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There is a common trope in fiction: when a character find themselves in a dark place out of this reality (somewhere in the eldritch realms, dreamscapes, lost cities of Carcosa or R'lyeh, etc), at first they do not fully understand their dire situation thinking that they are simply in a strange and unfamiliar location on Earth. Eventually they look up to the night sky and see that the stars are wrong in this place, so it cannot be anywhere in our universe.

I am trying to incorporate this trope in my story. A modern citizen got lost beyond space and time. They suddenly find themselves in nighty wilderness under the starry sky (the moon or any other celestial bodies are not visible). At first sight everything seems at least familiar, but... Then, they notice something eerie with the stars above, so they realize that they are likely not even anywhere in our reality.

What can possibly be wrong with the stars in the night sky?

There should be something with the stars themselves (brightness, colour, luminosity), their positions or relative movement, that is visible with the naked eye and that is commonly and widely known not to be present in our world. Also, I would like the answer not to be too difficult to imagine/visualize and to sound a bit lovecraftian-style scary.

I personally came with the idea that every time the person looks up to the sky, the constellations are entirely different, but for me it seems a bit too straightforward and silly.

Assume the person in the story is somewhat clever and observant, yet has no special education related to astronomy, stellar mechanics or something, neither they have much experience in spotting constellations.

PS: Bonus points for a subtle and uncanny feature that allows for more gradual realization, but something that is obviously wrong is OK, too.

Some updates based on the comments:

UPD1: Strict scientific accuracy is not required, I don't mind moderate handwaving for the sake of storytelling. Yet, I don't want the solution sound entirely magical.

UPD2: The lights visible in the sky may not neccesarily be actual stars. I'd like them to be some stellar/atmospheric phenomena, though.

Brainstorming should produce the most horrific moment, from almost familiar to absolutely eldritch horror in shortest amount of time. Preferably without talking the eyes of the constellation while stumbling around.

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The color of the stars. Other answer(s) explain a situation where the observer is in our Universe, but not in the Earth. You ask for stars which are wrong and it is clearly visible that it cannot be our Universe. Thus, the stars in the sky must look somehow from which already an educated layman can say, it cannot happen.

If only the constellations differ, that can happen anywhere - already at ten light years away, most constellations become unrecognizable. If the milky way looks different, galaxies are visible in the sky, or there is more or fewer stars - these all can happen inside our Universe, only in different places.

However, what cannot happen is certain colors of the stars. The stars are plasma, with a roughly black-body temperature spectrum (not exactly, but the difference is not visible to us). Thus, their temperature can affect their color, but it cannot be any color. How a human free eye sees a star, that depends on its temperature. It cannot be any color, only a color along this line:

enter image description here

Thus, a star can be red, orange, yellow, white or light blue. No star can be green, blue or purple. It is impossible. At least, in our Universe.

P.S.: if you see the sky, you will see that most stars are white or yellow. That is because most stars we see are roughly Sun sized. There are some exceptions and with a good eye, in a clear sky you can see it.

P.P.S.: There might be rare phenomena, such as having an oxygen nebula around the star, that could make a star green. But that is rare. It cannot happen in all stars.

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    $\begingroup$ Most stars look white to the naked eye, since only a very few are bright enough to activate the color-sensitive cones in the eye. And an inexperienced stargazer like the one in the question might not know that green or purple stars don't exist. Maybe if they were all a sickly green, say, that would be eerie enough, but that seems like a fantasy scenario rather than something with a scientific explanation. $\endgroup$ Commented Mar 24 at 22:31
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    $\begingroup$ @ChristopherBennett to be honest, I don't think its possible to have an answer to this that isn't more or less a fantasy scenario. Stars do follow general rules but, to a naked eye observer, most of those cannot be accurately evaluated. So, you kinda have to make something super obvious and super impossible. $\endgroup$
    – Topcode
    Commented Mar 24 at 23:16
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    $\begingroup$ When the observer is on a rogue planet that travels through a galaxy at relativistic speed, then they would see the stars in one direction red-shifted and those in the other direction blue-shifted. That could reduce in unusual star colors without requiring to be in an entirely different universe. $\endgroup$
    – Philipp
    Commented Mar 25 at 10:46
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    $\begingroup$ @ChristopherBennett if I saw a sky full of green stars (or of any other color), my first thought would be that there is some glass or gas cloud or any other transparent substance between me and the stars filtering the light. $\endgroup$
    – phoog
    Commented Mar 25 at 11:46
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    $\begingroup$ @Philipp :-) You have spot a very nice and very surprising point of the physics :-) Fun is that normally doppler makes a red or blue shift. But, the black body spectrum has a very funny feature: if you doppler shift it in any direction, you get yet another black body spectrum, only warmer or colder one! It happens only with the black body spectrum, it has a deep mathematical reason and it has very nice consequences (for example, the cosmic microwave background does not really cool, it is doppler shifting). $\endgroup$
    – Gray Sheep
    Commented Mar 25 at 11:47
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There are patterns in the star positions that are very obvious, and which the observer would inescapably have heard of had they been visible from Earth. For example:

  • A large area of the sky, in a regular geometric shape, with no stars at all.
  • Stars arranged in several regular parallel lines, with similar changes in brightness along each line.
  • Stars that flash on and off like fire-flies.
  • Stars that change in colour over periods of only a few minutes.
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    $\begingroup$ The question is whether op wants to incorporate a reason for this into the story. $\endgroup$
    – DonQuiKong
    Commented Mar 25 at 8:13
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The stars are slowly moving relative to each other

That movement is slow enough that you would not notice it with a single glance at the sky. But over a few minutes, constellations shift and warp until they are completely different from what they were before.

