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Old 26th February 2012, 01:39 PM   #31 (permalink)
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Re: Sky colour

No I agree, I picked that depth and pressure as I believe most people can operate at that with no serious ill effects. What I don't know is how much gravity you would have to have to give that sort of atmospheric pressure. I suspect it would take a lot of gravity and a lot of atmosphere.
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Old 26th February 2012, 02:21 PM   #32 (permalink)
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Re: Sky colour

Not, I suspect, much more gravity. Take Venus as an example; surface gravity 10% less than Earth's, atmospheric pressure 92 times higher, with high temperatures (several hundred degrees) which should increase the number of molecules reaching escape velocity and – er – escaping.

Certainly you need a high percentage of heavy gasses; the amount of carbon dioxide in Venus' atmosphere would make it unhealthy for unmodified humans (as would the temperature and the lack of free oxygen; Venus is possibly not the optimal spot to set up homesteading), but assuming the partial pressure of oxygen was roughly right, you could bulk it out with chemically inactive gasses. CFCs such as freon (dichlorodifluromethane, and no, I didn't have to look it up) would be good, or noble gasses such as neon (not chlorine, methinks)

Blue light is scattered by atmosphere most as it is about the shortest wavelength we can conveniently see. If you have less atmosphere (for example on top of a mountain, where there is still enough air to breath, as long as you are of Tibetan or Andean ancestry and don't exert yourself too much) the sky goes a glorious velvety violet and you can see some stars through it in daylight.

For longer wavelengths, reds and oranges, molecular scattering doesn't work as well. so particular scattering (either solid particles of standardised size, or drops of liquid, generally on a human habitable world water) is recommended.

A cool enough star that doesn't produce much short wavelength radiation (say Marion Zimmer Bradley's "Darkover" Cottman's star) would probably give you a greenish sky (and not much of a suntan, and vitamin D deficiency).

If you think of the low angle light of evening, where the energy has to traverse a lot more thickness of atmosphere than usual, it is not really the sky that goes red. The sun itself does, as short waves are scattered, and the clouds reflect or diffuse this back, but the sky overhead goes an ever darker blue, verging on high altitude violed, before trying "transparent, let darkness through".
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Old 26th February 2012, 04:27 PM   #33 (permalink)
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Re: Sky colour

A (slightly) denser and deeper atmosphere can have some noticeable effects. (I'm someone who burns easily even in the sun of southern England, but I had no such problems at the Dead Sea, because the UV component of the light is attenuated there.) However, I didn't notice that the sky had a different colour. Not that I was looking for a difference at the time.


Nothing to do with sky colour, but you might have to tone down the effects on skin colour of living on a world with high air density and/or deeper atmospheres. (Would these worlds be places where tanning parlours were actually fashionable? )
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Old 26th February 2012, 07:06 PM   #34 (permalink)
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Re: Sky colour

Chrispy: I sort of suspected that the amount and composition (heavy molecules etc.) of atmosphere had more effect on atmospheric pressure than the gravity but wasn't sure. So thanks for that! And you are absolutely right about high altitude, I have spent a fair bit of time at altitudes above 5000m and apart from anything else it played merry hell with colours on photographic film - snow tended to go a blue-violet colour. Can't say I ever noticed seeing stars in daylight but I can well believe it; might have needed to go higher than my max which is only about 6000m.

Ursa: that's a good point actually; if you have a significantly thicker atmosphere than ours then the amount of UV getting through would be much diminished. They'd probably all be pale and pasty like me (I too burn really badly and even with so called 100% sunblock on when altitude climbing have been known to burn).
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Old 26th February 2012, 07:41 PM   #35 (permalink)
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Re: Sky colour

Another option may be have your planet extremely close to another planet so as to be looking up at another world throughout the day. Your planet may have to be fairly small though I don't know though.
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Old 26th February 2012, 08:03 PM   #36 (permalink)
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Re: Sky colour

This has started to become a very interesting thread topic. Thanks for the input everyone, given me a lot to think about. David also put a thought in my head:

There is a creationist theory that the earth used to be surrounded by a canopy of water until it was torn down to create a worldwide flood. It is often used to explain the drop in lifespans because now radiation gets through, where before it was blocked by the water vapor.

But there is also scientific arguments refuting it, so not sure whether it is actually possibly or not. If so then the planet could still have its canopy which means light my be seen differently. Which brings me to this:

Quote:
Originally Posted by chrispenycate View Post
For longer wavelengths, reds and oranges, molecular scattering doesn't work as well. so particular scattering (either solid particles of standardised size, or drops of liquid, generally on a human habitable world water) is recommended.
Are you talking about having water in the atmosphere here Chrispy, or just meaning surface water? Or have I completely misunderstood?

