This seems like an appropriate place to ask. A lot of arguments I have seen regarding time/weather/environment seem to dredge up "The sky is artificial" as an "answer" to the question or problem. My question about this, is it a KNOWN fact that anyone in game will know as a truth of the world? I figured that one would simply adjust psychologically to the circadian rhythm naturally as it is presented to them (hence why all my characters are absolutely culture shocked or even mad the first week of inception.) But that doesn't account for the answer being that the sky is artificial. How would my character know that as a truth, or would he know it at all? Does it take some kind of special scholar to know it? is it written somewhere? I'd imagine its a pretty convincing sky, fake or no, and that non of my guys have the itch to climb up to the ceiling and test it.
It is not a known fact that anyone in-game will know as the truth. Most outlanders will no doubt feel that the world is off and slightly unnatural, but there's no way to tell you're on a demiplane because it's a fairly convincing facsimile. The sky seems real enough. The stars shine, the moon has phases, the sun shines and feels warm. For all intents and purposes, it looks just like the real night sky. The celestial objects just aren't "there": the sun isn't a nearby star a million miles away, the moon isn't a giant hunk of rock exerting gravity, and so on, but nobody knows this because there's no way they could confirm it. If, by chance, and character from the Spelljammer setting (D&D in space!) was brought into the demiplane along with his/her spelljamming vessel and tried to fly to the moon, the ship would just end up in the Mists. But that would be a rare occasion, and nobody would believe a bunch of outlanders who claim they can travel the stars in a magical flying ship.
The only NPCs that would be aware of that the sky is fake are probably Azalin and Strahd, and they're not going to discuss it with your characters.
Guessing from the era the setting is based on, I highly doubt anyone even knows the actual science behind the moon and the stars.
So would it even matter? The common people probably have all sorts of different superstitious explanations for the appearing of stars.
They most likely have no clue that the sun is also a star, nor that the world may be round, if that is even the case in this setting at all.
I'm also curious what the commonly known explanations are though. Or the most popular theory to explain the stars and the moon.
It's not touched upon too much. Gazetteer II has a diagram of the universe according to Lamordians, though.