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There are a LOT of unworked holes in this, but the basic premise is my planet is a P-type orbit in a binary star system where these two stars have close orbits around each-other in order for the planet to maintain a safe, habitable distance, and it's atmosphere is purple/violet. In short, this planet is similar to Earth save for external changes. It's still habitable to humans, terrestrial, mostly water with continents and the atmosphere has nearly all the same substances. Now from my understanding and helpful comments, Earth's atmosphere is actually violet, but we can't perceive that. So I was wondering, what physical variable could possibly change that so it appears violet to the naked human eye? I ask this so I can apply the same logic to the planet I've created. I hope this makes more sense than what I've previously asked, and I apologize for being misinformed.

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    $\begingroup$ @JBH: "If the color is scientifically measured, our atmosphere is actually violet": this is not even wrong. Color is a sensation which exists in the mind; any color measuring device which produces a value different from what an average human observer sees is broken and ought to be sent back to the supplier for a refund. You probably wanted to say that there is more energy at wavelengths corresponding to the violet part of the spectrum, which is true; but that has nothing to do with color. $\endgroup$
    – AlexP
    Oct 2, 2022 at 22:45
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    $\begingroup$ @JBH: You are confusing power spectrum with color. There is indeed a relationship between them, but the relationship is complicated and depends on very many factors. Colorimetry (= the measurement of color) should, by definition, agree with the perception of a human average observer, otherwise it would be utterly useless. If you look at the "Ask Question" button at the top right of the page, it is orange. Any non-defective colorimeter should say that it is orange, although in the actual light produced by the computer display the power corresponding to the orange wavelengths is exactly zero. $\endgroup$
    – AlexP
    Oct 2, 2022 at 22:55
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    $\begingroup$ @JBH: The question is based on false premises; no idea how it could be salvaged. The sky does not turn red at sunrise or sunset because the atmosphere is blue. First, the air is only very faintly blueish; the intrinsic color of the air is so faint as to be completely irrelevant. Second, the sky does not turn red at sunrise or sunset. (Except a portion near the horizon, smaller or larger depending on how much dust or fog or other particulates are there.) (The clouds may become intensely red.) $\endgroup$
    – AlexP
    Oct 2, 2022 at 23:45
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    $\begingroup$ @AlexP, you are definitely being pedantic. Talking about the color of the atmosphere is pointless. The OP is asking, "what atmospheric changes would make the sky look purple?" We could even include changes in the space between the planet and the star. Nattering about the definition of "color" is not useful. $\endgroup$ Oct 3, 2022 at 0:44
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    $\begingroup$ Raevynn, well done. And I apologize about the spat. They occasionally happen. We have some fantastic people on this site and one of them is @AlexP. You'll come to appreciate his depth of knowledge. I do believe this is a better starting point for your train of thought. $\endgroup$
    – JBH
    Oct 3, 2022 at 0:56

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How purple do you want it?
Daylight on any planet is the result of a 3 step process:

  1. The star gives off light.
  2. The light changes as it travels through the planets atmosphere.
  3. The light is perceived by a creature.

According to Hyperphysics, "Stars approximate blackbody radiators and their visible color depends upon the temperature of the radiator." This means there are very strict limits on what can happen in step 1. Also, I am assuming that there is nothing in step 3 you can change [i.e. the creatures on the planet are humans with normal color vision], because otherwise you enter complicated philosophical questions about what color even means.

In step 2 you, as the writer, have complete freedom. You can change what the atmosphere does to be whatever you want. The catch is that the less the planet is like Earth, the less likely your readers are to accept it. So we need to get a purple sky while minimizing the change in the atmosphere. That leaves the question: how purple do you want your sky?

Option 1: The first human to walk on the planet's surface says, "The sky is a little darker, just barely. Almost like a really, super light purple."

Option 2: The first human to walk on the planet's surface says, "Man, that sky is $^#&ing purple. I mean kids are going to be grabbing that purple crayon so much!"

Option 1: The super easy
The CIE diagram shows where colors are in CIE color space, and fortunately for us, there is a curve that goes through it that corresponds to every blackbody radiator called the Black Body Locus or Plankian Locus [image source]:

CIE diagram with Blackbody Locus

This curve does start to dip down toward the line of purples, which is that flat edge right on the bottom of the colored region. For step 1 simply pick a temperature for your star that corresponds to the color on this curve you want. Wikipedea currently has a GIF showing several examples. Then have the atmosphere do very little in step 2. A close up on the Black Body Locus is available here, and in it you can see that even Earth's atmosphere does not change the color of sunlight that much. The Daylight Locus depicts what a black body would look like after Earth's atmosphere changes it:

Close up of Black Body and Daylight Loci

As long as you are careful with the habitability zone of your star and the UV radiation, you can get the desired result even with an Earth-like atmosphere!

