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If a planet were to have a very thin atmosphere how thick or thin would it have to be to allow there to be a daytime and a nighttime given human or near human ocular sensitivity?

In other words, there should be no differentiation between day and night unless there's enough of an atmosphere to facilitate dispersion of the light and such. Is this true and if so, what's the boundary point of this?

What might it look like just before and past this boundary point?

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  • $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. Given that the surface will light up just fine without any atmosphere, are you asking how to get an illuminated sky? If so, that's an issue of ocular capabilities because any atmosphere will illuminate with enough ocular sensitivity. In otherwords, this could be a simple question with a complex answer. $\endgroup$ Jul 23 '18 at 6:23
  • $\begingroup$ Good call @JBH. Yes, I'm referring to the illuminated sky and having ocular abilities or sensitivity similar or the same as humans. $\endgroup$
    – ylluminate
    Jul 23 '18 at 7:10
  • $\begingroup$ Can the question be interpreted as "At which atmospheric density Rayleigh scattering becomes visible to unaided eye"? $\endgroup$
    – Alexander
    Jul 23 '18 at 20:52
  • $\begingroup$ @Alexander excellent observation. Sure helps to know key terms like this and so yes, I think you have nailed down the proper terminology and research for this! $\endgroup$
    – ylluminate
    Jul 24 '18 at 16:34
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The question is a bit difficult to answer, but for comparison, Mars has atmospheric pressure about 0.6% of Earth's, and Martian skies are indeed colored, but the color is butterscotch. The Rayleigh scattering effect on Mars is very weak, though, and sky color is due to iron compounds molecules in air, not the air gases itself.

So I think that 0.6% of Earth's atmospheric density is very near for human perception limit to notice daytime sky color.

Martian sky

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The difference between day and night is due to being exposed to the light of the main star, and also of defining what EM spectrum you are interested in (in the gamma ray region most of the Earth surface would not be in the darkness).

Atmospheric refraction accounts just for the few minutes around the transition from day to night, those moments are called dusk and dawn on Earth. No atmosphere means no swift transition from day to night.

Having no difference between day and night can either mean:

  1. permanent illumination
  2. permanent obscurity

Let's go through the two points.

Permanent illumination

For the half of the planet facing the star, that'd be easy. For the other half, the only way to get light from the other side would be having the atmosphere acting as a wave guide, trapping light thanks to total internal reflection. This can happen for some radio wavelengths on Earth thank to the ionosphere, but otherwise it's not achievable due to the small difference in refractive index between gas and vacuum.

Permanent obscurity

This is more easy: just have an atmosphere which is totally opaque to most of the star radiation you are interested in, and the difference between day and night will be almost zero. For the Earth-Sun case, volcanic hashes can do a good job at shielding the visible light.

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  • $\begingroup$ Let's take the case of earth and an earth-like planetoid. What if we were to reduce the atmosphere of earth by 100x or 1000x, would it still have a daytime? Would the atmospheric thickness be enough to support light diffusion and diffraction that seems to lend to the daylight levels we would consider "daytime"? This also effects this like sunrise and sunset since the atmosphere thickness would reduce or extend the amount of time the light reflects and refracts within the atmosphere during those very "grey" hours. $\endgroup$
    – ylluminate
    Jul 23 '18 at 7:08
  • $\begingroup$ @ylluminate, see my edit $\endgroup$
    – L.Dutch
    Jul 23 '18 at 7:15
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There is no boundary point. The sky is gradually going to get darker the lower the atmospheric density, all else being equal. What you would consider "lit up" will be subjective (and also dependent on whether your eyes are adjusted to the dark or not.) Furthermore if you vary the atmospheric composition you can greatly change the visibility of the atmosphere and its ability to scatter light at a given density.

For Earth atmosphere at least, you can get a pretty good idea of density/brightness relationship by watching a video taken from a high altitude balloon launch (or rocket launch, but I recommend balloon since it's more gradual and tends to be more stable.) If the video includes altitude telemetry you can calculate what the atmospheric density is at a given altitude.

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