So I'm trying to determine if it's realistic for a planet to experience daylight for most of the year while experiencing nightfall only a few times within that same year? Say, less than 10 times? Also, while still remaining habitable. I'm thinking this would call for a multiple stars situation? What sort of effects/problems would this setup cause the inhabitants?
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11$\begingroup$ Maybe you're aware, but there's a famous scifi short story about this topic. it's called Nightfall, by Isaac Asimov. In the story, nightfall occurs only once every 2000 years. Caused by the fact that the planet is in a system of multiple suns, so there's always atleast one visible. I won't go into too many details, since it's a rather famous story and you'll find them easily. What you're asking about is a simpler system than that, since nightfall in your system is much more common $\endgroup$– user1878648Apr 14 at 13:12
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2$\begingroup$ @Trish that question has one possible setup that could answer this question, but this one is open to more scenarios. $\endgroup$– The Square-Cube LawApr 14 at 14:19
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1$\begingroup$ @TheSquare-CubeLaw The answer to that question contains about 5 setups that solve this, including a 300 year days solution. $\endgroup$– TrishApr 14 at 14:30
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2$\begingroup$ ... but it is a different question, "How would this Hexa-star system have to be set up" the configuration is specified... For a good answer, 5 additional suns are needed because the story refers to such a system. I've tried to find an answer not requiring additional suns. $\endgroup$– GoodiesApr 14 at 19:11
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$\begingroup$ The whole planet? Or just the part with the people on it? The polar regions of Earth are like that for half of the year, so if your rate of precession were locked with the orbital period, you could have part of the planet be like that year-round - and if you wanted, you could make that the only part of the planet that's inhabited. (Maybe the rest is ocean, or too cold to support life since it never gets any sun?) $\endgroup$– Darrel HoffmanApr 15 at 13:41
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4 Answers
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Eternal day, but mega structure blocks your sunlight..
Suppose your planet has a tidal lock so inhabitants on the sunny side would enjoy perpetual daylight. That side always faces the sun.
There's only one thing spoiling the party,
https://en.wikipedia.org/wiki/Topopolis
https://en.wikipedia.org/wiki/McKendree_cylinder
Long time ago, the system your planet resides in was inhabited by a high-tech species. About 900 million years ago, they built a toroid mega structure around the sun, to harvest its energy. But in some unfortunate period of their history, their civilization declined, but their toroid solar light collector still hangs there, inside your planet's orbit. It is about 900.000 miles across, about 60x the diameter of your planet.
Luckily, this Dyson torus has a very tilted orbit compared to the planet's orbit. Its inclination is ca 30 degrees, so it won't block the sun on your planet permanently. It's not a heavy construct, but it is big. Very big. Twice per year, the megastructure will eclipse the sun and there will be nightfall on your planet.
(NOTE: if you need more nightfalls, suppose it's a mega structure ruin.. some huge parts are still intact.. some day, the planet will collide with these remains, but that is still a few million years away)
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1$\begingroup$ Good answer. If megastructures don't work for the OP, the planet's moon (if it has one) can also block off the sun once every 30 years or so, maybe shorter if the moon has the right orbit. $\endgroup$ Apr 15 at 17:45
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$\begingroup$ Thx @Brinstar77, I've considered a moon or an asteroid belt.. issue is, that moon would need to be very big, or the asteroid belt very near, to cause actual nightfall.. the Earth's moon is already very big.. but a full solar eclipse is only a subtle darkening, for a few minutes. A dual planet orbiting a sun would be an option too.. but in that case the tidal lock would involve both planets rather than planet and sun. The eternal daylight would not be applicable, there would be a night-like, periodic occlusion. $\endgroup$– GoodiesApr 15 at 22:16
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The Demon Star could be the key to what you seek. If a planet could orbit Algol C, and either Algol A or Algol B were sufficiently brighter than Algol C (and the other sufficiently dim to be less luminous at that distance), you could have true day follow the main star, and alternate day follow the non-eclipsed bright orbital star.
Your rare night would occur when the brighter orbiting star is eclipsed by the dimmer star, which could produce moonlight level darkness on the planet. for this to work, the orbits of the binary stars would have to closely align with the orbit of the planet around the primary star, and the co-orbit between the 2 distant stars would have to be relatively slow (a single period would have to be a single day-night cycle, accounting for rotation of the planet as well). This would not allow for nightfall for the whole planet, only nightfall for the side opposite of the primary star, like on Earth.
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MULTIPLE STARS
The planet is in a system that has 3 or more stars in relatively close proximity. As a result of this, for the period of the year where the planet is in-between its star and the other stars, every inch of the planet is exposed to the suns and their rays. It's only when the planet's sun is inbetween the planet and the other suns that night actually happens.
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1$\begingroup$ Please include a diagram and the calculations showing that orbits are stable, or a reference to a reputable source. Thank you. $\endgroup$– AlexPApr 14 at 14:39
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$\begingroup$ Sorry, the calculations are a bit beyond me. I'm imagining a system where there is a 'primary' sun that the planet is orbiting around, and two 'secondary' suns close enough to shine light on the planet. $\endgroup$ Apr 14 at 14:47
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$\begingroup$ And the orbits of three massive bodies (and of the planet) are stable why? $\endgroup$– AlexPApr 14 at 15:01
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$\begingroup$ @AlexP I have no clue, I will let people who know more about the physics of celestial objects come up with an answer rather than talk out of my arse. However, a newly discovered planet by NASA does have 3 suns. See this: nasa.gov/feature/goddard/2016/… $\endgroup$ Apr 14 at 15:06
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1$\begingroup$ @AlexP so shouldn't it now be on you to show what the minimum distance condition is? Brinstar77 has cited a high profile scifi trope (see Star Wars, Nightfall) AND backed it up with at least one real life system with three suns. This doesn't have the hard science tag on it. FWIW, maybe the system isn't stable, just long lived, maybe only a few centuries long. Who cares? The OP didn't. $\endgroup$ Apr 15 at 5:37
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Space Mirrors
An advanced enough society might have mirrors in space. They will orbit around the planet in such a way that those who are "behind" the planet in relation to the Sun will reflect light back at it. A sufficiently large amount of mirrors could cause night to look like day.
Natural darkness from night could happen when the satellites are out for scheduled maintenance.
If you don't want your planet's inhabitants to know why this happens, the satellites could be the work of another species, from a long time ago. The aliens either went extinct or quit the system. Alternatively, a previous civilization of the same species built the system, but war or something else wiped them out. Society rebuilt itself from scratch and now there are at most legends about the space mirror system, if anything at all. The system is self sufficient and can do its own preprogrammed maintenance routines, which is when people experience night.
answered Apr 14 at 14:23