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I'd like to build a world that has one pole where it's constantly day (all year long) and the weather is very hot, and one where it's constantly night (all year long) and the weather is freezing.

I'd like for the area in the middle, especially around the equator, to be more temperate and have a day/night cycle of sort (most settlements would be there).

Is there a way to explain it scientifically, maybe through the positioning of multiple stars or other celestial bodies?

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    $\begingroup$ The day/night pole is easy to do with a tidally locked planet, but that has... impliciations (Crazy winds all the time, among other things) Perhaps a super-bright moon could provide the day/night cycle for the equator, but I'm not sure if that's possible (Else this would be an answer!) $\endgroup$ – Andon Jul 19 at 21:34
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    $\begingroup$ This questioned has been asked numerous times, or the gist of it, regarding tidally locked worlds. The short answer is no, it's mechanically impossible for a planet to have a pole fixed toward the sun as it orbits. $\endgroup$ – rek Jul 19 at 21:36
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    $\begingroup$ I know it's impossible if you have only one star, that's why I asked if something can be done through other stars and celestial bodies (either shedding light or casting a shadow). As for tidal lock, that doesn't give you a day/night cycle at the equator, does it? $\endgroup$ – Mr_Bober Jul 19 at 21:52
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    $\begingroup$ It is possible to have a consistent day/night cycle at the equator only, with all other places on the planet only having balanced days and nights at certain times in the year. If that is acceptable, I will write an answer. $\endgroup$ – Bilbo Baggins Jul 19 at 22:07
  • $\begingroup$ @rek I believe it's possible ir the planet has an 8-shaped orbit around 2 stars. $\endgroup$ – Renan Jul 19 at 22:19
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Your planet is a rogue planet near the center of an active galaxy.

Active galactic nucleii, AKA quasars, produce enough light to create a habitable zone tens to hundreds of light years wide, rather than a measly fraction of an AU. Orbits in that habitable zone can take thousands or millions of years--i.e., timescales that are relevant to ecology, and much longer than any human civilization has lasted. You just need to set your story close to one of the solstices, so it's summer at one pole and winter at the other. On the scale of the lifetimes of everyone in your story, and their grandparents and grandchildren, it might as well always be light at one pole, dark at the other, and cyclic at the equator.

There will also be random fluctuations in light levels as larger or smaller masses get sucked into the quasar's accretion disk, varying on timescales from hours up to months, but you can just say by way of authorial fiat that the particular galaxy this world is in happens to have a remarkably stable quasar which has been putting out roughly the same power for millions of years, and will continue to do so for as long as your story lasts.

If you want to have well-defined years, just make it orbit a small star that provides insignificant light and heat (like a red dwarf, or even a brown dwarf), or a dead star like a neutron star.

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  • $\begingroup$ Voted this up because I think it actually works better than my answer. A solar year millions of years long effectively does the trick. Only thing that might be an issue is that the density of stars near the center of galaxies is very high, so I find it kind of strange that a planet revolves around a galactic center without its orbit being interrupted by anything else. Also, that life there isn't constantly being wiped out by nearby supernovae. $\endgroup$ – Gilad M Jul 20 at 10:21
  • $\begingroup$ What kind of effects would this have on climate and seasons, especially around the equator? (Should I open a new question for details about this?) $\endgroup$ – Mr_Bober Jul 20 at 13:51
  • $\begingroup$ I don't think this would work; see quora.com/…. Quasars occasionally have massive gamma ray outbursts that would fry all life forms anywhere near it. $\endgroup$ – Bilbo Baggins Jul 20 at 18:42
  • $\begingroup$ @BilboBaggins True, it's marginal. But that's why I specified that this world just happens to be in a galaxy with a remarkably stable quasar. It may be unlikely, but it is at least physically possible. $\endgroup$ – Logan R. Kearsley Jul 20 at 19:02
  • $\begingroup$ @Mr_Bober Definitely open a new question for that. The details are too involved to explain in a comment. $\endgroup$ – Logan R. Kearsley Jul 20 at 19:04
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Your unfortunate world is tidally locked... and had a relatively recent collision with another planet.

The planet orbits close enough to its star that it would normally be tidally locked, so in the absence of other factors, one side of the planet (which we'll call the south side) would constantly be facing the sun, and the other side (which we'll call the north side) would always face away. That gives you your eternal day and night at the poles.

For millions of years, it was tidally locked. Life sprang into being along its temperate equator, never knowing anything but eternal dusk, sandwiched between hemispheres of cold darkness and scorching daylight.

Then another planet, small compared to your world but still more massive than any asteroid ever to strike the Earth, collided with it. Since then, the momentum of the impact caused the planet to wobble. Not enough to fully break the tidal lock, but enough that a band near the equator spends half its time facing the sun and half its time facing away. This, effectively, gives you the day-night cycle you're looking for. The life that miraculously survived the impact has since recovered and has adapted well to the pattern of night and day, just like life on Earth has.

Eventually, tidal locking will slowly take over again, causing the wobble to fade, but this could take anywhere from a few million to hundreds of millions of years, depending on various parameters of the planet's orbit. In the meantime, this scenario fits your requirements to a T!

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    $\begingroup$ That doesn't give a you a day/night cycle along the equator--it gives you a cycle along the terminator, which is at 90 degrees to the equator. $\endgroup$ – Logan R. Kearsley Jul 19 at 22:40
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    $\begingroup$ A collision with a body massive enough to turn the planet, even a little bit, would completely shatter and melt the crust. Life would have to evolve after the planet cooled and the crust solidified again. $\endgroup$ – A. I. Breveleri Jul 20 at 2:48
  • $\begingroup$ A habitable planet is a massive object, and only very powerful forces would cause it to wobble in the manner you describe. The planet would have to be subject to a strong gravity gradient, so it would have to be very close to a massive primary, and you would need to explain or at least consider why the flexing due to tidal forces did not keep it molten. $\endgroup$ – A. I. Breveleri Jul 20 at 2:54
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    $\begingroup$ A tidally locked planet with a highly eccentric orbit would present a terminator that moves back and forth, at least a little bit, due to "libation". The rotation period of a tidally locked planet is equal to its average orbital period, but its speed along its orbital path is not constant, being greatest when closest to its star. $\endgroup$ – A. I. Breveleri Jul 20 at 3:01
  • $\begingroup$ @A.I.Breveleri: That of course libration (as in equi-librium, from libra, a weighing balance) not libation (a pouring of a liquid as an offering to the gods etc.). $\endgroup$ – AlexP Jul 20 at 6:55
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A habitable moon of a moon of a gas giant.

The gas giant would have to be tidally locked or else the habitable moon would not have the eternal day/night.

The moon of the gas giant would need to rotate at 90° axial tilt. The axial tilt would be needed to create a stable orbit in the right spot for the habitable moon.

The habitable moon of the moon would not have a lot of axial tilt (around 10°) but it would have to have some or else there would be no day/night cycle along the equator.

enter image description here

The moon of the moon would have a regular day/night cycle exactly on the equator, with the length of days increasing when traveling north and decreasing when traveling south. From 10° N up to the north pole would be eternal day and from 10° S down to the south pole would be eternal night.

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    $\begingroup$ Sorry -- no matter how complicated the orbits, revolutions, and rotations involved, there is no mechanism that will prevent the pole of the habitable body from always pointing to the same place in the sky (on the time scale of observable days and seasons). In your diagram, a quarter of a (gas giant) year later, the arrangement of bodies and orbits will look the same but the star's light will be shining from the side instead of the top. $\endgroup$ – A. I. Breveleri Jul 20 at 2:34

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