I'm drafting up a world for a pet project of mine. I came across the idea of "what if there was a much longer night every X days?" I thought that would be an interesting theme for the world, so I'm trying to explore it.

My first thought is that if there was a very large cloud of debris in just the right orbit, it might cause an extended eclipse, of sorts. To someone living on the surface, without telescopes, it would seem like there was a regular period of darkness.

But, I'm not very familiar with cosmology/orbital mechanics, and I wanted to see if this was a plausible. And if plausible, what the necessary conditions might be.

  • 1
    $\begingroup$ Floating debris doesn't orbit. $\endgroup$
    – RonJohn
    Jun 17, 2022 at 1:52
  • $\begingroup$ (Wouldn't it be "naturally" instead of "artificially"? 🤷‍♂️) $\endgroup$
    – Jason C
    Jun 17, 2022 at 15:27
  • $\begingroup$ This might be worth looking into: youtube.com/watch?v=1VPfZ_XzisU ... it wouldn't be caused by orbiting debris but could create a much longer day / night cycle every once in a while. Though you would have to have an oddly shaped planet. $\endgroup$ Jun 17, 2022 at 16:38
  • $\begingroup$ I imagined a planet orbiting a star that used to have a Dyson Sphere, or had an unfinished one. Or even better, has one in construction in that very moment, in detriment of unaware planet inhabitants. Edit: I see someone else commented that :P $\endgroup$
    – Opazo
    Jun 17, 2022 at 18:04

5 Answers 5


Your world could be a moon of a (large) planet.

That way, if your moon rotates along it's own axis, you would have a day/night cycle, but whenever it's behind it's planet it turns dark. You wouldn't get very long nights this way, even from Io, the closest large moon to Jupiter, Jupiter only takes up about 1/18 of the elliptic. I could do the numbers, but I doubt you would get more than a few hours of extra night time, tops, if you want to keep things moderately realistic.

If you want to handwave how your world sticks together under the tidal forces, you could use this mechanism to make very long nights, though!

Edit in response to @DarrelHoffman: Kepler's third law tells us that the period $P = \sqrt{k a^3}$, where $a$ is the average distance and $k$ some constant. Assuming a circular orbit for simplicity, the length of the orbit is $O = a \pi$, and we may divide these numbers to find the speed of the moon $v = \pi^{-1}\sqrt{k a}$. Specifically, the speed is monotone in $a$. If we assume the light rays are parallel, the length of the part of the orbit that's behind the planet is fairly constant, unless the moon is so close it's ridiculous. All in all, the time spent behind the planet thus is decreasing. So sorry, there doesn't seem to be any sweet spot!

What occurs to me now would be to let the moon have a very eccentric orbit! That way, you could let it periodically end up behind the planet when it's at its furthest away, and therefore when it moves the slowest. You probably couldn't make it that excentric in practice, but possibly enough to make some different. This orbit would also make for very interesting seasons, and would likely make astronomy and/or astrology develop in interesting ways!

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    $\begingroup$ Io is nowhere near the closest moon to Jupiter! en.wikipedia.org/wiki/Moons_of_Jupiter#List $\endgroup$
    – levininja
    Jun 17, 2022 at 3:10
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    $\begingroup$ @levininja while technically correct, you comment seems besides the point, considering there are only four closer moons on that list, and all of them are tiny. We're talking about a world that presumably has some civilisation on it, not some random chunk of rock. $\endgroup$ Jun 17, 2022 at 9:31
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    $\begingroup$ Wouldn't it be better to pick a more distant moon? Sure the planet would appear smaller the further away you get, but the orbital period also increases for more distant moons. Obviously if you get too far out, the planet doesn't appear large enough to block the sun for very long, but there must be some sweet spot that maximizes the degree of sun blockage on the far side. $\endgroup$ Jun 17, 2022 at 13:42
  • $\begingroup$ @JordiVermeulen the question didn't specify anything about what kind of world or what civilization needs to look like (in domes or whatever) and that's irrelevant anyways as this answer just made a carte blanche statement that Io was the closest moon to Jupiter $\endgroup$
    – levininja
    Jun 17, 2022 at 14:44
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    $\begingroup$ @levininja There. Is this better? $\endgroup$
    – EdvinW
    Jun 21, 2022 at 21:55

Natural debris? No, not really. At least, not unless you are in a very dangerous star system with lots of asteroid impacts--and even then, it would be a periodic dimming, not a total blackout.

