I have read many posts about planets in perpetual sunset or twilight. Tidal locking, perpetual sunset at the poles of a planet with zero axis tilt, and so on. Great information by the way. This question is similar. Can a planet that is mostly covered in water be in a state of perpetual sunset? And can the sunset be significantly larger than our own sunsets and still be habitable by humans? My short story takes place on a water world in perpetual sunset.

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    $\begingroup$ If the question is similar, why would not the answers be similar? Water should not affect axial tilt, and if anything it should make tidal locking happen faster. $\endgroup$ – SE - stop firing the good guys Sep 5 '19 at 8:55
  • $\begingroup$ @Hohmannfan because water bends light, and a planet with an (largely) unbroken curved surface of water could have longer periods of twilight, especially with greater amounts of water vapor in the air. $\endgroup$ – HA Harvey Sep 5 '19 at 9:54
  • $\begingroup$ Hmm, what is a "larger sunset"? $\endgroup$ – Alexander Sep 5 '19 at 16:22
  • $\begingroup$ Can you provide more information as to why tidal locking or zero axis tilt would not do the trick? Do you want the water everywhere on the planet always to be liquid? Also an entire planet can not be in the state of perpetual sunset (unless you abandon physics and go for full magic), you will always have zones of light and darkness, so can you clarify that part as well? $\endgroup$ – Khris Sep 6 '19 at 8:13

It depends on the nature of your story.

Fantasy: go for it! In the bible, God made the sun stand still so his prize fighters could finish whooping some ***. No reason divine will of some sort could engineer something of the sort!

High Sci-Fi (meaning "more out-there"): Yes, you might want to set the stage a bit more (a closer, ice moon that reflects more light than we are used to, crystalline formations at the poles or in the air, water vapor, etc.) to make it feel plausible. But look at worlds for sci-fi settings like Halo . . . if your story is good enough, your audience will help you with the suspension of disbelief.

Low Sci-Fi (like low-fantasy): Maybe. If your sci-fi story is trying to stay grounded, you will want to set the stage with extremely plausible reasons why the planet is perpetually in twilight. Just "mostly water" won't cut it. You need something that almost constantly eclipses the sun, and something else (a not-very-distant second star system?) that provides weak light to the dark side of your planet. Or, perhaps the planet is tidally locked and the dark side is a frozen wasteland? That opens more cans of worms for the planet. To sum up, it may be doable, but it won't be easy.

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Give the planet an ring set askew of the sun-ward plane such that its shadow is constantly cast upon part of the planet. Have your "twilight" be caused by sunlight reflecting off of (and through) ice particles in the ring. This would definitely meet your desire for "significantly larger" as it could fill most of the sky.

Now for the perpetual part, set your story on a floating resort city which deliberately cruises through the shadowed waters, constantly keeping up with the planetary rotation and/or currents, such that its' guests are always under the "twilight". Imagining how beautiful the skies under the shadow might be, it is easy to understand why they might choose such a course, purely for aesthetics and guest appeal.

Alternatively, the shadow could be the only habitable part of the world, with everywhere under direct sunlight too irradiated and with the nighttime side too cold for easy living. The city ship's efforts to stay in the shadow would therefore be for survival sake. Whenever the ship is out of shadow, it needs to shield all its windows and cover its food-growing fields to save them from the sun or the frost. Only in the twilight shadow is it safe to grow food and pursue other activities on the deck.

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  • $\begingroup$ The ring's shadow moves with the seasons. $\endgroup$ – Anton Sherwood Sep 6 '19 at 18:00

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