As you may have guessed, this does not work for stars in our universe or under our current rules of physics, but here are some semi-scientific explanations that may work in your universe:

  • The "stars" are not actually stars, but small moons orbiting the planet. They shine because they are illuminated by whatever passes as the sun there. This situation is unlikely to occur naturally, or remain stable in the long-term in our universe, but this is not our universe.
  • The "stars" are living organisms, like fireflies, living in the upper atmosphere. The eldritch horrors are not FROM the stars, they ARE the stars!
  • The "stars" are much smaller than in our universe, allowing them to be packed much closer together. Needless to say, the rules of gravity (and others) will need to be modified. Depending on how much modification you think you can get away with, it might still not be enough to produce easily observable apparent movement speeds.
  • There is some heavy-duty optical lensing going on, projecting minuscule movements across the whole sky. No idea how this would work, but fits the idea of "warped dimensions".
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  • $\begingroup$ Another explanation: a lot of big artificial satellites. Starlink 20.0. $\endgroup$
    – Pere
    Commented Mar 25 at 19:08
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    $\begingroup$ The planets or small stars explanaqtion could be discounted if the motion of each 'star' follows random wiggles or pirouettes through the sky rather than any likely 'orbital' $\endgroup$
    – Penguino
    Commented Mar 25 at 20:21
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    $\begingroup$ had a similar idea but you can also see them slowly consume one another some stars disappearing in darkness while the others grow larger. $\endgroup$
    – Josh King
    Commented Mar 26 at 1:05
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    $\begingroup$ Of the answers I see, this one would generate the greatest feeling of eldritch horror in myself! $\endgroup$
    – Cort Ammon
    Commented Mar 27 at 4:54
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    $\begingroup$ The slower the stars move, the longer it takes for the horror to set in. If it takes several days for the stars to noticeably move, the protagonist might very well start to question their sanity before they could convince themselves they really were in a different reality! $\endgroup$
    – Malcolm
    Commented Mar 27 at 5:45
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The stars don't move

As the earth rotates throughout the night, the stars appear to move from one horizon to another. It almost seems like a celestial sphere is turning above the earth. But in this world, for some reason, they don't, and stay immobile throughout the night. The same stars and constellations are always there in the same fixed location, almost like they were just painted lights in the ceiling of a massive cave. It can plausibly take many days (nights) until your character notices that, specially if he comes from somewhere with plenty of night pollution and is not used to stargazing.

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  • $\begingroup$ I would never notice that, never noticed the stars move at night in 26 years of life...and as a child I grew up in a village with no electricity. $\endgroup$
    – Xenophile
    Commented Apr 10 at 17:56
  • $\begingroup$ @Xenophile I'm amazed by that. I don't pay a lot of attention to the stars, but I know one or two constellations. And they're not always visible. It seems to depend on the night, or the time, or the season, or all the above. It's certainly not always in the exact same spot every single second. $\endgroup$
    – ojchase
    Commented Apr 10 at 19:21
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Minor Frame Challenge: You are using this trope wrong

The OP wants the stars to tell a person that they are in a different universe, but this is not what the trope is supposed to do. When people look up at the stars to see familiar constellations, they know that they are on Earth. But, when they look up at the stars and they are not the Earth's stars, it only tells us we are on another planet. Assuming you've been jumped into an alternate universe falls short of every reasonable explanation.

Reasons why Most answers don't work:

  • Different Constellations: You are on any non-Earth planet.
  • More and Brighter or Less and Dimmer stars: You are on a planet closer to or farther from the galactic center.
  • More and Dimmer or Less and Brighter stars: You are in a more or less differentiated part of the galaxy.
  • Green Stars: Since the physics of this place are not so different that I am instantly dead, I'm gonna assume a world with a different atmosphere or nebula gasses that filters the starlight, or I might mistakenly believe that a visible star might be able to achieve a doppler effect of a red star getting closer or blue star moving away... something someone with some but not advanced knowledge of astronomy might think.
  • Stars don't move at all: The planet has a negligible spin relative to the galactic background.
  • Stars move fast enough to see move or orbit in different directions from one another: Those are just tiny moons reflecting the sun.
  • Stars don't move in smooth curves: This planet has a strong wobble.
  • Stars move completely randomly: Clearly, I am hallucinating. Avoid eating any more suspicious plants.

The fact of the matter is that no matter how weird you get, there is always going to be a more reasonable explanation than you are in another universe. Your reasoning does not even need to be scientifically accurate. Most people will simple put "I'm in another universe" at the very bottom of theories to try to explain anything unusual they encounter... unless...

Change the Earth, but NOT the stars.

The Stars tell us where we are in the world. With a sextant, watch, and compass a savvy survivalist can tell just where he is in the world. Even without these tools, you might be able to get a decent approximation, but it will help the narrative if he has some kind of survival kit to work with. So imagine you are lost in the wilderness. You phone is dead, but no worries, you have all the tools you need to find your way back to civilization. You just need to follow the stars South for 50 miles, and you'll be at the nearest town... only when you get there, it is more wilderness. So, you keep going until you are certain you've gone too far, you circle back, and no matter how much you search you can't find it.

What's worse would be if the stars eventually lead you to find a geologically significant landmark. A distinct mountain or lake that you know for certain should be surrounding by buildings, but it's just wilderness here.

The constellations tell you that you are certainly on Earth, but things are not where they should be. At this point, it is starting to become a reasonable assumption that you are on Earth... but not THIS Earth. At this point you are really suspicious, but maybe you traveled through time? Sure, it sounds ridiculous, but no more so than inter-universal travel; so, at this point all madness is equally viable. One thing you can reason though is that the stars are telling you that you've not traveled very far in time because constellations change over time... but maybe you went back by just a few centuries to before that town was founded? Maybe the stars have not moved enough yet to be noticably different?

The final nail in the coffin of your reality will be when you encounter people or wildlife that are not where, who, or how they should be. This could fully disprove time travel leaving jumping into an alternate reality (aka: another universe) as the only possible option left... that, or maybe hallucinations or a dream. Different people have different breaking points of disbelief; so, at this point it becomes an author's choice if your protagonist feels more comfortable believing that he is in an alternate universe or that he can simply not trust his own senses.

