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The Problem

I had in mind giving my world a permanent full moon for a long time. Though I generally ran into 2 problems along the way.

  1. Firstly I wanted of course to make it appear as if it was always full moon.

    • My solution for this was to make the moon glow on its own. Problem is I have no idea if that doesn't screw up my next point.
  2. Secondly I wanted this full moon to be visible on the entire planet at least every night.

    • Problem here is I have no idea how to do this and everything I looked up about it did not give me a clear answer.

So how can I use various magical shortcuts to cause my planet to have a permanent full moon that is at best always visible?

Additional Information

  • The moon can if necessary be not visible during the day. Though ideally it would be optimal if it were visible.

  • There is really no limit to magic though keep as grounded as possible. So nothing like a magical moon projection.

  • Natural conditions can be changed through not to much. So nothing like a second moon.

Special Note: The quasi continuation of this question can be found here:

Moon calendar with a permanent full Moon

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    $\begingroup$ just a story suggestion, but I don't think physics is something you necessarily even want here. Perhaps Moon is hiding in Earth's shadow for fear of Sun's vengeance after she stole some of his light while he was sleeping. Perhaps the ancients used the last of the True Magic to bind the moon to the dark side of the Earth to keep the Moon-Dragon eggs from hatching, not realizing that the Moon-Dragons were Earth's protectors as well as her tyrannical tormenters... $\endgroup$
    – g s
    Mar 21 at 6:38
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    $\begingroup$ While it's not what i had in mind parts of this really sound like a good story for the mythology of my world. $\endgroup$ Mar 21 at 6:42

10 Answers 10

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If you want it to be visible every night and don't mind it not being visible during the day, then it's easy: the Moon stops orbiting the Earth at its normal speed and instead takes up a position almost exactly opposite the sun in the sky. Magic holds it up and makes it orbit the Earth once per year instead of once per month, which would otherwise be completely impossible. The Moon does't glow by itself, it just gets lit up by the Sun as always (it's not quite exactly opposite the sun in the sky, so that it doesn't get eclipsed).

Then from every point on Earth, as the Sun sets every night, the Moon will rise on the opposite side of the sky at the same time, and it will always be a full moon. Then, every morning, the Moon will set as the Sun rises.

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  • $\begingroup$ Really like this solution. Do you think it could any adverse effects implementing something like this in my world? Like for example messing up tides. $\endgroup$ Mar 21 at 12:45
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    $\begingroup$ @Fallenspacerock it would affect the tides, yes: usually there are Solar tides and Lunar tides, which generally happen at different times of day, although it varies depending on the Moon's position in its orbit. But now the Moon's orbit has changed, in such a way that the Solar and Lunar tides always happen at the same time. This will mean that there will be only one high tide and one low tide every day, and they will be a bit bigger than normal. $\endgroup$
    – N. Virgo
    Mar 21 at 12:49
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    $\begingroup$ At the moment when this first starts to happen there will probably be some chaos, since the forces involved are huge - I imagine it would cause earthquakes and such, possibly very severe ones. But you can always magic that away if you want to. $\endgroup$
    – N. Virgo
    Mar 21 at 12:51
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    $\begingroup$ @N.Virgo there will still be two high tides and two low tides, with the high tides occurring at noon and midnight every day. $\endgroup$ Mar 21 at 14:53
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    $\begingroup$ @TomJNowell that's addressed in my answer already - the moon tracks a point almost opposite the sun but not quite, so it doesn't get eclipsed. $\endgroup$
    – N. Virgo
    Mar 22 at 3:49
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The moon orbits the L2 Lagrange point. L2 orbits conserve energy and momentum, but they are not stable equilibria; over large periods of time they will always evolve into eccentric elliptical orbits. Thus angels occasionally have to nudge it back on course when other interactions, such as the distant gravitational influence of Jupiter, tug it away from the L2 orbit. This makes this answer both and !

The Moon is always full because it is always more-or-less exactly opposite the Sun.

L2 is about 4 times farther from the Earth than the moon is. So if you want a big moon-like moon with moon-like light and moon-like tides, and it's at the L2 point, you'll need a moon that's about the size of Mars. I doubt that there's any science-based way for this to plausibly happen naturally.

