What do you need to get a purple sunset

[Hans]

I was reading a fine piece of science fiction that had one paragraph in it that described a purple sunset on a far away planet.

What color/classification/type of sun and what type of atmosphere would you need to obtain a purple sunset?

[ppaulk]

I believe that the primary light that is scattered be nitrogen and oxygen in the atmosphere is blue; but much of that scattered blue light still reaches the surface, making the sky appear blue. In the morning and evening the light travels through a thicker portion of the atmosphere before reaching the surface: most of the blue light is absorbed or reflected, but the less scattered longer wave length red light still makes it to the surface causing the sky to appear red.

I guess that if we had different atoms and molecules in the atmosphere that scattered more violet light and less blue and red light (or all wave lengths other than violet), then the sky would look more purple.

I hope my limited knowledge is both correct and helpful.

Philip

[grant hutchison]

Pretty much any classification of star will appear white if you're close to it: your eye readjusts to the illuminant so that it can correctly judge colours in the environment. For instance, the filament of an electric light bulb has a very similar power spectrum to a class M red dwarf star, but it still looks white.

Molecular atmospheres preferentially scatter blue light in the way ppaulk describes: it's not a feature of the specific molecules, just their size in comparison to the wavelength of light.

So there's a problem: the "sun" will look white, and the atmosphere at sunset will trim off blue wavelengths, making it look red. We really need some mechanism that selectively removes green wavelengths from the lightpath between your eye and the sun, to leave purple behind.
Some sort of purple vapour might do it (iodine?), but when you looked towards the sun you'd be seeing the shadowed side of any purple particles, so the preferential colour effect on the sun's disc would not be strong.
Hmm. I suspect purple may be the least natural of all sunset colours ...

Grant Hutchison

PS: I'm making a distinction between violet, which is the spectral colour we see when our blue receptors are stimulated very much more than our red and green receptors, and purple, which is a non-spectral colour we see when our red and blue receptors are stimulated in excess of our green receptors.

[Sam5]

So there's a problem: the "sun" will look white, and the atmosphere at sunset will trim off blue wavelengths, making it look red. We really need some mechanism that selectively removes green wavelengths from the lightpath between your eye and the sun, to leave purple behind.
Some sort of purple vapour might do it (iodine?), but when you looked towards the sun you'd be seeing the shadowed side of any purple particles, so the preferential colour effect on the sun's disc would not be strong.
Hmm. I suspect purple may be the least natural of all sunset colours ...

Grant Hutchison

Very interesting question and interesting answers. I live in a Southwestern US state where we occasionally have some deep purple in our sunsets.This is an unusual color that I’ve never seen anyplace else. However, I notice that the purpole tends to be seen mainly around bigger cities. I wonder if the air pollution of a big city puts something into the air that causes the purple sunsets? I think purple is a mix of red and blue, with no yellow and green.

[ppaulk]

I don't know if it is a mixture of colors, like paints, but the atmopheric influence on the different spectrums of visible light, the highest energy of which is violet.

Philip

[grant hutchison]

I don't know if it is a mixture of colors, like paints, but the atmopheric influence on the different spectrums of visible light, the highest energy of which is violet.

Violet wavelengths are preferentially scattered by the atmosphere, but our eyes are quite inefficient at seeing violet. It takes much more energy at short, high energy wavelengths to excite the sensation of violet in our eyes than it does to produce the sensation of blue.
So in Earth's atmosphere, the scattered blue light overwhelms the violet, and we see blue sky. It requires a very strong dominance of very short wavelengths to produce the colour violet.

Grant Hutchison

[mugaliens]

I was reading a fine piece of science fiction that had one paragraph in it that described a purple sunset on a far away planet.

What color/classification/type of sun and what type of atmosphere would you need to obtain a purple sunset?

Purple Haze?

[alainprice]

The only colors I have yet to see in a sunset are green and yellow. It is possible to catch some green in the sun's disk as it passes the horizon, but no yellow when the sunset is in full force.

Right now I can spot purple in the clouds, no violet though.

[Zamise]

A red giant setting in an oxygen rich atmosphere like earth's maybe would produce a more purple sky at sunset? Or maybe a blue giant on a planet like like Mars's which has a lot of red dust in a thin atmosphere? Just thinking of possibillities.

[Maksutov]

1.5 ounces of vodka and 8 ounces of prune juice. Pour over ice, garnish with a ripe cherry and a pitted plum, and stay close to the restroom.




Seriously the atmosphere would have to contain an unusual chemical soup that absorbed almost every wavelength except those out in the far blue/violet portion of the spectrum. Not aware of any gaseous chemicals that would do that.

[snarkophilus]

We had a very purply post-sunset here a few nights ago. I remember it was quite hazy, because I was rushing home to grab my camera to take a picture of the moon, which was a bright orange and had a double rainbow around it. Very neat (but my batteries were dead).

Anyway, as the sun was down in the west, the sky went all rainbowy in the east: blue at the top, then down to red. As the sun continued to set, the colours shifted toward it, and beneath the red it was purple. Not a bright, glowy purple, nor a deep, shadowy purple, but a Purplesaurus Rex (Kool-aid) purple. Darker than lilacs, but not as dark as Barney.

The purple colour lasted for half an hour or so, eventually covering about a quarter of the sky. Then came what passes for darkness at this time of year.

Kind of cool how that works when the sun sets. Blue above, red toward and away from the sun, and orange on the sides. The atmosphere is neat.

[Maksutov]

[edit]The purple colour lasted for half an hour or so, eventually covering about a quarter of the sky. Then came what passes for darkness at this time of year.