It will actually be the fact that the stars remain the same that your protagonist can rule out most other possible explanations of what is happening to him, and it will be the little epiphanies of his journey slowly invalidating his expectation of the stars that will lead to the rising action of realizing something is off instead of there being something about the stars themselves that should be immediately evident.

What if you need the stars to be different for plot purposes?

If you can't rule out all other possibilities, then give your protagonist a reason to place inter-universal travel as a more likely conclusion. In real life, the idea of interplanetary travel seems the most possible because other planets are things we can see and measure whereas other universes only exist in vague theoretical terms. However, if the scientists in your world have already made functional inner-universal looking glasses to see into and confirm the existence of other realities, then even if inner-universal portals are not yet a thing, the possibility of inner-universal travel would suddenly seem far more conceivable.

There could even be published papers based on scientific observations about what makes these other realities unique from our own. In this case, whatever is different with the stars could be exceedingly subtle, but still enough to clue in your traveller because it would be the exact thing he'd be looking for. If you go this route though, there is no "best possible answer" to your question, because it would be up to you to come up with what science has defined as the key difference between realities.

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    $\begingroup$ Heh. I'm reminded of Wen Spencer's Elfhome. They're still on Earth; the stars haven't changed (except for oh so much less light pollution you can actually see them), but... is that a tree walking? That's odd, it looks like OH MY GOD IT JUST ATE BOB!! ("Bob" is made up. The walking, carnivorous trees genuine Elfhome natives.) $\endgroup$
    – Matthew
    Commented Mar 26 at 18:09
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    $\begingroup$ If "Clearly, I am hallucinating" is considered a more reasonable explanation than "I'm in an alternate universe," then no evidence of any sort (earth or sky) can lead to the latter conclusion. $\endgroup$
    – LarsH
    Commented Mar 26 at 20:20
  • $\begingroup$ Upon noticing that the stars are moving randomly, what's preventing him from following the reasoning of "maybe hallucinations or a dream, but at that point, you've been in this place for way too long and thinking way too clearly for that to sound right"? $\endgroup$
    – Mutoh
    Commented Mar 26 at 20:30
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    $\begingroup$ @LarsH Stars slowly moving in random directions would look like pattern sliding which is the most common form of hallucination. Non-schizotypal people can experience it for a wide range of reasons including various intoxicating substances, acute head trauma, sleep deprivation, stress, or prolonged isolation. Most people who experience it are quick to disbelieve that they are actually seeing movement. $\endgroup$
    – Nosajimiki
    Commented Mar 26 at 22:00
  • $\begingroup$ In contrast, things that you can physically feel and interact with are far more convincing. $\endgroup$
    – Nosajimiki
    Commented Mar 26 at 22:01
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The stars are toroidal (doughnut-shaped)

At first glance, taking in the night sky as one whole picture, nothing seems off. Actually, it seems slightly off. But you can't put your finger on it. There are lots of twinkling stars of varying size and luminosity. It looks like a fairly normal night sky, you can just rationalize away any slight unease because you're viewing it in an unfamiliar place.

However, when you start to look at each star individually, you start to notice something very wrong. None of the stars have a uniform, symmetrical appearance. Most are actually fairly oval-shaped, a few look like straight lines, and your eye catches one that you swear might have a pinhole. Upon realizing that the stars you're viewing are not spherical, it dawns on you that what you're seeing can't be explained by physics as you understand it and you are farther away from home than you could have possibly imagined.

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One possibility is if the planet they're on is significantly closer to the galactic core, or in something like an elliptical galaxy where the stars are more closely packed than they are here in the Orion Arm. The night sky would simply have more stars in it, and would be considerably brighter. Even someone who didn't know the constellations could tell that there was a greater density of stars than they were used to.

The catch here is, does your character come from the city or the country? The night sky out in the country, without urban light pollution, is far more jam-packed with stars than a city-dweller is used to, so a city-dweller might not immediately know the difference between just being out in the country and being in another reality. In crossing between worlds, they'd have to cross from countryside to countryside, or city to city, to cancel out that variable, so that they'd know the stars themselves had changed instead of just the seeing conditions.

Another possibility is that the planet is close to a nebula that spans a significant portion of the night sky. Nebulae generally aren't as bright to the naked eye as they are in the colorful astronomical photos we're used to seeing, but as the person's eyes adjusted, they might gradually realize there was a broad patch of fuzzy light across a fair portion of the sky.

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  • $\begingroup$ good anwer imo, on the other end of the spectrum, having the star system being the most remote of its host galaxy, allowing either almost starless nights or fully populated galactic picture depending on annual orbital position around the star. $\endgroup$
    – user109900
    Commented Mar 24 at 20:52
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    $\begingroup$ Good point about a starless night, but it'd only work in the wilderness or a preindustrial city, otherwise the lack of visible stars could just be chalked up to light pollution or air pollution. As for seeing the whole galaxy, remember that the Milky Way is a view of the galactic disk from directly inside it, so a view of it from outside would be dimmer than the Milky Way -- something you'd only see once your eyes adjusted to the darkness. $\endgroup$ Commented Mar 24 at 22:25
  • $\begingroup$ Urban stargazing: the star is out $\endgroup$
    – phoog
    Commented Mar 25 at 11:54
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    $\begingroup$ This seems to be a valid scenario that allows the protagonist to realize slowly that they're not on earth (or in the solar system) anymore. But isn't the story requirement that they have to realize that they're not even in our reality anymore? $\endgroup$
    – Schmuddi
    Commented Mar 26 at 9:07
  • $\begingroup$ If it's another planet in our reality, odds are it would have different gravity, a different moon or moons (or an absence thereof), a different atmosphere, different plants and animals, etc. that would make it immediately obvious that it isn't Earth. If it's not easily distinguishable from Earth yet exists in a massively different astronomical context, that's getting into isekai territory. $\endgroup$ Commented Mar 26 at 11:01
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Green is no Good

I dont think making your Stars Green is a good option here. Green stars suggest Planck´s Law is different. Which, if you know how it is derived, is a big uff. Planck´s Law being different suggests several fundamental aspects (Including $\pi$), are different. Which ultimately translates to a universe in which not a single law of physics applies as we know them.