A mysterious alien megastructure, like a partial Dyson sphere? Sure. Heck, there are even proposals to that kind of thing on purpose for climate control.

If you want an entirely natural explanation for varying day/night lengths, you could just put your planet in a binary star system. When the stars are far apart in the sky, days will be long and nights will be short. When the stars are close together, days will be short and nights will be longer. In this case, however, the maximal length of a night would be the "normal" nighttime--half a rotation period--with nights usually being shorter.


Very Possible!

Depending on your definition of "extended," this already happens with the moon. Consider also that Saturn's rings do cast a shadow on the planet. In any case, nothing in orbital mechanics prohibits this!

As a side note, I doubt a cloud of debris would have the same effect as a solid body like the moon unless it was REALLY BIG. Smaller debris clouds would dim light shining through, but the light would still go through at random points. There is also the issue of multiple reflections within the debris cloud, which allows more light through than a solid body.

How to Solve This

In theory, any orbiting body of sufficient size could extend nighttime with the right size, distance, and speed.

Specifically, if you wanted exact numbers, you need an angular velocity of the orbiting body relative to the perspective of the planet to extend this darkness time. Once you have this (and how long it extends night by), you can then solve for mass and distance of the orbiting body.

This will give a family of solutions, actually, because of the combined effects of mass and distance. So you will need to get an equation for the apparent size of the orbiting body, and fitler down your family of solutions by parameters that fit. To an observer on the surface of the planet, full eclipses are only possible when the apparent size of the sun is equal to the body providing the eclipse.

You may still have multiple solutions here, so this is where you get to choose a mass or size and solve for that.

Some Examples

A large enough sun-shield in a particular orbit would permanently shade a spot on the planet: specifically a Lagrange point of the planet-sun system. Go a little higher or lower in that orbit and this dark spot will move (more or less slowly, depending on the orbit).

Another option is a moon in a highly elliptical orbit: it will spend a lot of time in the outer range of its orbit, potentially causing longer nights/eclipses. This adds a little complexity to our approach by varying the apparent size of the moon, as it looks smaller the further it is from the planet and still needs to dim/block the sun. There is a much smaller "sweet spot" for this situation than if we decided to use mostly circular orbits.

  • $\begingroup$ "A large enough sun-shield in geosynchronous orbit would permanently shade a spot on the planet." - not quite. An object in geosynchronous orbit merely stays above the same spot on the surface, but the sun's rays do not usually fall perpendicular to that spot on the surface. The object only casts a shadow on the spot it orbits above at solar noon. At any other time, the shadow falls elsewhere. $\endgroup$ Jun 17, 2022 at 13:50
  • $\begingroup$ @NuclearHoagie Sorry, my bad. Editing... $\endgroup$
    – PipperChip
    Jun 17, 2022 at 22:36

Natural cause? Can't come up with a new one.

Consider a previous civilzation that put up a sun shield to reduce overheating. At the height of the civilization, it was kept perpendicular to the line between the star and the planet, and it was kept at the L1 lagrange point between the star and the planet.

L1 is unstable, so it now drifts. And the control mechanism no longer keeps it perpendicular. So you get periodic partial eclipses. If it was highly reflective, you would also have a real glare when the sun and it made the right angles. Be difficult to predict.


This would happen even without debris cloud. If rotation speed of planet decrease causes nights to last long. Another dense planet might cause the reduction of rotation speed.

And for the debris cloud, here is my answer, a debris cloud can cause long nights, but it depends,

  1. Size of the home star of the planetary system.

    • If the star is small, more light would block by the debris cloud.
  2. Distance between planet and star.

    • When distance increase, the amount of light fall on to the planet decreases, so when the planet encounters the debris cloud it is more likely to have darkness.
  3. Thickness of debris cloud.

    • The more thickness have, it is harder for light to penetrate.
  4. Materials that debris cloud made up of.

    • None reflective dark-colored (black) opaque debris would do the job.
  5. And for longer nights, the debris cloud and the planet should revolve around the star towards the same direction with different speeds.

    • This make nights that exist for days, even weeks long. (Relative motion)

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