Sky colour

[Ray McCarthy]

* Indigo really is Violet which really is dark Blue but Isaac Newton, for mystical reason of his own wanted the visible spectrum to have 7 colours.

Asimov complained about that.
Radio <-- Infra Red <- Red, Orange, Yellow, Green, Cyan, Blue, Violet*-> Ultra Violet
Magenta isn't in visible light, it's a vision artefact caused by the Red part and Blue parts being similar level and little light in middle (green) part of spectrum. It can't be created by less than two colours of light.

Rayleigh scattering
https://en.wikipedia.org/wiki/Rayleigh_scattering

Interestingly, the illumination is bluest at dawn and dusk, especially if the sunset is obscured and least blue at noon, even though the UV is highest at noon (because it's brighter overall). Switch off auto white balance on phone/camera and point at white surface.


*for a particular definition of violet.

 

[Brian G Turner]

Rayleigh scattering

Damn! I told my daughters it was due to refraction, and "proved" this by showing the rainbow effect above the horizon at sunset. Oops! Hopefully they'll forget I said that.

 

[Ray McCarthy]

Along with Kennedy claiming he was a donut and glass in church windows flowing.
I used to think my dad was infallible but ignored his advice anyway.

 

[Mirannan]

Asimov complained about that.
Radio <-- Infra Red <- Red, Orange, Yellow, Green, Cyan, Blue, Violet*-> Ultra Violet
Magenta isn't in visible light, it's a vision artefact caused by the Red part and Blue parts being similar level and little light in middle (green) part of spectrum. It can't be created by less than two colours of light.

Rayleigh scattering
https://en.wikipedia.org/wiki/Rayleigh_scattering

Interestingly, the illumination is bluest at dawn and dusk, especially if the sunset is obscured and least blue at noon, even though the UV is highest at noon (because it's brighter overall). Switch off auto white balance on phone/camera and point at white surface.


*for a particular definition of violet.

About blue illumination: Certainly. I have a number of photos which were taken a few years ago by yours truly, which look seriously creepy because of the "unnatural" cast to the light. The reason is that they were taken with long exposures, at the bottom of the rather narrow, deep valley leading up to Ingleton Falls in Lancashire. It was a fine winter's day, so the sun was very low all day and didn't actually reach to what I was photographing; so the light was extremely blue because all of it was coming from a very blue sky.

I actually got the effect in my photographs, because I was using a film camera. Using slide film, too.

 

[Piousflea84]

IMO,

A realistically breathable atmosphere will always appear blue, because it is by definition a nitrogen/oxygen atmosphere. Any other color in the atmosphere itself would be some kind of dust or smog, and would diminish the breathability of the atmosphere.

However, because the atmosphere is blue doesn't mean the sky is blue. An Earth-like world could orbit around a Jovian-scale gas giant, with an orbit close enough to make the gas giant very large in the sky. Gas giants can vary dramatically in color (just look at Jupiter, Saturn, Neptune and Uranus) so the sky would take on whatever color the gas giant is.