A Closed Universe

There are much more reasonable options at your disposal. One of which is a closed Universe. The ultimate curvature of the Universe is related to the Cosmological Constant $\Lambda$. As the derivation of $\Lambda$ proves, it can be 0 or any arbitary value without violating any core principle of physics.

Right now, we think the universe is flat. However, this dosnt have to be the case. Your Alt-Universe could have negative curvature, creating a big hypersphere.

One of the interesting properties of such a closed universe would be that for one, Energy is conserved on a global scale which is very interesting. But even more interestingly, if you walk long enough in one direction you will end up where you started.

As such, the Stars in the sky would appear mirrored, each reflecting getting bigger and fainter.

enter image description here

Finding proper simulations of this is hard so use your imagination. The stars would be fainter because each reflection looks further in the past and covers a greater distance. The Inverse Square law still kind of applies.

Of course, if the Universe is sufficiantly small each reflection would add more energy than is lost (at least till some point) so the stars may appear a lot brigther than usual.

The point is, there is a balance where everything looks more or less right but you kind of get the feeling that parts of the Sky (especially bright stars) are mirrored and seem to have ghosts around them.

The Big Crunch

Another more sane option is to have a universe which is actively in the process of imploding.

Due to how that works, your MC would be under a bit of a time crunch. As spacetime collapses into itself, all radiation ever emitted is Blueshifted. So everything gets cooked.

But, there is a short window of time in which your MC could make some interesting observations. If the implosion is fast enough, and there are not too many stars, they will notice that the night is pretty warm for the fact there is no sun. This is all the radiation being Blueshifted into the Infrared range.

They might also notice that the stars sure appear to get brighter as time goes on. Not just that, you kind of cant shake the feeling they are getting closer.

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    $\begingroup$ The light emitted by the stars could still follow Plancks Law, but be "modified" afterwards. Perhaps the eldritch entities moving through space selectively blue- or redshift certain frequencies, so that the only equilibrium is a sickly green... (they might either permeate all space, or just surround the planet you are on, and it would suffice if most photons encountered a few entities through their whole journey). $\endgroup$
    – sh4dow
    Commented Mar 26 at 13:34
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    $\begingroup$ And if the MC feels it being warm in the night, the actual sun of the system would have to be very weak(/far away) to prevent the planet quickly warming to lethal temperatures (and even then, the local temperature would tend to something greater than that of the blueshifted background radiation). Also, any perceptible warming of the background radiation would strongly lower the local entropic gradient, meaning any life on such a planet would die out over a relatively short peroid of time (weather and other dynamic pattern would also cease to exist). $\endgroup$
    – sh4dow
    Commented Mar 26 at 13:39
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It might not be what you're looking for, but let me give it a shot.


(...)

Then I looked up.

The galactic globes of fiery gas that I so dearly loved to watch from my balcony were gone. In their place, an army of impostors - made of colors and lies and smells that couldn't be, that didn't exist and yet presented themselves to me in a blanket of defiance against anything I knew I knew.

The liars were uncountable. Bright grains of sand against a beach of darkness, that webbed and flowed with tides of entropy and wrongness. They danced together in pairs and trios that split up and embraced again, ever so slowly yet so fast moving.

A noxious nebula took a large slice of the sky, glowing in blood-sick red with pockets of pustulent sickness. From its womb, dark stars glowed in reverse, sucking in the glow of the red and spitting out glimmers of pure shadow.

A single, lonely blue moon cast its light from above, so immense that it could dwarf my own home planet, yet here it was nothing but a companion for whatever world I've found myself in. I couldn't take my eyes off it once we locked gaze - it felt entrancing, calming, yet so wrong - like the embrace of a forbidden lover of the indulgence of the addict. I shouldn't be there. It was telling me so.

Once I blinked, it blinked back to me - the moon now a single massive eye of red and blue and purple and pain and sorrow and joy and fury and a dance and hope and a scream and me and itself and nothing more but everything.

I've intruded into the domain of Gods.

And I wasn't welcome there.


I see two paths here.

  1. Our adventurer doesn't know it, but they ventured inside a very advanced civilization's domain. They managed to cover multiple words with dyson-like spheres, enabling them to create whatever skybox they want for no other reason but for their own art. Unfortunately for our hero, their minds are quite different from ours, and what they consider art might be quite unsettling for those not used to it.

Or

  1. Our adventurer stumbled into a different universe with different physics, different powers, and natural laws far more flexive and permissive from what he could find at home. Here is but a mere bug on the back of a living planet - and this living planet is just one in a swarm of beings like it.
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  • $\begingroup$ I don't think looking from a balcony you can measure the absolute size of a moon, and derive "it's bigger than my home planet". That always depends on the distance. $\endgroup$ Commented Mar 26 at 0:35
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    $\begingroup$ @PaŭloEbermann That's the inner mind of the character, not the author. He is allowed to be wrong! $\endgroup$
    – Mermaker
    Commented Mar 26 at 3:38
  • $\begingroup$ @Mermaker I firmly believe the author is allowed to be wrong as well. Just has to be willing to take crap for it. $\endgroup$ Commented Apr 10 at 20:14
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Context: Lovecraft wrote in the 1920s. Many of his protagonists are explorers, navigators, etc. Also, cities were smaller and had far less light pollution. Combine these two facts and you have people who know the night sky fairly well. These people in that time would notice if the constellations are all messed up in a way that is not just an unknown place on Earth.

