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Trying to form a mostly logical basis for a world that would have one region always in darkness while also having a day-night cycle.

My first thought was to ask if a planet could have an axial tilt always facing away from its star, but upon reading other questions on the site it seems that's not possible.

My most recent idea is a moon that is tidally locked to its planet with its rotation around its planet just happening to correspond with the time the planet's orbit takes around its star. Don't know if this is logically possible?

Thoughts on that idea and/or any other possible explanation for such a world?

P.S. Just read about precession while writing this post, but it seems that would be is the realm of millennia on such a planet to reasonably occur although the length of year on this world can be just about anything if it allows for this to work and for the planet to be habitable.

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    $\begingroup$ Welcome to Worldbuilding! Kind of an important question: how big is the "region" in question? A valley? A country? A continent? $\endgroup$
    – jdunlop
    Sep 2 at 20:55
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    $\begingroup$ Also, your moon suggestion is already covered on this site. Quick summary: such an orbital configuration is impossible. $\endgroup$
    – jdunlop
    Sep 2 at 20:57
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    $\begingroup$ (Also worth noting that even if that weren't the case, you specify that the planet is not tidally locked to the star, which would mean that if the moon held its position between the star and its primary, the region in darkness would rotate out from under it.) $\endgroup$
    – jdunlop
    Sep 2 at 21:06
  • $\begingroup$ It seems that toroidal planets without axial tilt have been discussed for similar reasons. $\endgroup$
    – Caesar
    Sep 4 at 5:50

4 Answers 4

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You can achieve this, but not via your moon idea. You need your planet to have very little axial tilt, and a deep basin at or near one of its poles, surrounded by mountains.

Within the basin, the sun will never rise, although if there is an atmosphere, it will never be truly dark in the basin, due to scattering in the atmosphere. There will likely also be reflection from mountains, but that depends on the exact topography.

Earth's Moon has quite a few places like this. Of course, they're mostly craters.

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    $\begingroup$ It doesn't need to have exactly 0 axial tilt, but it needs to be small relative to the size of the mountains. The Moon has at least 300 such regions, mainly near the poles but some surprisingly far from it. $\endgroup$
    – Cadence
    Sep 2 at 21:23
  • $\begingroup$ @Cadence well, the moon is tidally locked. $\endgroup$
    – Topcode
    Sep 2 at 22:40
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    $\begingroup$ @Topcode the moon is tidally locked to the Earth, not the sun, so it still experiences a day-night cycle. $\endgroup$ Sep 2 at 23:16
  • $\begingroup$ @Cadence: Thanks, incorporated. $\endgroup$ Sep 3 at 15:30
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If a planet is tidally locked and has an eccentric orbit, then it can experience libration causing some parts of the surface to experience a day/night cycle.

The Earth's moon exhibits an eccentric orbit causing it to rotate faster around the Earth at some times and slower at other times. Thus, the point on the moon facing towards the Earth will rock back and forth slightly over the course of an orbit. As a result, some regions of the moon can permanently view the Earth, some regions never view the Earth, and some regions can view the Earth rise and set over the horizon with each orbital cycle.

If our planet is tidally locked to its star, then we could get a similar effect, where part of the planet is always in daylight, part of the planet is always in night, and a region in between has a day-night cycle. In the case of the moon, the effect is about 8 degrees, so the star would stay near the horizon and the region would experience varying strengths of twilight, but a more eccentric orbit could result in more pronounced effects, potentially even going from fully day into fully night (though observers would see the sun rise, pause, turn around and set, and then repeat, on a yearly cycle). If the eccentricity is too large, then it will likely end up in some other spin-orbit resonance (like Mercury spinning 3 times for every 2 orbits around the sun), and I'll admit that I don't know where exactly that line is.

I know you asked for a solution that doesn't tidally lock the planet, but having the planet be tidally locked in this way causes regions of the planet to have a day-night cycle and other regions not to, which might work for your purposes.

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  • $\begingroup$ Kudos for submitting your two very different suggestions as separate answers instead of throwing them into a single post. $\endgroup$
    – Tom
    Sep 3 at 1:20
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If parts of the planet experience a day-night cycle while other parts don't, then there must be some local phenomenon blocking out light from entering a region that doesn't. I propose smoke from naturally-persisting fires.

It's possible for reservoirs of fossil fuels to catch on fire and burn until they run out of fuel to burn.

For example, there is the "Door to Hell" in Turkmenistan, formally called the Darvaza gas crater, which has been burning for over half a century: "Door to Hell" gas fire still burning This particular incident was caused by human activities, but natural geologic and meteorological phenomena could cause the same effects (not likely, but it can happen).

We need two circumstances to line up: the fire needs to produce a bunch of smoke, and the local conditions need to trap that smoke in.

For the fire producing smoke, we can use a burning coal vein, like at the Centralia Mine Fire in Pennsylvania, USA, or we can burn impure natural gas that contains contaminants that produce sulfurous and nitrous oxides.

To trap the smoke in, all we need are nearby mountains that make it harder for air to leave. For example, the LA basin had this effect, with mountains on three sides and an ocean on the fourth, that trapped smog in very effectively (and caused LA to have massive issues with smog before modern clean air regulations). You could get a similar effect with mountains on all sides, if you have a place like that.

If those conditions line up, then you can have a fire burning that fills an area with smoke. If the smoke is strong enough, we might even lead to night-like conditions permanently in an area. Though do note that the smoke will also result in very poor air quality, with people living there also inhaling a bunch of smoke (expect a lot of lung problems from anyone living there).

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shade

enter image description here

It is shady on the forest floor. Shadier yet under the kelp forest. Your planet has floating photosynthesizers greedy for the light. Underneath is always crepuscular and gloomy.

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  • $\begingroup$ Good start of an answer, but OP specifically requested that one region of the planet be in permanent darkness. Why are the floating photosynthesizers particularly concentrated in one part of the globe? $\endgroup$
    – Kirt
    Sep 4 at 19:22
  • $\begingroup$ @Kirt - why are the redwoods only in one part of the globe. Or the kelp. Soil is good? They like the climate? High winds are rare? Outcompeted everywhere else? $\endgroup$
    – Willk
    Sep 4 at 21:59

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