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(I was recommended to ask this question here by the guys on World Building)

I'm starting a whole new planet for a story and would like it to have an orange sky during the day.

At a basic level (I am not a smart man) what chemical composition would be most conducive to Orange sky colouring? What colour would that appear during sunrise/sunset (Our blue turns orange due to some science magic, what effect would this same process have on an Orange atmosphere?

The air doesn't have to be breathable, I am fine with whatever implications this will have on human characters re: Breathing gear, space suits, etc.

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  • $\begingroup$ The sky is blue due to Rayleigh Scattering; having an orange sky would simply require smaller molecules (than water) in the atmosphere. $\endgroup$ – Kyle Kanos May 22 '15 at 3:36
  • $\begingroup$ @KyleKanos Shouldn't it require bigger molecules? $\endgroup$ – Jimmy360 May 22 '15 at 4:57
  • $\begingroup$ You might want to google Sydney's Orange sky, or Australia's Orange sky, or check out the photo gallery on the bottom: en.wikipedia.org/wiki/2009_Australian_dust_storm $\endgroup$ – userLTK May 29 '15 at 5:00
  • $\begingroup$ That's the colour I was looking for, but I was hoping for more of a general colouring more than extra dust/contaminants in the air. I understand it has something to do with the size of the molecules that are refracting the light... I'm just wondering what would be the right size to refract orange light without the air becoming opaque like that. $\endgroup$ – Wompguinea May 29 '15 at 5:04
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I'm not a professor of chemistry or anything, but on a computer screen, Orange equals about 2 parts red and 1 part green on the RGB scale.

Using the chart in the link below:

http://www.scienceclarified.com/Ga-He/Gases-Properties-of.html

You can get green from Chlorine gas and Red/Brown from NO2. Chlorine is highly reactive so in reality, I don't think it would stay in an atmosphere, but for a simulation, . . . why not.

A mix of Redish Brown and Green lit by a bright sun would equal an Orange like color.

It's worth noting that the sky isn't colored gas but only a tiny refraction effect that's spreads the blue light around the sky. It's noticable cause the atmosphere is miles thick. A colored gas and you'd only need trace amounts of it with an atmosphere a few miles thick and it would have a filtering maybe clouding effect so you wouldn't see the stars very well at night.

I couldn't find any data on which gases refract Orange light more than other colors, but that would work too. A thicker atmosphere might also work where the blue light doesn't make it all the way through that much atmosphere (like what happens at sunset when the sky/atmosphere turn Orange/Red).

Another way to do it would be orange dust, orange clouds or maybe an Orange star (there are many Orange stars) and less refraction - but the science of exactly how coloring of an atmosphere works is kinda complicated. I'm not smart enough to give you a complete answer.

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