If your protagonists are familiar with the night sky and routinely use it for navigation, it doesn't take any of the more exotic answers here to make them realise that they cannot possibly be anywhere on Earth. Equatorial constellations like Orion or Virgo are visible from most of the Earth's surface most of the year. And a set of just a few constellations can quickly tell me where on Earth I am. Finding Polaris will tell me I'm in the northern hemisphere, and with Polaris I've found the Big Dipper and its height in the sky gives me an idea of my latitude. If there's no Polaris, I can look for the Southern Cross and likewise establish my general latitude.

People who navigate by the stars - which was still common practice in the 1920s about - would know these things and the major constellations and important stars.

It seems logical to me that Lovecraft and other authors are referring to this knowledge rather than some obscure physics properties.

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  • $\begingroup$ Not only logical, but blindingly obvious, basically the OP is unreasonably ignorant not to understand that this is all there is to this particular 'trope' to the extent that they even thought this was a question worth asking. $\endgroup$
    – Pelinore
    Commented Mar 26 at 9:27
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    $\begingroup$ I thought the OPs question specifically asked to distinguish being able to tell "somewhere not on earth" from "not in any place that fits our known laws of physics" (or, to quote, "they are likely not even anywhere in our reality"). So this isn't about applying the existing trope, but extending it. $\endgroup$
    – sh4dow
    Commented Mar 26 at 13:45
  • $\begingroup$ @sh4dow the "existing trope" doesn't stop working when it's 'another world' or universe, it's the same stars and constellations you're used to or it's not, that's all there is to it .. so it doesn't need "expanding" to fit other universes or realities, the question is, to put it mildly, a bit dumb if the OP really thinks it needs any expanding beyond that for 'other' worlds .. and deciding precisely how it differs is story based so entirely up to him and off topic for here anyway. $\endgroup$
    – Pelinore
    Commented Mar 26 at 22:50
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    $\begingroup$ @Pelinore perhaps so, but that isn't the topic of the question (though the title alone is ambiguous) $\endgroup$
    – sh4dow
    Commented Mar 27 at 23:22
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There are about a billion different ways a person might notice the stars seem off. I don't want to insult Christopher Bennett or The Author but generally I disagree with a few of their assumptions.

Generally having different constellations or less light pollution would most likely be easily rationalized a way. As most people don't know enough about this stuff to instinctively know they are in a different world.

So here is what I think would cause a very instant reaction:

  1. There are more stars: Just like Christopher Bennett suggested if there are more stars in the sky or they are brighter,it would have a rather instant effect. Though it would probably need to be a lot brighter to have a noticeable effect. Probably the most immediately noticeable or if it is not enough,it just gets completely ignored. So probably not the most ideal solution.
  2. There are fewer stars: Also similar to that of Christopher Bennett. Unlike his suggestion though we could generally assume that the world is in a galactic super void or something similar. Meaning there's a lot less light from stars so this would probably give off an instant bad feeling. Though there may be still enough stars for them to slowly feel more and more uncomfortable.
  3. Some star are very different: There could be a more subtle way to show they are in a different world. You may just place a few odd very bright stars in the sky and anyone would instantly notice something is off.

Ofcourse, beyond these there are a lot more possibilities. if we want to include the supernatural. But I left them out as this question did not specifically ask for it.

Note:English is not my primary language so please excuse any grammar or spelling errors.

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    $\begingroup$ If I would find myself under a night sky with very few stars, then my first hypothesis would be to blame the weather. When the stars are just in wrong locations, then my first assumption would be that I traveled to the southern hemisphere. I wasn't there yet, but I know that the night sky is totally different there. But someone who was already on both hemisphere would notice that the sky can't be either hemisphere. $\endgroup$
    – Philipp
    Commented Mar 25 at 10:48
  • $\begingroup$ Yes i would probably rationalize all of them if they are not too extremely obvious. Number 3 is a lot different because something like 3 to 5 very bright stars in the sky would definitely give me a feeling of dread. Though while i would not automatically conclude i was in another world it would likely still lead to me thinking about this possibility constantly. $\endgroup$ Commented Mar 25 at 11:06
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Same but different

As @Nosajimiki sagely points out, my first instinct (make the sky so radically different that an Earth person would immediately know it was different) would be the wrong way to show a different universe.

Most reasonably educated people, even non-astronomically-inclined city dwellers like me, know at least one constellation: The Plough/Big Dipper/Ursa Major, Cassiopeia, Orion, the Pleiades, the Southern Cross... something.

Bonus points for a subtle and uncanny feature that allows for more gradual realization

Given this, let's make the reveal a multi-step thing.

  1. A pretty Aurora: weird but OK. Glowing with colours, a pretty light show. They look at it for a few seconds, but they are too busy with other things to think much about it. They would perhaps be surprised to see an aurora. ("Aurora Borealis!? At this time of year, at this time of day, in this part of the country?..."). It'd be weird, perhaps a little unnerving in the context, but not supernaturally so.

  2. Known constellations: comforting touchstone. Let the protagonist spot a couple of the constellations that they know. Preferably two well-known ones which are a decent distance from each other. So, say, Orion and the Ursa Major. Someone reasonably educated would know two constellations, and would understand that being able to see two that are at a significant angle to each other means they can't be too far from Earth in any direction. Seeing only one would mean they're in a line from earth to the midpoint of that constellation. Seeing two means that you're somewhere in the intersection of the two lines, which gives probably less than a lightyear of scope for movement, especially for those constellations with fairly close stars.

    Be sure to pick two constellations that are visible from wherever the protagonist is on Earth: the Southern Cross is only visible from the southern hemisphere, and Ursa Major from the northern!

    So at first, seeing their familiar constellations, they would be comforted. I am comforted whenever I see the Plough. It's like a touchstone to me.