NASA has helpfully provided some animations (not to scale) of such an orbit: the James Webb Space Telescope follows an orbit around the Earth-Sun L2 point. See also here. JWST is tiny enough and has a short enough planned operating lifetime that it can use rockets instead of angels to stay on course.

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    $\begingroup$ While not the ideal solution it sounds very interesting nonetheless. And it probably is a really useful idea for my worlds moon based religion. $\endgroup$ Mar 21 at 5:49
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    $\begingroup$ The unstable orbit is actually pretty interesting story-wise. Religious authorities can argue that they are essential in encouraging folks to pray (aka, donate their magic energy) so the moon doesn't crash down on us all! $\endgroup$ Mar 21 at 10:29
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    $\begingroup$ @Fallenspacerock IMO this solution is better (i.e. more ideal) than the one in the answer you accepted. That answer describes the moon being somewhere on a line opposite to the sun. The Lagrange L2 point is actually a specific point on that line, with the great property that it is an unstable equilibrium. Keeping the moon on L2 would require far less energy or 'magical energy' than on any other point on that line. $\endgroup$
    – Chris_abc
    Mar 21 at 13:23
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    $\begingroup$ I never said it's a bad solution it's just so that the one i accepted is the most simple to implement in my world. If you like this one better you are entitled to your own opinion and you can use it for whatever world you are working on. $\endgroup$ Mar 21 at 13:53
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    $\begingroup$ Bonus points for this answer: if the angels are disturbed or otherwise prevented from precisely keeping the moon in place around L2, via demonic intervention or otherwise, a blood moon might form as the moon passes through the planet’s umbra, which is a nice little indicator of “something is going wrong in Heaven”. $\endgroup$ Mar 21 at 15:52
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A Few Issues

The largest is this:

Secondly i wanted this full moon to be visible on the entire planet at least every night.

It can't be "at least every night". It has to be at most every night. If you only have one moon, and your planet is a sphere, only half the planet can possibly see any of the moon at any time. If we limit ourselves to "only half the day", then we can move on to the other issues.

Energy

This is both better and worse than it could be. If we're using Earth as an example, our moon only reflects about 7% of incident sunlight. Sunlight at Earth's distance is about 1370 $W/m^2$. The circle of the moon from Earth's perspective is 9.4 trillion square meters. So you'd only need to radiate $1370 \times 0.07 \times 9.4 \times 10^{12}$ = 105.3 TW. You've got magic, so that would be fine. But why waste energy? You don't care if any of the light missing your planet is radiated.

So if you can magically restrict the direction of radiated energy to just the energy incident on the planet, again using Earth as a model, that's about 0.01% of the light reflected by the moon. So if you turn the surface of your moon into a spotlight aimed directly at Earth, it would only need to be a 14.5 GW light. Much more manageable.

As for the "how"... well, you already have magic.

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  • $\begingroup$ I didn't thought about redirecting the light but it sounds very neat. It would be very useful for my physics based magic system. Also if we assume this wouldn't there be a way to easily reflect the light over the entire planet? $\endgroup$ Mar 20 at 23:01
  • $\begingroup$ Only if you're willing to assume the giant mirror from M S's answer. If light isn't travelling in straight (accounting for the curvature of spacetime) lines, we've moved a long way from "physics-based magic". And there's no redirection here, only restriction, like a spotlight. $\endgroup$
    – jdunlop
    Mar 20 at 23:12
  • $\begingroup$ Yeah with physics based i meant more losely based. I might in that regard consider the mirror idea though of course i would need to come up with an explanation to not make it appear to weird. $\endgroup$ Mar 20 at 23:16
  • $\begingroup$ It could orbit the Earth very fast (magic), it could do multiple orbits of the Earth per night. I think this should count as being visible multiple times per night. Although, you'd really have to get it zipping along pretty quick. $\endgroup$
    – Zwuwdz
    Mar 21 at 15:47
  • $\begingroup$ Solution: 2 nearly identical moons in nearly opposite orbits on either side of the planet. That way everybody gets a visible moon almost all the time. (I say "almost" because each moon would be visible to slightly less than half the planet - At any given time there'd be a ring where one moon has set and the other hasn't risen yet, but that can be a relatively short period. A primitive society might take a while before they realize the two moons are not the same. (You'd need to invent telescopes at least.) $\endgroup$ Mar 22 at 3:20
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In Need of a Moon are You?