Option 2: I have been working on this answer for 4 hours because of this option

We are now going to have to talk about why violet is cursed and evil and makes colometry an absolutely horrible field to study. To oversimplify, a spectral power distribution is a graph showing how much light of each wavelength is produced by a light source. You can go from an SPD to any color, but not the other way around. Here are 3 hypothetical SPD and my guess as to what color they produce [It's 12:40 AM, don't judge me]:

Some very badly drawn SPD

If the SPD is flat with one peak, then the color will be near the curve on the CIE diagram. This is where those charts showing what wavelength correspond to what colors come from. If the SPD is flat with one trough, then the color with be near the line of purples. If there is more than one peak or more than one trough, then there is no easy way to predict the resulting color and you need to do the calculations.

The fact that purples are caused by troughs is horrible news for us because black body radiators always look like hills, with a single peak and slopping sides. This means that your atmosphere is going to have to do a lot of work.

The star would need to produce a roughly flat SPD over the visible spectrum and the atmosphere needs to absorb a absolutely ton of light across a small band of frequencies. These do exist and are called absorption bands. However, every absorption band is associated with a specific molecule, so you can't just casually make them up. This would give you a SPD like my second example, but you would have to do a lot of research to find the right chemical and a lot of world-building based on what this chemical does in the atmosphere, or your readers may not accept it.

Unfortunately, we cannot take advantage of the fact that this is a binary star system to get an SPD that looks like my third example. The CIE diagram has the extremely useful property that if you pick any two points and find their midpoint, then mixing those two colors will result in the midpoint's color. This means the convex hull of the Black Body Locus is all the colors that could possibly result from combining starlight. As there are no colors there I would call purple, this super cool option that I was extremely pumped for would fail.

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    $\begingroup$ You can't casually make up the molecule, but you don't have to bother your readers with the chemical formula either. "Plant pheromenes. Won't kill you immediately, but wear a gas mask if you're going to be outside for a longer period of time. On the plus side, no need for sunscreen"- helpful advice for new immigrants $\endgroup$
    – MSalters
    Oct 3, 2022 at 13:14
  • $\begingroup$ You are getting purple if the negative peak is green-ish. If the negative peak is in red, you would be getting teal, and yellow if the dip is in blue. This no idea is fairly simple to guesstimate as well - those two peaks seem light blue + yellow (assuming the whole x axis is what human eye sees), so you are seeing white-ish type of color due to how processing in eyes goes. $\endgroup$ Oct 4, 2022 at 6:50
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    $\begingroup$ For readers who aren't doomed to study chromaticity: that "line of purples" is not merely E Tam tossing out a casual description, either; that's its formal name. en.wikipedia.org/wiki/Line_of_purples $\endgroup$
    – Ti Strga
    Oct 5, 2022 at 16:57
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Your planet is a bit warmer than Earth

Okay, so just being warmer alone will not give you a violet sky, but it does two really important things. It will increase the amount of water vapor in the air and it will make the polar regions the primary habitable zone of the world since the tropics will be too hot for any Earthlings. Here on Earth, our poles get a perpetual twilight that typically looks like this:

enter image description here

Because of how cold the air is you don't get a lot of water vapor to scatter the light, but if the poles had a temperate environment, it would look more like this for the 6 months of the year that you get sunlight:

enter image description here

Purple skies here on Earth are caused by the way light scatters when sunlight hits a sky full of water vapor and other "large" particulates (aka: clouds) in the air at an acute angle. The acute angle causes the sunlight to scatter via the Tyndal effect such that the blue wavelengths are filtered out before they reach the sky overhead leaving only light in the red spectrum to illuminate you and any cloud cover. So we get red clouds and a generally pinkish ambiance. However, the sky above the clouds has smaller particulates; so, it is not effected by the Tyndal effect. Its primary mode of light refraction comes from Rayleigh scattering which means that the sky above the clouds will appear blue. So, even though you have 1 light source, its light is being filtered differently at different layers of the atmosphere causing the light reaching you to be mostly blue and red with very little in the green range giving the sky a violet hue.

Technically speaking, this world would still have generally blue/white or maybe even entirely white sky at lower latitudes, but in the regions that people could safely call home, a purple sky would be the norm.

This could even be Humanity's actual future here on Earth

If the runaway greenhouse effect goes as predicted, then we may not actually need to go to another planet for our decedents to eventually experience this. As global temperatures rise from greenhouse emissions, an increase in atmospheric water vapor will go hand in hand with the increase in atmospheric CO2. The poles of Earth will become increasingly humid while the tropics become barren wastelands. Eventually more humans may live under a violet sky than a blue one as we flee to the only remaining habitable parts of the planet.

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    $\begingroup$ Please note that in the second picture supposedly provided as an example, the sky is blue, with a small gold area near the horizon around the sun; the royal purple things are clouds. $\endgroup$
    – AlexP
    Oct 3, 2022 at 19:32
  • $\begingroup$ @AlexP Exactly, it would be a planet somewhere between Earth and Venus if that helps clarify what I mean. A hotter Earth like world would have perpetual cloud cover allowing perpetually violet skies when then sun is at an acute angle... which it would always be at near the poles. The "sky" would not be blue because cloud cover would be the norm. You would at best get the occasional hole in the cloud cover letting a little bit of blue through, but this would be the exception, not the rule so we would identify the "sky" as violet. $\endgroup$
    – Nosajimiki
    Oct 3, 2022 at 21:01
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    $\begingroup$ I have personally seen violet sunsets where there was enough clouds to not only fully illuminate the sky in a purple/magenta hue, but to actually color the whole landscape pink. $\endgroup$
    – Nosajimiki
    Oct 3, 2022 at 21:06
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Violet filter

violet fileter

https://lensesforsnap.com/purple-snapchat-lens-filter/

The problem is that those pesky other colors clamor for our attention so we cannot appreciate the violetness. Violetiosity, as it were. So: filter those other chumps out and let only the violet thru!