    Perhaps a couple of stars in the constellation are hidden behind the Aurora, but it's still easily recognizable.

  3. A Nebula: the penny drops. But over time, because it is static, with features and details that remain constant over multiple hours, the protagonist will slowly realize that the glow in the sky is not an aurora, a localized atmospheric phenomenon, but something bigger in scale.

    One way to play with this is to have savvy readers spot this before the protagonist. Perhaps, in their initial glance, have them notice a curl that appears to wrap around the moon. In a later scene, describe that the moon has moved out of that curl, then that the moon's setting, and maybe even have the protagonist calculate time's passage by comparing the distance from the moon's position to the curl, dividing the arc of the sky into 12 to get approx time in hours, or treating the moon as a half-degree (fairly common knowledge I think), and counting the distance to the curl in moon-widths to get time in 720ths of a day, without explicitly mentioning or making any point of the fact that the curl (and hence the aurora) is stationary, until the protagonist goes "hey wait, why isn't this aurora moving?"

    However they realize, the protagonist can then look closer, and see more static details, sweeping tendrils of glowing gas across the sky. The moon isn't just "bright enough to shine through" the aurora, it's in front of the aurora. Has something happened in space? A big solar flare, or even something hit the far side of the Moon and filled Earth's orbit with dusty ejecta...?

  4. Behind the stars: unimaginable scale. To really drive home the scale to the reader, the protagonist may investigate further, in which case they will realize that the nebula is BEHIND the constellations we all know and love. Encompassing the whole Milky Way, that it's not just planetary, interplanetary, or interstellar, but intergalactic. Or at least, too nebulous to have a significant effect within just the few hundred light years that contain the stars of the constellations.

    So the constellations would be visible as they are all made from nearby stars, only a few of them more than about 1k light-years of us, in our local area of the 100-light-year-across Milky Way galaxy. But Andromeda (another galaxy, 2.5 million light-years away) would be hidden by the glowing nebula. The Pleiades (442,000 light years) could be hidden, or just somewhat faded.

    You might argue that knowing where to find galaxies or star clusters in the sky is a bit of a stretch, and I agree. I recognize the Pleiades when I see them, but I wouldn't be able to tell if they were missing, because I don't know where they are relative to other stuff. I suggest having them find it out from their phone's star map app. If the alternate world is similar enough, they can always just connect and download one, but otherwise, you can handwave that they have one that has the star maps cached and the last known geolocation cached, so being unable to connect to GPS and cell data would be another hint they aren't in the same reality, but wouldn't prevent the app from working. Either way, that'll show them where they SHOULD be able to see the various constellations, and they should match up perfectly... except for the missing distant stuff.

  5. Science: Is that even possible? Further investigation could show them that the largest known nebula is the NGC_262 halo cloud, only 1.3 million light years across, so what they're looking at is plausibly big, but its existence around the Milky Way implies at least subtle differences in the physical laws or formation of this universe, or some natural event at intergalactic scale, or some immense supernatural or alien influence, far beyond the normal imaginings of man.

This kind of scale should be a good place to stat with Bramblesandian "eldrich madness":

People, especially games, get eldritch madness wrong a lot and it’s really such a shame.

An ant doesn’t start babbling when they see a circuit board. They find it strange, to them it is a landscape of strange angles and humming monoliths. They may be scared, but that is not madness.

Madness comes when the ant, for a moment, can see as a human does.

It understands those markings are words, symbols with meaning, like a pheromone but infinitely more complex. It can travel unimaginable distances, to lands unlike anything it has seen before. It knows of mirth, embarrassment, love, concepts unimaginable before this moment, and then…

It’s an ant again.

Echoes of things it cannot comprehend swirl around its mind. It cannot make use of this knowledge, but it still remembers. How is it supposed to return to its life? The more the ant saw the harder it is for it to forget. It needs to see it again, understand again. It will do anything to show others, to show itself, nothing else in this tiny world matters.

This is madness. -- Bramblesand, tumblr (worth reading the rest of that thread).

Why does this work?

It doesn't require "navigator"-level sky knowledge. I think enough of the readers can recognize two constellations that even a city dweller knowing them should be believable and relatable to the reader, unless your world is similar enough that their phones can connect, in which case they can always just Google it.

Some non-Earth planet would not have the same constellations. Earth in some other time would not have the same constellations. And nowhere near Earth would have the same constellations, and still have the galaxy be within a hugely vaster nebula.

This is unlikely to be the same reality as they were in the day before, as that would require the nebula to have come into being across the entirety of the night sky in one day, which would require a faster-than-light spread of the nebula, so seems like an unlikely possibility. If there are other people in that reality, or internet sources there, who treat the nebula as if it has always been there, that would solidify this conclusion.

They can't fully eliminate the possibility of hallucination or insanity or other mental effect that causes them to either imagine the nebula, or to imagine that it once was not there. But some reasoned thought and confirmation from third parties (eg the cached star map, other people who came over with them) should allow them to rank that as fairly low on the probability list.

The only "likely" conclusion, then, however improbable, should be that this is Some Other Earth, in Some Other Reality. Maybe another timeline, a simulated reality, an alternate universe... but it is Other.

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This is a common trope because, until recently, most people were well-familiar with the night sky, enough to notice small differences. Even now, there are people, like myself, who do still know constellations and would notice if they were off. I don't know them all, but I always look for Orion, and if it had 5 stars in its belt, or something, I would notice it.

But for a modern city-dweller, distanced from nature, the most immediately obvious change would be if there were a lot more stars, perhaps so many that they light up the night. Another easily noticeable celestial change would be multiple moons, or a moon that was a different size, color or shape. For maximum horror, you could use an image from one of my childhood nightmares, of the moon being a blank-eyed mask hanging in the sky.

A more subtle, and perhaps creepier change would be if there were no stars (or few stars), just velvety blackness. You might at first mistake that for an overcast sky, but I think that would stand out pretty quickly.

