LOT of mostly pure ice asteroids (maybe failed water worlds, that ended up in an erratic orbit and got a way-to-close transit around KoI or FoI) at the same ecliptic as GoI, with intersecting orbits.
  8. Change the inclination of PoI, so it does not share an ecliptic with the Ice astroids... and the ring system!

The end result

What does this setup offer you?

Three light sources (GoI, KoI and FoI), that - although predictable - will slowly change while KoI and FoI are dancing around each other, at least if observed from PoI. Over the course of thousands of years. Well, yes, both stars will stick around at the same position somehow, but PoI will observe a change during its seasons. Some 500 years ago, KoI and FoI did an eclipse during summer, now its during winter.

Thinking about it, a orange on white star eclipse must look phenomenal during sunsets! Also, during a time, where most people thought the sun revolves around the earth, having two (and a half) suns will be a tough riddle to solve for their Copernicus and Galileo copy-cats.

But we are not done yet. You also get an ice ring system, that will reflect light. Because PoI has a different inclination compared to these, they will be visible all over "a year". Also, keep in mind that these rings will not be as stable as the ones Saturn owns. I think his ring is made from a moon that got to close, which was already in some kind of stable orbit. Your rings will be made of ice asteroids that are caught and ripped apart mid-orbit. Their... decay might even be visible during a human life time, or faster

While the ring system will fade over time, crossing orbits with an erratic system of ice meteorites will offer a lot of replenishment to that ring system. Which will be non predictable to anybody with medieval technology. Hell, it would be hard to predict for us too!

And as the end result, your PoI folks will end up with two (a half) suns that reflect on ice rings which come and go without being very predictable. Also, they will get barraged by ice meteorites twice a "year", when PoI passes through the ecliptic of the rings. But thankfully, we thought to bring a strong magnetic field and a bit more air to fend off these. In most cases...

Happy new year!

Bonus: add eccentric planets to the system, that can send even more irregular ice asteroids in the general direction of GoI; if the albedo allows it, they ALSO can offer a nice shine during the... night. Do not forget, that the KoI star would have only a small area around it, where its plants would remain stable! So everything may be more packed. Still, GoI would claim most of the habitable zone around KoI for itself.

... now I feel the urge to model this system in Universe sandbox. I may come back to edit a screenshot of that system in here.


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