Why are earthly sunsets never green?

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Ann
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Why are earthly sunsets never green?

Post by Ann » Sun Jul 04, 2010 7:57 am

I recently asked a question which wasn't an honest question at all, namely, what color the Sun is. I felt I already knew the answer, and I really just wanted people to admit that I am right - that the Sun is white, that is.

Well, now I have another question which is an honest question, because I don't know the answer. It is why earthly sunsets are never green. Chris answered that question in another thread. He said:
Just the nature of how the spectrum gets scattered and absorbed.
Unfortunately I don't understand that answer. I think I understand atmospheric scattering of different colors moderately well, but I wouldn't mind having that explained to me again. What I don't understand at all is why atmospheric absorption of colors would prevent sunsets on the Earth from ever looking green.

Can anyone explain that to me?

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Re: Why are earthly sunsets never green?

Post by Henning Makholm » Sun Jul 04, 2010 10:11 am

Ann wrote:Well, now I have another question which is an honest question, because I don't know the answer. It is why earthly sunsets are never green.
I would say it's because green is in the middle of the visible spectrum.

It is fairly easy to to imagine a physical process that attenuates shorter wavelengths more than longer ones across the entire visible spectrum; that will make the sunlight reddish or yellow. It shouldn't surprise one either to find one that attenuates light more the longer its wavelength is, which makes the light bluish (though I haven't actually heard of blue sunsets).

However, green light requires that you remove some of the short wavelengths and some of the long ones but leave more of the middle wavelengths through. And the peak needs to be fairly accurately located in the green interval. Red and blue can result from the tail of a spectral distribution that peaks outside the visual range, which makes it a lot easier to happen accidentally than green, where the peak must be located precisely on the green spot. For this reason green is a rare color.

(It is possible for sunsets to produce green light, but it requires fairly rare meteorological conditions and doesn't last for more than a few seconds. Google for "green flash".)

Wait a minute, green is "rare"? Plants seem to manage to be green fairly consistently, don't they?. Yes, but that is arguably not just a freak accident. On the contrary, our color vision has probably evolved to perceive the color of leaves as a distinct primary color -- it stands to reason that the ability to detect living, thriving foliage from a distance would be a survival trait in many natural environments. Where there are growing leaves, there's likely to be water and food. If you can't eat the leaves themselves, you can probably eat the animals that graze on them.
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Re: Why are earthly sunsets never green?

Post by Ann » Sun Jul 04, 2010 6:03 pm

Thanks a lot for your answer, Henning. It clarifies a lot, but not everything, at least not to me. You said:
However, green light requires that you remove some of the short wavelengths and some of the long ones but leave more of the middle wavelengths through. And the peak needs to be fairly accurately located in the green interval. Red and blue can result from the tail of a spectral distribution that peaks outside the visual range, which makes it a lot easier to happen accidentally than green, where the peak must be located precisely on the green spot.
I definitely understand what you said about red and blue colors resulting from a tail of a spectral distibution that peaks outside the visual range. It means that a predominantly ultraviolet light source may produce some blue light too, but not a lot of longer-wave visual light, so that a mostly ultraviolet light source may look blue. Similarly, a mostly infrared light source may look red.

I also sort of understand what you said about green requiring the removal of both long and short wavelengths and only leaving the middle wavelengths through. Yes, I can see that this kind of selection of middle wavelengths would be hard to achieve. On the other hand, wouldn't it be just as hard to only let the yellowwavelengths through? They, too, can be regarded as middle wavelengths. And there is no shortage of yellow sunsets:
Image

Image

Why do we see yellow sunsets but not green ones? Or maybe there is a hint of green in this yellow sunset? What do you think?

Image

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Re: Why are earthly sunsets never green?

Post by Ann » Sun Jul 04, 2010 6:17 pm

I couldn't help googling for "green sunset", and I found several pictures. Some of them look absolutely improbable. When was the last time you saw a sunset look like this?

Image

And why is it that sunsets don't look like that, even though sunsets often look yellow?

Some of the pictures I found do show sunsets which seem to have a green tinge, and they don't look improbable. I thought this one was beautiful:

Image

Really beautiful and a bit green, too. But can sunsets ever look greener than this?

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Re: Why are earthly sunsets never green?

Post by Henning Makholm » Sun Jul 04, 2010 7:49 pm

Ann wrote:I also sort of understand what you said about green requiring the removal of both long and short wavelengths and only leaving the middle wavelengths through. Yes, I can see that this kind of selection of middle wavelengths would be hard to achieve. On the other hand, wouldn't it be just as hard to only let the yellowwavelengths through?
That gets back to the physiology of color vision. There are three kinds of color receptors in the retina. Each has a rather broad frequency response, but their sensitivities peak at blue-violet, green, and red-orange wavelengths. The sensitivity curves of the two latter ones overlap significantly, so any monochromatic stimulus in the cyan-green-yellow-red range will trigger both kinds, but to slightly different degrees.

At the retina level, then, "yellow" is not a primary stimulus, but simply the perception that results when the green-peaking and red-peaking cells register about equal signals, and much more than the blue-peaking ones. This can be caused by monochromatic yellow light, but it also results from a continuous distribution of "white light minus most of the blue end" (where the loss at the blue end happens after both "red" and "green" sensitivity have tailed off) -- which can be perceived as a fairly saturated yellow, because the eye cannot distinguished between a broad and a narrowly peaked spectrum.