Allow us of Bespoke Worlds to introduce you to our astounding line of Subplanetary Objects! Moons of every shape and description from large planet sized objects right down to the tiniest of asteroidal moonlets, our Catalogue of Moons is sure to offer your local planetary council a suitable solution for all their lunatic needs!


All of our products are lovingly crafted in house. We never contract out to sub-par creation gods whose fakery is obvious on first glace. No indeed! Bespoke Worlds has aeons worth of experience in the planetary system design, fabrication, repair, upgrade and removal industries. Our innovative engineers and designers have been meeting and exceeding the expectations of our clients for more than seven billion years, and we are certain that we can offer an absolutely gorgeous and unique moon for your absolutely fantastic and beautiful world!


Let us take a look in the Catalogue for moons that meet the "perennially visible as full" requirement. Several options exist for fabricating a moon that is always perceived to be at the full. However, due to the rudiments of elementary physics, there may still appear to be "moon phases", a scarcely discernable to mildly noticeable alteration in the overall brightness of a moon due to ordinary planetary motions. However, we strongly believe that moons of our Living Light and Biota ranges especially will overcome and perceived defect in apparent brightness overall.


For example, here is one of our classic offerings, delivered quite recently to the developers of F34-4096-32, otherwise known as Oscun:

Oscun


Here we can see Oscun continually bathed in the gentle glow of the red bioluminescence of the moon's native flora. Due to the relatively harsh conditions preset for this moon, its overall climate can hardly be considered edenic. Plant life on this type of moon is extremely hardy, lichen-like, and requires no external care or maintenance plans. Oscun's moon was originally seeded with four standard strains of algofung base matrices, designed to provide wide coverage over the entire surface of the moon. Our most recent voyage to Oscun, for routine follow-up, was a mere thousand years ago, and showed curious signs of base matrix recombination. Further study is warranted and is planned for over the course of the next myriad.


This type of moon is truly a one-and-done! The engineering and planning are simple; the fabrication processes are quick. The length of time such a moon's life flourishes can, to a certain extent, be engineered into the original package configuration. Of course, no planetary engineer can guarantee the absolute viability or terminal conditions of life! It is tenacious after all! But we can project, within a billion years or so, the likelihood of life continuing to flourish on any of our sufficiently large celestial objects.


Slightly more popular in recent eons are greenish moons. The view from 9a66-1121-77, also known as Larabii, is quite spectacular!

Larabii


Warm shallow seas teeming with a variety of blue and green bioluminescent creatures, broad grasslands and cane forests rustling in the gentle breeze shall provide a pleasant retreat for the people of Larabii in several myriads of years when once they reach those first wondrously tentative steps towards the stars!


The contrast between red and green is due in part to the hardiness of the life forms selected for each kind of moon. Red is quite suitable for planetary systems with a darker aesthetic: cooler climate, harsher conditions, not suitable for higher life, but quite adequate for certain kinds of plants and fungi. Bespoke Worlds can offer a broad range of reds from the virulently bright all the way down to the most baleful of black-reds!


Greens and blues on the other hand are more suitable for planetary systems with a more tranquil aesthetic: milder climate, more temperate conditions. These moons, while suitable for higher life, do not necessarily need to be provided with such. Again, a broad range of hues from the darkest to the brightest are available, based on your --- our potential client's --- specific needs and tastes!

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    $\begingroup$ An absolute fabulous selection of celestial objects. I am sure there is one in your sortiment that will fit into my world perfectly. Are about 100 million years an acceptable time for delivery? As i think my planet should have mostly finished forming by then. Do you accept whole planets worth of resources as payment. And no need to be so rude about creator deities i am sure they do there best out there. $\endgroup$ Mar 21 at 17:25
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What if it's not actually a moon, but another planet orbiting the Sun on the same radius as your world. It's a very big white planet that's always aligned behind us. People would see it every night and only during the night, since it's opposite to the sun.

PS: If it's not possible astronomically and you feel like you have to give an explanation why they are not collapsing into each other, you could say that some magic prevent that, barely keeping it at bay and stable.

