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It would be convenient for both astrophysics and for the plot of the story set in this world if there exists a ninth planet in the Solar System far outside Pluto’s orbit (with appropriate mass, Ap, Pe, etc., since we can explain TNO clustering and big plot points in one go). Story aside, in this world there is such a ninth planet, which I have taken to calling Minerva.

I know this’ll probably be screwed up by the actual discovery of a ninth planet (or an exhaustive search to determine that none exists), but still in this world there is an mini-ice giant of eight Earth masses (essentially a small, dense rocky world with a really thick atmosphere) which has nine smaller moons like the Galilean moons of Jupiter, orbiting with an aphelion of 460 AU and a perihelion of 290 AU. The moons also are very convenient for habitation (mostly similar to Titan) and for this reason the humans of this world want to go and colonize the moons of Minerva.

Slight issue: Minerva’s so far away from the Sun (around six times as far from the Sun at its closest as Pluto is at its farthest) that it’s not very well sunlit and is pretty much dark most of the time. This would be fine, except for the fact that the human colonists want to have a regular day/night cycle that everyone can agree on; just setting up lights on timers doesn’t work because independent colonies can have their “days” and “nights” be at different times which is a pain for colonists.

So, how do you establish a global day/night cycle on an otherwise dark Titan-like moon of a gas giant?

Some notes and requirements:

  • Days and nights can be as fast or as slow as required (i.e. days and nights can each be either one hour or four hundred hours long, or anywhere in between), but they always have to be the same length day-to-day. Maybe not longer than 1,000 hours, though, that's a bit much.
  • There should not be any confusion as to whether it is day or night; no matter what language is spoken by the colonists or what schedule their colony is on, everyone should always agree “yes, it is day” or “no, it’s nighttime”.
  • It being light outside in addition to light inside is critical for growing crops and walking around; it has to be light outside, at least 50% as much irradiance as Earth receives (at least 700 watts per square meter) for half the surface of any given moon at any given time.
  • Each moon can either be linked to the same day/night cycle or can have something different from all the others.
  • Humans have access to high-efficiency fusion reactions, in-orbit megastructures of arbitrarily large size, and a virtually-unlimited supply of materials.
  • The moons are all tidally locked to their planet and orbit with periods between 2 days and 60 days, and are all functionally similar in mass and radius to Titan.
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    $\begingroup$ Why can't you use lights on timers? If you can build orbital megastructures, surely you can manage to turn some lights on and off at the same time. If the colonies are so independent they can't even accomplish that, it's not clear why they'd want planetwide synchronization in the first place. And if they do want synchronization, they just have to schedule their lights appropriately. $\endgroup$ Mar 20 at 13:23
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    $\begingroup$ "independent colonies can have their “days” and “nights” be at different times which is a pain for colonists." is it though? Is it a major pain for me in the US that people in China don't agree with me on when night is? Off hand I don't feel like it's affected my life all that much, and we're even living on the same planet. $\endgroup$ Mar 21 at 19:50

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it has to be light outside, at least 50% as much irradiance as Earth receives (at least 700 watts per square meter) for half the surface of any given moon at any given time.

This is ruinously wasteful of limited resources. What's the point of cooking a frigid wasteland? More importantly, the moon you're cooking is a giant frozen ball probably made of many different icy volatiles, and by heating it up you're generating a huge foggy chilly cloud which will eventually rain back out of the sky or condense as a frosty layer on the outside of your domes.

And yes, there will be domes, because those worlds will be cold and a Titan-like atmosphere will not remain gaseous. Nitrogen freezes out at ~63K at earthlike pressures, and Pluto's surface temperature is a chilly 40-60K. You'll get a tenuous nitrogen atmosphere due to sublimation, but honestly I really do not think you should be trying to heat an open surface to keep the atmosphere from becoming a lithosphere!

Instead, you should have some nice big domes... perhaps even an entire worldhouse, though I'm not quite sure why you'd want to build that all the way out there. Use suitable materials and designs to keep the contents safe, at which point there isn't really an "outside" that most people will even be visiting... only "inside the dome" and "inside a building in the dome". The domes can be insulated (both the canopies and insulated from the frigid ground) and pressurised and can be lit and heated to suit the inhabitants' needs and wants.

how do you establish a global day/night cycle on an otherwise dark Titan-like moon of a gas giant?