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If you significantly change the positions of the stars and either their density or brightness, your protagonists will pretty soon realise that they are not on earth, but could still be somewhere in our universe.

Changing the colours of the stars as suggested by Gray Sheep will likely convince an amateur astronomer that something is impossibly wrong, but may not be the case for an observant non-astronomer.

So how about de-randomizing their positions in the sky. Maybe they produce an image of a face or message (or maybe just a geometric pattern) if you connect the dots. It could be subtle enough to not be initially obvious, but once pointed out would convince even the most skeptical.

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    $\begingroup$ I kind of like this. How about this variation: the stars slowly (over the course of a week, or month, or something) cycle between randomized positions and perfectly aligned to a grid? $\endgroup$
    – Dúthomhas
    Commented Mar 25 at 9:55
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One) If the new location is in a multiple star system, the other stars might be so far away from the planet that they don't appear as discs and suns but as dots of light in the sky. But they might appear many times brighter in the sky of the planet than any stars in Earth's sky.

Since other stars in the star system should be hundreds or thousands of times closer than stars in other systems, they should appear tens of thousands, hundreds of thousands, or millions of times as bright as the stars in Earth's sky.

Maybe when it or they are above the horizon they dim the distant stars in interstelalr space considerably and are all that someone can notice or pay attention to, but when they go below the horizon the other stars in interstellar space become very noticeable.

Two) Maybe, as others have suggested, the planet is in a denser region of space where a lot more stars are visible and some of them are much closer to it than any stars are to Earth, and thus appear much brighter.

Three) You may have heard about a science fiction novel, The Mote in God's Eye (1974) by Larry Niven and Jerry Pournelle. When I read it I tried to think of an astronomical phenomenon which could be called "the Beam in God's Eye".

Suppose there was a world in the Virgo Galaxy Cluster where the great elliptical galaxy M87 appears as a circle of light in the sky, brighter toward the center, and the world is close enough to M87 that the light emitted by the relativistic jet of matter from the core could be seen. Thus if the natives call M87 the eye of their god, the jet could be called "the beam in God's Eye".

A parsec is a distance at which 1 Astronomical Unit (AU) would appear to be one arc second wide. A parsec is 206,265 AU long. The jet of M87 is about 1,500 parsecs long, and thus, seen from the side, it would appear 1000 arc seconds long at a distance of 3,093,975 parsecs, 10,000 arc seconds long at a distance of 309,397.5 parsecs, 100,000 arc seconds long at a distance of 30,939.75 parsecs, etc.

The maximum resolution of the human eye is about 28 arc seconds or 0.47 arc minutes or about 0.0078333 degrees. Thus at a distance of 3,093,975 parsecs the jet would appear to be 7.8333 degress wide if seen at a right angle, which is about 15.666 times as wide as the Moon seen from Earth. I note that an astronomer is said to have seen the jet with his eyes through the 100 inch telescope, instead of in phtographs, so someone close enough to the jet should be able to see it with their unaided eyes from the surface of a planet.

Four) Suppose that two spiral galaxies were headed for each other on a collision course, like the Milky Way and the Andromeda Galaxy are. If a planet is on the opposite side of its galaxy from the other galaxy, it would see both, one behind the others. If that planet orbited in the plane of its galaxy, it would see its galaxy's disc as a band of light across the sky, like the Milky way is seen from Earth.

And if the other galaxy's plane was at an angle, it would be visible as an oval area of light in the skay. But the other galaxy could be oriented with the edge of its disc facing the planet. And the other galaxy's disc would probably be inclined at an angel to the disc of the nearer glaxay. Thus two milky ways would seem to cross each other in the sky.

And I have some other ideas.

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Negative space

Instead of something being different with the stars, something could be different with the space between the stars. There are a couple of possibilities here.

The easy one is "no Milky Way." The person stares at the stars for a while, then realizes that they're weirdly homogenous. Not rectilinear (that would be super-weird, but also super-obvious), but no patches of brightness or darkness. When they get to an area of open sky, they realize that there's no milky band of stars stretching across the sky. On closer examination, there are no nebula or other galaxies.

If you want to get creepier, you can leave the galaxies and nebula, but have shadowy tendrils that block out strips of them as if giant, black octopuses were grabbing stars and consuming them.

Both of these are physical impossibilities in our universe, making it clear that the laws of physics are no longer being obeyed.

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  • $\begingroup$ You couldn't distinguish stars from galaxies with the naked eye, and a more homogeneous distribution of stars wouldn't necessarily preclude you being in some other place in our universe - the distribution might not fit any place we know of/can think of, but that's difficult to judge for a layman. And even the most stunning arrangement (eg. rectilinear) might be explained by the existence of type 3 civilizations creating artificial structures of multiple stars, eg. for travel (a 100x100x100 grid of stars might be a comfortable "generation ship")... $\endgroup$
    – sh4dow
    Commented Mar 26 at 14:05
  • $\begingroup$ @sh4dow, Andromeda has an apparent brightness of 3.4, so it's well within naked eye visibility. The Large Magellanic Cloud is even brighter, and is clearly not a star. Although notable homogeny could be accomplished through artificial means, it would clearly violate most people's intuition. $\endgroup$ Commented Mar 26 at 19:44
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    $\begingroup$ I kinda like the idea of giant black tendrils blocking out stars. They could be subtle at first, especially if not too wide, and if they move slowly. $\endgroup$
    – LarsH
    Commented Mar 26 at 20:45
  • $\begingroup$ @RobertRapplean not every galaxy necessarily has satellite galaxies, and whether you are located in an isolated galaxy or some more unusual collection of stars (perhaps the remains of a galactic collision, with the original galaxies indistinguishable from stars?), not seeing any non pointlike object doesn't immediately preclude being in this reality. Also, while there is certainly some amount of homogeneity that effectively precludes a natural (or at least random) origin, I think most people would have difficulty making remotely accurate judgements about that (unless you literally have a grid) $\endgroup$
    – sh4dow
    Commented Mar 27 at 23:40
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    $\begingroup$ @sh4dow, I think we'd have to generate images to actually test this. My supposition is that there's an uncanny valley to the pattern of stars in the sky, where an artificial arrangement looks unaccountably wrong. If you were to adjust physics so that the stars arranged themselves similarly to how atomic nuclii spaced themselves via electromagnetic repulsion in a non-crystaline structure, I think people would notice. $\endgroup$ Commented Mar 29 at 19:02
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The stars are slowly going out. It's gradual enough that the protagonist doesn't notice the first few times they look up, but maybe they happen to catch one get snuffed out. Then they notice that each night there are less and less in the sky. This helps to establish that something dreadful is happening and I need to get out of here.