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  • $\begingroup$ While i generally want to keep the moon as a satellite of my planet your answer is indeed going to be helpful with my final model. $\endgroup$ Mar 21 at 10:20
  • $\begingroup$ Insert obligatory Star Wars reference? $\endgroup$
    – Guran
    Mar 21 at 12:10
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    $\begingroup$ This is possible astronomically - such a planet could occupy the L4 or L5 Lagrange points, which are stable equilibria 60 degrees ahead/behind the other planet in a slightly larger orbit. The planet would always appear about 2/3 full, but it would only be a small dot in the sky, since it would be about as far away as the sun itself (Mars and Venus both get several times closer to earth at their closest approach). $\endgroup$ Mar 21 at 13:43
  • $\begingroup$ @NuclearHoagie That's why I added the PS. If we want the planet to appear the same size the moon appears to us, I don't think it'd be stable. $\endgroup$
    – Jemox
    Mar 21 at 15:50
  • $\begingroup$ If the other planet is always behind the occupied planet in its orbit, then part of the surface lit by the sun won't be visible from the occupied planet, part of the surface in line of sight from the occupied planet won't be lit by the sun, so you won't have a full moon. Same problem if the planet is ahead in the orbital path. For the moon to appear full merely by virtue of reflected sunlight, it has to be on a concentric orbit with a larger radius, neither ahead nor behind the occupied planet. $\endgroup$
    – David K
    Mar 28 at 15:57
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The Moon In Your Head

Do you really need a physical moon?

Unless your world needs a moon's influence specifically for the tides and similar geophysical phenomena, who says the moon in question has to be a physical object at all? Ancient mind-affecting magic projects the glowing lunar orb onto the vision of the world's inhabitants. Those around the same area will find that they agree on its relative position in the sky so long as they don't measure it too closely, but those far apart will easily find contradictions in its position - perhaps implying false conclusions, such as that the world is a different shape than what it actually is.

In order to create the phenomena of a physical full moon's light, their vision could also be made slightly more effective at night, or light could be cast by an atmospheric phenomena. (Countless bioluminescent organisms photosynthesizing in the upper atmosphere, and releasing their stored energy during the night? And, at that altitude, there's no requirement that those organisms are small, either.)

MindMoon can exist alongside a physical moon

If a physical moon IS required, someone could have sewn particularly powerful invisibility magic around it, or the same mind-affecting magic could make the true moon functionally invisible to the world's inhabitants. Their eyes can physically see it, but their minds are prevented from recognizing it. Perhaps by using physical, camera-like devices, they could find evidence of the physical moon's light and phases, and evidence that the mentally-projected false moon did not actually create physical light (because their nighttime photography is less illuminated than it seems like it should be, compared to taking photos in apparently comparably-dimly-lit areas during the day). For that matter, can other animals see it, or is it only mentally-projected into the minds of humans? Presumably, ancient magic would not prevent humans from recognizing these secondary effects of the physical moon and missing interactions of the nonphysical always-full moon, just directly perceiving the physical moon itself.

"In your head" doesn't have to mean "unreal"

There's also no reason that the mentally-projected moon has to be something unreal; just because the world's inhabitants can only see it through mental projection, and it doesn't have a local physical presence or reality, it could still have a mental, spiritual, or magical reality. It could be a figment woven into the collective psyche of the inhabitants of the planet, where it exists in some shared underlying mindspace. Sight, after all, is a mental process that apparently happens in our brain / mind after our eyes receive the light reflected off of something. In this sense, people who can see it will all agree on the mentally-projected moon's attributes, because they're all seeing the same thing. (Or, the mental projection could simply force everyone to see the same thing.) Is it possible to venture to that nonphysical moon's position through mindspace? Could be.

Why, though?

But why? Why use magic to project a moon into everyone's minds? This is a huge amount of effort to go through, and could even be a fair amount of effort for a god to go through, . I can see several scenarios in which this might make sense:

Something to hide

If an ancient, powerful civilization wielding powerful magic wants to live on the moon and not be bothered, this is a (somewhat) reasonable way to do it. Why make the moon full all the time? This could be to give the humans / other species of your choosing an advantage by only making the mental projection affect them, and grant them better night vision. Adding phases to the moon just makes the whole thing that much more complicated, really, and such powerful magic written into the shared unconscious of an entire planet would be a huge undertaking as it is.