It seems weird that you'd want to do this in the first place, but you can either a) do this the hard way, which is you get everyone to agree on a standard cycle, use a shared timekeeping system to synchronize everyone's automatic light timers, and then enforce it by whichever means seem appropriate (carrots and sticks) or b) the other hard way, which is to build some fusion candles and drop them into the upper atmosphere of the central gas giant. These were first dreamed up by Larry Niven in A World Out Of Time, but Orion's Arm seems to have the most convenient reference at the moment.

These are autonomous aircraft that stay permanently in a gas giant's atmosphere, sucking in gas and filtering out the interesting bits (eg. helium-3, deuterium) to run fusion rockets to keep themselves aloft, and unlike the original design also run massive plasma lamps to cast heat and light at the moons above without providing significant thrust. (Orion's Arm also has sunlines as a related technology, which might be worth a read).

You can cycle them on and off in whatever pattern you like, have as many as you like, and control the illumination as you like. Maybe you even do something even more gonzo and hazardous, like have the illuminators be massive laser arrays to ensure light and heat isn't wasted on outer space, but collimated nicely on the moon surfaces. This gives you a standard diurnal cycle that everyone has to put up with, and can potentially provide the ridiculous, destructive and wasteful levels of external heat and light you want.

There may be a settling in period as the moon surfaces thaw. I'm not sure how long this will take, but the giant fog clouds that arise will interfere with the amount of heat and light that actually reaches the surface. I'm not sure how long it will take to warm everything up to a uniform 70+K to maintain a nitrogen atmosphere, but it could take centuries... there's a lot of cold ice to warm up out there.

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Frame challenge

just setting up lights on timers doesn’t work because independent colonies can have their “days” and “nights” be at different times which is a pain for colonists.

It works perfectly, it's the day/night that messes up with our clock, not vice versa. Whoever has travelled and experienced the jet lag can confirm this: our body is pretty good at keeping an internal clock, it's the outer day/night that messes up with it, making you awake when the rest of the people around you is sleeping.

As long as the first colonist decide on a clock and keep the synchronization of all the clocks on the planet, they do not need an external day/night cycle.

People working as submarine crew live perfectly fine with their life set up on an arbitrarily chosen clock, completely unrelated to their physical location, while the submarine can spend months without surfacing and seeing the light of the sun.

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    $\begingroup$ A side note: perhaps OP is asking about time sync for different colonies on Minerva's different moons. Illuminating one moon won't largely effect other moons, and they could have different day/night cycles. If that is the case, th only way to sync their clocks is by force (administrative application of governmental decisions). Exact RTC could also be synced should you establish a set of atomic clocks on each moon, and periodically sync em by PTP or similar tech. $\endgroup$
    – Vesper
    Mar 20 at 13:39
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Single moon/single colony: why not timered clocks

With a single time-consuming entity, there is no problem using timers; just enforce an exact length of day and night (say 43200s for either) and let it be.

Multiple colonies/single moon: use NTP on top

You can make clocks tick in unison on all colonies, so the day would start on each colony in the same physical time (relativity aside). Use an already established time sync protocol that's working here at maximum distance to synchronize base clocks, and go with it. If a (new) colony has decided to not use the already-established interval, either let them have it but use your own time to direct communication, or convince them to join the global "time zone" right away. If they persist, see below.

Multiple moons: expect them to have different day/night cycles within each colony or moon

While clocks can still get synced over NTP or PTP, there is less effective force to make them use the same day/night cycle as us, also the other colony/ies could have opposing governments, or at least be actual concurrents (assuming humanity did overcome general belligerence, or else BOOM there's no them), or there are other reasons to have different day/night cycles on different colonies or moons. Perhaps even astronomical, as you can still discern the Sun from that far away as an outstanding star, and one colony controls motherland uplink so it's forced to sit at 23h56m day length, or they would get out of sync with Earth's transmissions. So abandon that problem, and instead cooperate with a remote colony by accounting for their day/night cycle, as they have their reasons to have exactly the selected day length, and you are to respect them.

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  • $\begingroup$ This mostly says what I would have written in my own answer, so have a +1. Shared timekeeping is basically required for a whole bunch of networked protocols (is this security token still valid? when should this financial instrument execute?) so some flavor of NTP will probably already exist. As for diurnal rhythms... who cares? Day/night cycles are for people on Earth who can't get away from them. Why reinvent inconveniences elsewhere? Sync your calendar with Earth and go to bed when you please. $\endgroup$ Mar 20 at 14:50
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Megastructures of unlimited sizes you say? Let's have a mega lens or mirror then in a Lagrange point so that it's in line with Minerva and Sun. Since you are on average 250 times further than Earth, the optics should be 250x the diameter of Minerva to get concentrated Earth-level sunlight on whole hemisphere at once. Having 50% irradiance means half the area, that is 71% smaller size. The day length is determined by Minerva's own rotation.