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  • $\begingroup$ Outer Wilds be like-- $\endgroup$
    – value1
    Commented Apr 19 at 12:38
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I needed exactly this, something immediate with the night sky that is just wrong. I went with a cosmic TV set, the night sky currently has a battle going between a dragon and knight(with bits flying off into the distance from previous attacks and hits). It changes slowly enough that a few seconds glance is alright, but is noticeably different over the course of a night. This particular sequence has been playing out (according to the locals) for a couple months. Before that it was the knight getting married. The newcomer arrives in the middle of summer, they are told that in winter the night sky shows colored ribbons dancing. And over the course of my story they do move far enough (several thousand miles) that parallax should be apparent if all this were not very far away.

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In one of Greg Igan books, IIRC - Dichronauts, different wavelength of light have slightly different speed.

The stars with high proper motion look like colored lines - blue at one end and red at another.

If star is faint enough then you cannot see the color with naked eye but you can see the dash-like shape anyway. Though in order for the dash to be long enough to be seen with naked eye you need to have slow speed of light or relatively high star speed.

(though in Dichronauts there will be A LOT of other tell tale signs that you are in another world :) )

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    $\begingroup$ Dichronoauts is the ++-- universe, with the hyperbolic sun. Orthogonal, the ++++ universe, is the one with the rainbow stars $\endgroup$
    – No Name
    Commented Mar 26 at 10:59
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Given these two key points:

... A modern citizen got lost beyond space and time.

... so they realize that they are likely not even anywhere in our reality.

You could interpret "beyond space and time" as "not in our universe." You, as the author, have a lot of wiggle room with this concept. Strictly speaking about our current understanding of the universe, there is no "outside." You get to decide what "outside" looks like.

The second point about being outside our (the readers') reality basically gives you a blank check to make something up. Of course, this doesn't provide a concrete answer. The key here is defining what "beyond space and time" means, and how that should look.

If the protagonists are "outside" of the universe, and our universe has no outside, then we are looking at some sort of multi-verse. Perhaps the sky is blanketed in stars from every universe. It's like the sky is broken up into distinct patches of stars and galaxies, where each patch is one universe. The boundary between universes has some sort of visible distortion as spacetime gets warped — like a bunch of curved and warped mirrors melded together as a rolling tapestry across the sky.

This would certainly stand out. We are accustomed to seeing the same thing in every direction. Nothing but a sea of points of light without significant bending or distorting.

As an illustration, imagine the sky covered in Cloud Gates:

Photo of the Cloud Gate sculpture in Millenium Park in Chicago, IL, USA

Credit: Wikipedia

Except you see stars in the reflections instead of a city.

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Disturbingly Orderly

One possibility in some settings: they look up and the stars are not randomly placed. There's a pattern to these stars: anything from a tesselated geometric grid pattern to something more subtle like a fractal pattern.

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You awaken under a night sky untarnished by man's artificial light. How full the heavens are with stars, like a field at harvest! You lie on the grass to gaze, as we all should when shown those far-flung fires. As your eyes acclimate to the night sky, you are able to catch sight of ever dimmer stars. It is almost as if they are born in that moment.

Gradually, almost imperceptibly, the first colors of daybreak creep through the air. You cannot tell which way is east, for the morning light seems to be coming from all around you. You look back up at the stars, and then it dawns on you: there are even more stars than the last time you looked. They seem to be filling every corner of the sky.

Welcome to an infinite and eternal universe.

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  • $\begingroup$ Doesn't a theoretical infinite and eternal universe have an even, continuous luminosity at all times? I don't think in this model you see stars appearing in realtime to fill the blackness; there instead is no blackness, because of the literal infinite number of stars in the sky, no? $\endgroup$ Commented Mar 29 at 21:44
  • $\begingroup$ This particular universe is eternal, but not that old yet. There may be an infinite number of stars in the sky, but they're still working on filling the sky. $\endgroup$
    – skeep
    Commented Mar 29 at 21:47
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Just above the horizon, visible only from an elevation and on a clearest possible night, there is a new constellation in the sky.

This constellation is a perfect triangle, with dozens of stars aligned in perfect lines to form its sides.

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Mysterious Colors unlike Any Seen on Earth!‎

To borrow straight from Lovecraft (and take it a different direction), it would be pretty hard to ignore if the stars' white light contained strains of a color you could see but had no point of reference for.

You wouldn't be able to pick out the alien color individually even if you could comprehend it, but something seems off about the sky in a way you just can't stop staring at, and as over time you come to understand why, it captivates you even more. You stare at a particular star until the whole sky sears spots into your vision, and in the afterimage you see the faint, toothless echoes of a blackness that would sink your thoughts and memories into itself.

Anyway you've never seen stars do that before, so with bombastic confidence you declare something Pretty Weird probably afoot.

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There are no stars at all.

Not exactly an answer to your question as expressed, but if you change it to something like "How can the look of heaven [rather than the stars] make it obvious that the viewer no longer is in our Universe as we know it?" then I think it still catches the spirit of your question.

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