Quote:
Originally Posted by chrispenycate View Post
A cool enough star that doesn't produce much short wavelength radiation (say Marion Zimmer Bradley's "Darkover" Cottman's star) would probably give you a greenish sky (and not much of a suntan, and vitamin D deficiency).
Oh right, this is what I was talking about earlier, so maybe it is true. So much debate going on about this around the internet, I think possibly because most people just don't really know for sure.
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Old 26th February 2012, 08:37 PM   #37 (permalink)
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Re: Sky colour

So, Warren, your desert planet might just have a canopy of water around it. How did it get there?

Have you ever read Medusa's Children by Bob Shaw? A piece of alien technology (IIRC) generated a wormhole that stabilised Earth's coastlines by sending excess water out into space during periods of global warming, and retrieved it during periods of global cooling.

The thrown-into-space water formed a tiny water world. Complete with life. Some of it human.
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Old 26th February 2012, 08:46 PM   #38 (permalink)
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Re: Sky colour

Could explain why its a desert world, most of the water was drawn up into the atmosphere. The how is the question...

I'll have a look into that book, haven't read it.
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Old 26th February 2012, 08:49 PM   #39 (permalink)
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Re: Sky colour

As soon as I saw this thread I had to say the sky would have to be blue to work as it is light refracting (all well covered above).

However you could play with the shades of blue in the sky which would I think, still be correct and not lose the better informed readers. Sunsets also could be changed bringing in oranges, reds etc. to suit moods in the story.

Plant colour would also be affected by the different light. It is green on Earth as it makes the best of the light from our sun, (help from biology bods please) it could be a much darker green or possibly red, I think I have heard the Earth was covered in red bacteria for a while.

So still a blue sky, but there could be small changes which would make it clear to the reader this is not Earth we're not.
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Old 26th February 2012, 08:52 PM   #40 (permalink)
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Re: Sky colour

I haven't read it for many years, but I'm pretty sure of the facts I've laid out above.

You have that possibility (alien technology), or a variant on Dune (sandworms, big sandworms, and how the hell did they put all the water up there? Sandworms can't fly!), or something natural (a cometary halo orbiting the planet? I don't think that would be very stable. I see impacts. Many impacts. Unless...something stabilised the comet's orbits...we're looping back to alien technology here, perhaps).
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Old 26th February 2012, 08:58 PM   #41 (permalink)
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Re: Sky colour

Quote:
Originally Posted by Bowler1 View Post
Plant colour would also be affected by the different light. It is green on Earth as it makes the best of the light from our sun, (help from biology bods please) it could be a much darker green or possibly red, I think I have heard the Earth was covered in red bacteria for a while.
For binary stars: http://www.cosmosmagazine.com/node/4243

For single stars with different wavelengths of light form Sol: http://www.cosmosmagazine.com/news/1...r-alien-plants

And that research brings up another question or two, Warren. Is your planet's star single or binary? And if binary, how do the two different star's light/colour mix when both are in the sky?

You might have your blue sky while one star is up, and a different colour when both are above the horizon. Again, I'm awaiting a real scientist to tell me why I'm wrong, but it's an idea.

P.S. This was the story that inspired my comment above about higher gravity/thicker atmosphere. The scientists speculate about elephant-sized fliers on such worlds...

http://www.cosmosmagazine.com/node/3615/full
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Old 26th February 2012, 09:08 PM   #42 (permalink)
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Re: Sky colour

I was originally going with a single star, but binary gives me ideas too. It reminds me of the movie "Pitch Black". I believe that planet had 3 stars didn't it? I'll have to check it out again, don't remember the sky colour on the planet.
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Old 26th February 2012, 09:10 PM   #43 (permalink)
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Re: Sky colour

Or, you could go with this:

http://www.cosmosmagazine.com/news/5...t-a-waterworld

An exoplanet that might just be a hot and steamy waterworld.
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Old 26th February 2012, 09:10 PM   #44 (permalink)
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Re: Sky colour

If your planet was actually a moon orbiting a large gas giant, could that giant have an effect on the moons sky colour?

Would a large orange gas giant give the moon an orange tint ?
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Old 26th February 2012, 09:24 PM   #45 (permalink)
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Re: Sky colour

Quote:
Originally Posted by David Evil Overlord View Post
Or, you could go with this:

http://www.cosmosmagazine.com/news/5...t-a-waterworld

An exoplanet that might just be a hot and steamy waterworld.

That's a good find, David.

Quote:
230 degrees Celsius
- Apart from the burning to death bit...

will have to move the planet back a bit I think. Surprised to see they actually found a planet out there that might possibly support the theory of a water vapor canopy. Wonder if moving the planet would effect it, but then, this is such an unknown that not even educated readers would be refute the possibility.

Hot ice... interesting.

Oh and, after checking out trailers/screens of Pitch Black - Don't have the movie to actually watch at the moment - I believe the sunlight was almost florescent bright at points, the sky either white or brown/orange. - So multiple suns might work as well.
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