In any case, the mirror has more than million kilometer diameter, or there is a swarm of them with equivalent area. With some adaptive optics tracking and illuminating all the moons is possible at the same time, too.

Whether such energy input applied to the surface is advisable, that is another matter. It will inevitably cause violent weather and the thawing surface will be unstable for long time.

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  • $\begingroup$ It's the intuitive answer, but a mirror a million km across would outmass Minerva and everything around it by lots, no matter how lightweight the materials, and the Lagrange points will be far enough away that it will need to be even bigger. (A lens would be much, much more massive.) Extra anti-bonus - the light pressure on a mirror that huge would need to be countered with an equivalent force or it will just be a huge light sail that will accelerate into interstellar space. $\endgroup$ Mar 20 at 21:27
  • $\begingroup$ @KerrAvon2055 not necessarily, 1 micrometer thin mirror(considered feasible for solar sails today) that big needs only about 1000 cubic km of material (a cube with side of 10km ). Fresnel lenses can be made very thin too. The light pressure is not an insurmountable engineering problem, just balance it against planet's gravity. $\endgroup$
    – Juraj
    Mar 21 at 23:27
  • $\begingroup$ You can't just throw mirror material out there, it needs to be held in a structure that will have a diffuse focal point covering all of Minerva and now needs to be able to dynamically shift in a constant dance to balance the light pressure in order to maintain a forced orbit. The micrometer-thin mirror isn't the issue, it's the structural elements to focus it, point it the right way and keep adjusting how it's angled. $\endgroup$ Mar 22 at 2:12
  • $\begingroup$ Explain that to Starshot Breakthrough Initiative. They are planning mirror sails with features you wrote about, maintaining focal point and very precise attitude, all elements extremely lightweight. They will use light many orders of magnitude stronger, which makes it more difficult actually. All that with near-future's tech and it's serious. It's actually sad you don't want to allow science fiction writers to consider such things... $\endgroup$
    – Juraj
    Mar 22 at 7:39
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FRAME CHALLENGE: You don't need a common illumination source to keep everyone in sync.

So, how do you establish a global day/night cycle on an otherwise dark Titan-like moon of a gas giant?

People/employers/governments/etc will just pick a cycle and use it. Whichever cycle gets the most traction will eventually win over everyone else because it'll be easier to do business with others if your working schedules are roughly synced rather than flipped.


With a planet so far out that it's "high-noon sun" will be (at most) comparable to an Earth twilight, you're going to end up needing to use artificial illumination (flashlights, headlamps, headlights, etc.) to get any real work done. (To say nothing of the fact that most work will probably be done within buildings/vehicles/etc. rather than externally in EVA suits.) As such, there is no real hard economic benefit to setting your clocks around Minerva's actual day/night cycle.

Additionally, you have people, presumably from Earth and used to 24 hour days, that could maybe (with some work) get used to something like a Neptunian 16-hour day, but could never stay up for something like a Plutonian 153-hour day let alone survive staying awake for a proper 5832-hour Venusian day. Your planet Minerva (and it's moons) may not be so extreme, but the point still stands that their "day" cycles are unlikely to have a particularly Goldilocks Earth-like period that makes them convenient for your colonists.

Combine with all that the fact that your colonists still need to coordinate/schedule supply shipments or whatever with Earth (and/or other larger population centers) and you're likely going to find it easiest to just pick an Earth time-zone and get everybody on Minerva (and all the moons) to just adopt it. Presumably, just going off UTC+0h (London time) is most straightforward - though you could pick something like Houston-time if your worldbuilding assumes most of Earth-freight launches from the US.

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You make a giant shallow hollow mirror from a solar sail, focusing the beam far out towards your planet. You station it behind the sun, were input angle and output angle line up. And thats it. Add some navigation. It will not cover the whole planet though, unless you blow it up to ridiculous proportions. But it could be enough to have a city sized spot on the planet. So if there are two cities on the same height, rotating around the planet equator with a constant sunbeam on them. Outside the pillar of summer - eternal cold and frozen gas.. inside.. tropical heat.

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