Kind of cool how that works when the sun sets. Blue above, red toward and away from the sun, and orange on the sides. The atmosphere is neat.

Sounds like you were seeing the Earth's shadow.

The atmosphere is really neat, and it makes for pretty good breathing, too, depending on one's location.

[Hans]

Thanks for the answers Gentlemen!

[George]

Very interesting question and interesting answers. I live in a Southwestern US state where we occasionally have some deep purple in our sunsets.This is an unusual color that I’ve never seen anyplace else.

Please share a picture of it sometime. That is interesting. Perhaps there is a certain set of larger particles which would scatter the longer wavelengths but not the shorter -"selective scattering" (as Grant has appropriately called it). [This scattering may explain the blue halo around the Martian sun. Albeit, the halo never turns purple as the sun sets.]

Hmmmm...the martian sky may not get as red either. Here on Earth, the sky should allow much more scattering, I think, than on Mars because our atmosphere is much larger in size and mass. So, if somehow selective scattering were at work due to smog particles, or some other specific sized particle, selective scattering may allow the blue to pass but reject more green, yellow, and orange colors. Allowing for more blue to pass and with the greater intensity of red in our earthly atmosphere during certain dusty sunsets, we might have an explanation for Sam5's purple sunsets.

I think purple is a mix of red and blue, with no yellow and green.

Yes.
Some time ago, Grant presented a class T star which radiates a little in blue and a lot in red. This should produce a maroon or crimson color (depending on your college preference ). [More blue would produce purple I suspect.] This would make for interesting sunsets for us. Evolution, however, might cause the natives to see white.

If the sun were slightly hotter, say 6500K (instead of 5850K), we might always see a violet sky. This assumes our eyes would have adapted to utilize more of the violet color; our eyes, as stated earlier, do not normally see violet very well.

[ArgoNavis]

If the sun were slightly hotter, say 6500K (instead of 5850K), we might always see a violet sky. This assumes our eyes would have adapted to utilize more of the violet color; our eyes, as stated earlier, do not normally see violet very well.

why?

How would the colour spectrum be shifted if the Sun were just a little hotter?


An interesting image of a purple sky is here:

http://thorn.as.arizona.edu/~kak/

It is only an artisitc rendering.

[George]

How would the colour spectrum be shifted if the Sun were just a little hotter?

A 6500K mean temp. would shift the peak intensity to around 450nm which is in the blue. Since we can see deeper into the interior of the Sun's central region, this area of the disk would have a peak slightly in the violet.

Since violet actually scatters more than blue, violet will have the advantage assuming it at least matches the blue photon flux. This, as mentioned earlier, assumes our eyes are not insensitive to violet.

An interesting image of a purple sky is here:

http://thorn.as.arizona.edu/~kak/

It is only an artisitc rendering.

Very nice.

[Careless]

Very interesting question and interesting answers. I live in a Southwestern US state where we occasionally have some deep purple in our sunsets.This is an unusual color that I’ve never seen anyplace else. However, I notice that the purpole tends to be seen mainly around bigger cities. I wonder if the air pollution of a big city puts something into the air that causes the purple sunsets? I think purple is a mix of red and blue, with no yellow and green.

I think it's pretty widely known that air pollution creates prettier potential sunsets (at least, no one I've ever discussed it with has not heard of this)
The most amazing sunset i've ever seen was from outside Cleveland. The sun was setting on the other side of the city, and it was amaazing.

Disclaimer: I was 18 and had just left the girl I loved earlier in the day for the summer

[George]

Disclaimer: I was 18 and had just left the girl I loved earlier in the day for the summer

Does it help to know that fires and volcanoes create a similar effect.

[lti]

Is this an example of a planet with a purple atmosphere?
http://trekmovie.com/wp-content/uplo...title_card.jpg
http://trekmovie.com/wp-content/uplo.../orbit_new.jpg
http://trekmovie.com/wp-content/uplo.../halka_new.jpg
http://trekmovie.com/wp-content/uplo...or/halkans.jpg

Sourced from TrekMovie http://trekmovie.com/2006/11/13/mirr...ots-and-video/

edit: to remove hotlinking 24/11/2006

[sirius0]

Actually I think the scattering has more to do with the distance between air molecules and other suspensions than what type of molecules are present. Therefore when pollution particles are present in addition to the spacing between molecules then the distance has been reduced on average selecting for wavelengths that are shorter. This might explain purple sunsets in hazy conditions. Also humans are trichromats. We have cones that are sensitive to red blue and green light. We are more likely to see blue even if there is a reasonable amount of violet or indigo. Ever noticed how indigo seems a darker hue? There are creatures (tetrachromats 4 cones) birds etc; that can see a fair way into the ultraviolet. For us red & blue light make purple; green and blue make yellow. (additive not subtractive as in mixing paint) Some women may have yellow receptor cones (or is that raffia, amber, rich cream? it is all yellow to me without my X chromosome active)

It might be of interest to know that Dogs do appear to have some colour vision due to rods having a reduced but valid colour sensitivity. This possibly also expands the human range a little (yes it is rich cream!)

[Kaptain K]

O Henry (William Sydney Porter) called Austin, Texas "The City with the Violet Crown" in 1894.

[umop ap!sdn]

You can see a little bit of purple in this photo that I took. The most heavily populated part of the city (Phoenix metro area) is west-northwest of where this was taken, so it's pretty much looking through all of whatever smog there was on that day.

[Ozzy]

I moved to the Big Smoke for a year and the sunsets were amazing. Lots of purples and greens. Sometimes it looked very surreal.

[Hans]

Well a touch of purple there - well done