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On a moon with an thin artificial atmosphere (think force field filled with Earth-like atmosphere), would the sky be a blue colour or have a blue tinge, or would the thinner layer of atmosphere (about 1km) mean it would just appear clear?

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    $\begingroup$ Since force fields are made up, you could have the field itself produce any kind of sky effect that would help your scenario. $\endgroup$
    – Willk
    Jun 19 at 0:05
  • $\begingroup$ Yes, I suppose that's the case! I was imagining it possibly being clear, but it could be nice to add a tinge of colour from the force field as well :) $\endgroup$
    – user96461
    Jun 19 at 3:38

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One kilometer of air won't do much scattering of light. The sky will appear very dark, almost black.

In addition, because light won't be scattered, the shadows will be very crisp and very dark.

At sunrise and at sunset, when the light gets to cross a longer distance through the air, the sky around the sun would appear blue, as it does on Mars.

Unless, of course, the atmosphere has some significant amount of suspended materials, such as dust, or ice crystals, which would greatly increase the scattering; but then the aspect of the sky would depend on the specific material.

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  • $\begingroup$ or, of course, if the forcefield holding the atmosphere in is also tinged a particular colour $\endgroup$ Jun 19 at 2:07
  • $\begingroup$ Would you be able to see stars do you think? I was imagining it maybe being like under an eternal night sky, but I'm not sure if the sunlight interacting with the atmosphere would make this unlikely... $\endgroup$
    – user96461
    Jun 19 at 3:43
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    $\begingroup$ Sunsets on Mars appear blue because the Martian atmosphere scatters red light, so only blue light is left when you observe the sun directly through a thick layer of atmosphere. An Earth-like atmosphere would still scatter blue light, so the sunset would still appear red-tinged, not blue-tinged. $\endgroup$
    – N. Virgo
    Jun 19 at 10:38
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    $\begingroup$ I agree that the sky would be pretty much black though. It's probably also worth mentioning that such a thin atmosphere wouldn't stop much UV light, so going outside would lead to instant sunburn, unless the force-field bloks UV. $\endgroup$
    – N. Virgo
    Jun 19 at 10:39
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    $\begingroup$ @N.Virgo "An Earth-like atmosphere would still scatter blue light, so the sunset would still appear red-tinged, not blue-tinged." Wouldn't that mean you'd see a band of blue fading in at ninety degrees to the Sun? IIRC that's where the bluest part of the sky is on Earth. $\endgroup$
    – nick012000
    Jun 19 at 10:57
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The atmosphere could also be held in place by a mostly but not entirely transparent roof over the whole moon, with giant airlocks for spaceships to land and take off.

A shellworld[1][2][3] is any of several types of hypothetical megastructures:

A planet or a planetoid turned into series of concentric matryoshka doll-like layers supported by massive pillars. A shellworld of this type features prominently in Ian M. Banks' novel Matter.

A megastructure consisting of multiple layers of shells suspended above each other by orbital rings supported by hypothetical mass stream technology. This type of shellworld can be theoretically suspended above any type of stellar body, including planets, gas giants, stars and black holes. The most massive type of shellworld could be built around supermassive black holes at the center of galaxies.

An inflated canopy holding high pressure air around an otherwise airless world to create a breathable atmosphere.[4] The pressure of the contained air supports the weight of the shell.

Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called bubbleworlds or gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed; the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.[5][6]

https://en.wikipedia.org/wiki/Shellworld

And see the sources cited.

A transparent or translucent shell of material surrounding a world would be a vast project but wouldn't require inventing force fields that can hold in atmosphere.

Possibly a fictional moon might have its artifical atmosphere held in place by both a force field to keep in air while meteorite and asteroid punctures to the material shell are being repaired, and a material shell to hold in the atmosphere if the force field generator goes off line.

And possibly both the not one hundred precent transparent shell and the force field would modify the appearance of the sky. And if one of the characters is an expert in the subject, they might notice the color of the sky change slightly when the force field goes down unexpectedly - no doubt right before the moon is predicted to pass thorugh the annual meteor shower and get lots of punctures in the shell.

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An artificial atmosphere can be nearly any color the makers want it to be. By adding particulates to the atmosphere, they can shift the perceived color dramatically.

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  • $\begingroup$ If the atmosphere is thin, depending on the solar wind there could be increased Aurora Borealis due to the reduced pressure. The color of the Aurora would depend on the composition of the atmosphere. $\endgroup$
    – UVphoton
    Jun 19 at 12:02
  • $\begingroup$ Aurora is caused by the magnetic field focusing solar wind particles at the north or south pole. The moon doesn't have a magnetic field, so (when it is outside earth's magnetic field) solar wind will cause a very weak tinge of greenish color over the entire sunlit side of the moon. As that is the daylight side it won't be visible. $\endgroup$
    – JanKanis
    Jun 20 at 7:26
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Look at any picture of mountains.

enter image description here

Clearly, there is a lot of blue in front of even close-by hills. Mostly you're looking for Rayleigh scattering. There can be other sorts of diffuse sky radiation from any sort of humidity, haze, or particulates. In at atmosphere under so low a ceiling there is quite a bit of weather, but hopefully not much pollution on a a well managed space habitat!

Rayleigh scattering adds up over distance - the mountains you see are not so bright as the blue sky above them - but outdoor lighting is very bright. During a partial eclipse the sky looks dimmer, as if it were earlier or later in the day, but it doesn't seem black. You'll still have some sort of blue sky.

The most off-putting part will be if air is chilled at the boundary with space (since there is no infrared heat coming back at it), which could lead to a very visible artificial boundary of condensation.

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  • $\begingroup$ But how far away are those mountains? Chances are the ones looking blue are more than a kilometer away. So you might see a bit of blue when looking toward the horizon or to a lunar mountain, but not when looking up into the sky. $\endgroup$
    – JanKanis
    Jun 20 at 7:32

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