Once complete, the other inhabitants of the planet will notice only that the moon stopped having phases, and that it's always full now, a strict improvement from how it was before in a lot of ways. It's now easier for people to function at night. The constantly-shining moon will be seen as a point of celebration, and perhaps I've read, heard, and watched too much fiction, but myths of a beforetime when things suddenly got moderately better just seem fundamentally less suspicious to me.

Now, what necessitated hiding like this? Perhaps, ya know, ancient Other Man Grew Proud and spacewarred with the ancient Fancy Moon People, and after brutally quashing and exterminating Other Man, the Fancy Moon People decided to hide their existence so that the other civilizations don't come and bother them. Maybe they even feel bad now about raining destruction upon that terrestrial civilization. Maybe they realized that destroying an entire civilization, even in self-defense, is a fundamentally awful act.

Of course, if the terrestrial civilizations find their way to the physical moon again, will they realize that, in the countless years since the Fancy Moon People went into hiding, they may no longer be the same genocidal people who brought catastrophe on the planet? And, does that matter?

Something that we're better off not seeing

I previously mentioned the possibility of seemingly endless swarms of high-altitude bioluminescent organisms which, unlike terrestrial bioluminescent algae, don't necessarily need to be tiny in order to cast a fairly even glow. There's no reason those have to be nonintelligent, friendly, or native to this planet.

Hiding these eldritch swarms from view might benefit everyone's mental health and prevent a panic, and it might also serve to protect the planet's inhabitants in some way. Are these entities also native to mindspace in some way? Can they sense when they're being perceived? Are they attracted to the bright light of intelligence shining out into mindspace?

(Mindspace, in this context, would be the underlying abstract space in which this planet's collective psyche or collective unconscious is built upon - so it's not truly overlapping with the physical world, but each animal or being with a mind connects the physical machinery of the brain with part of mindspace. But, mindspace is an abstract space, so any coordinates in it use a fundamentally different coordinate system, and likely does have any particular relation to our dimensions of space, and possibly even our dimension of time.)

Then again, will these entities remain hidden and harmless forever? ...Did they D E V O U R the Fancy Moon People, who themselves may have dug Too Greedily And Too Deep into mindspace to escape from the entities themselves, and possibly significant parts of the physical moon itself? Are they sniffing around the perimeter of the world, knowing there's something there but being unable to perceive it directly, and therefore unable to navigate through the coordinates of physical space to find their desired prey? (Perhaps due to some effect of the planet's gravity well.) Does breaking through the illusion of the mentally-projected moon remove this protection, and if so... yikes?

Jealous moon deity

You mentioned a moon-based religion in one of your comment responses. An expansionist, interloping lunar deity coming in from another world might want to get rid of the physical moon in order to weaken belief in the planet's native moon deity, while also gathering worshippers among the inhabitants who now see their glorious full moon in the sky. Did they get jealous of their own system's sun shining fully every day, almost always being completely visible except during eclipses? Well, now they fit that bill too, and are even getting worshippers from other solar systems, who can all now gaze at their beauty every single night.

The native moon deity, on the other hand, will presumably remain nearly powerless (being starved of their symbiotic relationship with the planet's inhabitants) until and unless the true moon can be unveiled. (Unless, of course, people can believe in them without having to see them - which could go a C.S. Lewis-style religious fiction route, or it could go another way and frankly discuss whether faith in something for which we cannot directly observe is justified, and what actually qualifies as evidence for that. Could be philosophically interesting either way, especially if the nuance is explored.)

Unhelpful moon entity

The other deities may have hidden the physical moon because of its malign influence. Perhaps the physical moon is a sleeping eldritch abomination, which does not need to wake up or act on the physical plane in order to shed its particular form of uncaring radiance that's notably Bad For Humans. Responsible local creator deities wouldn't want to have that sort of thing messing with their planet and living creations. In the tradition of eldritch horror, such abominations don't have to be evil, intend any harm, recognize living beings at all, or even have intelligence of their own in order to be an apocalyptic threat to living beings.

Cast into the void

A planet that was cast out of its original solar system into the darkness of the interstellar void by a passing massive interstellar interloper (other exoplanets, black holes, destabilization of the solar system's magical weave by The Hubris Of Man, etc.) could have both its moon AND sun provided through mental projection. In this case, in the sun's absence, there would have to be something in place to provide heat, energy for photosynthesis or a suitable equivalent, etc..