One could filter out all frequencies except a single frequency - things would look dark and monochromatic but the sweetest chromatic of violet. Or you could let some of those other colors in but just a little, limiting their roles to the backup dancers of the lit world. And fully allow all that could be Violet!

Snapchat does this thru some digital jiggery-pokery. I propose instead sweet shades of violet hue.

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There are a few things that make the sky look blue, but it all really comes down to the amount of red and green overwhelming the blue end of things. There are two ways to shift this.

The simplest would be to increase the number of blue cones in the eye. If these aren't Earthlings, you can do whatever you want with the colors.

If they are Earthlings, then you need something that will block/reflect the red end of the spectrum before Reyleigh scattering takes effect. Alternately, you could have a chemical in the upper atmosphere that flouresces in the purple spectrum. How that chemical got there is a matter of story telling.

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Evolution is the answer. We don't perceive ultraviolet very well because of evolution. There are white (at least to our puny violet sensing eyes) flowers. Bees love them. They think that these plain, boring white flowers are awesome. Because they can see how amazingly purple they actually are. The flowers are colored in ultraviolet! The flowers evolved so that bees could spot them. And bees evolved so that they could see them.

Dogs, on the other hand, are infamously colorblind, they can't see green and red, rather they show up this weird yellowy blue color for them. We know this because, they don't have the right types of cones in their eyes to perceive these colors.

So, a couple of ideas. You have your humans face some sort of evolutionary pressure to develop a new type of cone in their eye that let's them perceive ultraviolet light. Or if technology is advanced enough and there is some reason to install ultraviolet sensors in their eyes that interface with their brain, essentially, artificial cones. Since your question involved, the "naked human eye", we'll eliminate artificial cones.

That leaves evolutionary pressure to develop those new cones. A good evolutionary pressure is a predator. This new world, perhaps, has an ultraviolet-colored predator that killed a bunch of humans. Slowly, the human colonists evolved to be able to better perceive the threat. And now, bingo, they can see beautiful purples. Potentially, these colonist humans could visit earth and be shocked that earth skies looked purple too, while all the earthlings insisted that the skies are blue.

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    $\begingroup$ "We don't perceive ultraviolet very well because of evolution." More like "because UV is ionizing; it destroys DNA". $\endgroup$
    – RonJohn
    Oct 5, 2022 at 0:57
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Ordinary human seeing purple:

To have an ordinary human magically teleported there and seeing purple sky, you mainly want to block a lot of green light from a sun-like star. You can push temperature higher to get bluer sun, it will be helpful to get deeper purple without blocking most of the light, but merely raising temperature never gets to purple - it remains stuck in blue.

So, what would absorb all that green? The easiest solution is to have a simple chemical that has correct absorption bands (you can handwave that part, there are bound to be many). Another, more complex and less plausible (but IMO more "alien" and possibly fitting) option - airborne photosynthesizing bacteria. Perhaps because the world has ever-present wind, one bacteria opted to leave confines of sea or ground, which offered a huge evolutionary advantage in amount of light the bacteria can gather. The problem here is that you want the bacteria to absorb green and pass most of blue and red (instead of reflecting green and absorbing blue and red like ours). Absorbing green to make energy is easy enough to handwave/ignore, passing a significant fraction of red is fine, but passing blue is tricky. So, another inspiration from nature - photonic structures eg morpho butterfly are used to reflect blue and prevent damage to the delicate internal organs (if you can call them that in bacteria).

A humanoid seeing purple:

To have a non-human seeing purple, you simply state they see blue and purple much better than we do, there is no need to explain further. We evolved from primates seeing 2 colors - blue and yellow. Then yellow receptors split to green and red. It wouldn't be a huge stretch of imagination to have evolutionary advantage of having blue split in the far future, like the yellow did in the past. There are many animals seeing in UV as well, such as bees, shrimp, birds, fish. As well as many animals seeing worse than we do, eg most mammals and fish.

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  • $\begingroup$ "you simply state they see blue and purple much better than we do". Purple is "red + blue", so you're just seeing blue and red better. $\endgroup$
    – RonJohn
    Oct 5, 2022 at 0:55
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    $\begingroup$ Well we see purple as red+blue. Extending vision towards 300nm would have you see something purplish as a pure color. Increasing number of blue receptors would almost surely extend vision in that corner of CIE chart, making it easier to distinguish shades of blue, teal, purple and whatnot. I don't think there is any need to improve vision in red to see purple better - red receptors are reasonably good already. $\endgroup$ Oct 5, 2022 at 8:07

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