However, note that faking a sun would likely be quite a bit more complicated, and require quite a bit more futzing (and magnitudes more powerful magic) to ensure the biosphere didn't utterly collapse. At least in terms of my suspension of disbelief, I could just about believe that one or more incredibly powerful yet still mortal spellcasters could write something into the collective mindscape of a world's inhabitants. However, I'd have a hard time believing that a mortal could use magic to stabilize the heat and energy requirements of a planet's biosphere. That seems much more in the realm of gods, and is a significant anchor of the possible lower limit on their power - it requires not only gods, but POWERFUL gods. And that can be a destabilizing element in itself, in terms of how balanced and believable a system of magic / deities seems.

Weirdly, this also seems plausibly doable by civilizations higher up on the Kardashev Scale, which can use megascale engineering technologies that might be able to effect an entire planet at once. Either way, it seems implausible for mortal entities who only have a single planet to play with to use magic to replace their planet's original sun. Maybe a sufficiently advanced ancient civilization with access to magic and technology could create a self-multiplying and self-perpetuating solution - perhaps the bioluminescent sky entities were created to be a replacement for the sun, and gradually bred to fill up the sky - perhaps they derive sustenance from a source outside of physical space - maybe they get sustenance from mindspace itself, and the ancients accidentally created their horrifying mind-devouring abominations as a last resort to save their planet from an icy death. But, note that breeding that many orbiting, mind-devouring space monsters would take quite a while, so the ancients would have had to have had time to prepare.

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    $\begingroup$ Wow very impressive. While it is certainly not what i intend to do exactly you do have put a lot of effort into it. I can basically use this for the basis of my moon religion. I am really great full for you effort thank you very much. $\endgroup$ Mar 23 at 0:21
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That's no Moon...

It's not a Moon, it's a big balloon several thousand meters up in the sky. For most people the distinction makes no difference whatsoever.

A single one doesn't work for the entire planet, but a truly powerful entity could put different balloons up above nearly all population centers, taking care that no two are visible from the same spot at the same time. Seeing the "moon" at different parts of the sky is normal, right?

Remove the word "nearly" from the paragraph above if you can control where people will live (and travel?) for the purposes of mandating sufficiently large gaps.

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    $\begingroup$ While this does sound funny i am more settled on having the moon be a moon and not a ballon. $\endgroup$ Mar 21 at 14:42
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That's no moon, that's a plot device

What if the moon is not a moon at all but a celestial traveler hiding in the shadow of the planet because the magical energy source of the sun is toxic to it. Something your moon based religion can focus on which is specifically not a ball of rock tumbling around the planet.

And if you want to provide some physics related drama, the heroes must do something about this situation because the mass of the traveler hiding on the far side of the planet is pulling the planet further out in its orbit and the world is steadily cooling. The traveler is ok with this because once the planet is pulled far enough away from the sun then it can continue on its way without getting a dose of sun poisoning when it tries to escape. It doesn't mean any harm to the tiny beings below but its plan will doom them.

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There's a gigantic mirror roughly on the opposite side of the Earth which reflects the moon's image to the hemisphere of Earth where it isn't directly visible.

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    $\begingroup$ I would rather opt for natural means or magical ones sorry if i didn't make it very clear. Such a construct would rather be useful for science fiction but not for my pseudo fantasy world. $\endgroup$ Mar 20 at 22:34
  • $\begingroup$ @Fallenspacerock this is a magical suggestion. I doubt an actual mirror would do the job well at all $\endgroup$
    – M S
    Mar 20 at 22:36
  • $\begingroup$ Ok though if you could provide at least a little of an explanation on how it could work would be great. I know it is magic but i want to at least want to know how you have imagined the mirror. $\endgroup$ Mar 20 at 22:56
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Bigger planet + closer moon

You can have Moon 20 times closer to Earth (without destroying it), so unlit by Sun portion of Moon will get 400 times more light reflected from Earth and will be visible. You may also be satified with smaller Moon in this case.

Bigger planet (maybe more clouds/snow for better albedo) is needed just to reflect more light in the direction of Moon to get even more light to shaded part.

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