So a perceived yellow can still be produced by a spectrum where intensity decreases monotonically as we move from long to short wavelengths. Not so for green -- green requires that the signal from the "middle" sensory cells is stronger than for either the "short" or the "long" ones. And that needs a spectrum with an actual peak in it.
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Re: Why are earthly sunsets never green?

Post by Ann » Mon Jul 05, 2010 6:41 am

Thanks, Henning, that makes a lot better sense. Interesting, by the way, what you said about green being a primary color to our eyes and how that has helped our ancestors to find food and water.

Not so long ago, I read that most mammals have only two color receptors in their retinas, one receptor for blue light (centered at, perhaps, 450nm) and one receptor for yellow-green light, centered at around 550 nm. Then, due to some kind of mutation, the yellow-green receptor was duplicated so that we got two of them, except the new receptor was placed a little bit "to the right" (toward longer wavelenths) of the original one. Say that the new one was centered at, oh, 560 or 565 nm. I don't think it was even at 580 nm, at primary yellow, but this tiny displacement nevertheless gave humans three-color vision. And it made red such an intense primary color to us, and it made yellow so bright. I find it fascinating.

Here is a graph showing the sensitivities of our color receptors. According to this graph, our receptors are "attuned" to even shorter, bluer wavelengths than I said before. I'm not altogether sure what the middle "white curve", centered at 498 nm, is doing there.

Image

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Re: Why are earthly sunsets never green?

Post by wonderboy » Mon Jul 05, 2010 9:30 am

This is the best picture I could find of a possible sunset, or sunrise.

Paul.
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Re: Why are earthly sunsets never green?

Post by Chris Peterson » Mon Jul 05, 2010 2:21 pm

Ann wrote:Here is a graph showing the sensitivities of our color receptors. According to this graph, our receptors are "attuned" to even shorter, bluer wavelengths than I said before. I'm not altogether sure what the middle "white curve", centered at 498 nm, is doing there.
The 498nm peak pigment is found in the rods, which are not color sensitive.

Charts like this are interesting, but you have to be careful with them. Although the one you link shows "response" on the ordinate, it is almost certainly a measure of the pigment absorbance, which is much easier to measure. In addition, the curves are normalized, so you aren't seeing anything close to the true response functions. In terms of actual sensitivity, the 498 nm peak is 1000 times higher than any of the cone peaks. The beta (blue) sensors are almost twice as sensitive as the rho (red) sensors, with the gamma (green) sensor sensitivity in the middle. The actual peak sensitivity of any of the cones is fairly unimportant, since the perception of color is determined by the ratio of all three cone types, modified also by the response of the rods when viewing most astronomical objects. Adding complications is the fact that there are mutated forms of the pigments, so not everybody has quite the same response curves, and some people (especially women) can have one of the pigments in two forms, resulting in a four-pigment primary system.

This helps show the relative response of the normal three pigment system.
spectsee.jpg
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Re: Why are earthly sunsets never green?

Post by Helio George » Mon Jul 05, 2010 7:18 pm

Ann wrote:Well, now I have another question which is an honest question, because I don't know the answer. It is why earthly sunsets are never green.
You have some nice answers and I'd like to add a little to them. [Your sunset images are outstanding, btw!]

The very tenuous layer of ozone does a great job of cutting out UV, but it also absorbs the middle wavelengths, including green. When the Sun is high in the sky, the ozone only attenuates the middle wavelengths by about 2% or less, I think.

However, during sunset, sunlight must pass through a great deal more atmosphere, which compounds the ozone's absorption. This is known as the Chappuis Effect. The result is that we see a blue sky even when most of the blue light has been scattered away by the time it reaches us overhead. Some notice the different shade of blue that is a result of this ozone absorption, apparently.

This greatly minimizes the chance of seeing a green sky.

But, never say never. Another interesting phenomena is selective scattering. When the number of particles are significant and of a size that favours the scattering of one narrow band of wavelengths, then some interesting things can happen. For instance, the blue halo seen around the Sun from the Mars rover images is caused by the larger particle sizes that scatter red light more than blue, just opposite of what we see here. Whether or not such conditions might take place here to allow a green sunset is unknown to me.

[A great book on this topic is "Why the Sky is Blue", Hoeppe.]
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Re: Why are earthly sunsets never green?

Post by Ann » Tue Jul 06, 2010 6:58 am

Thank you very much for your answers. Thanks, in particular, to henning and George.

I've chased down a few more sunset pictures that I find revealing. This one is interesting:

Image

It is a relatively cloud-free picture, so we can concentrate on the color of the sky. (Many sunset images, by contrast, show flamboyantly colored clouds.)

At the horizon there appears to be some clouds, nevertheless. Hugging the horizon is a thin orange-to-red layer. It is easy to imagine that here light has had to pass through so much atmosphere that even some of the red wavelengths are being scattered, though only horizontally (where light has to pass through the thickest layers of atmosphere), coloring the horizon orange-red. (Some orange and a bit of yellow light is being scattered here, too, but it seems probable that red light dominates.)

The orange-red layer is very thin. Directly above it the sky changes color to orange or yellow-orange. The atmosphere here is not so thick, and more orange and yellow light is being scattered here. Certainly some red light is being scattered too, but not enough of it to dominate over orange and yellow.

The orange-yellow layer is also quite thin. Above it, the sky changes color to something that I have no name for. It is a light color but somewhat "dirty", looking like something you might have produced as a child when you painted with watercolors and cleaned your brush by dipping it in a glass of water. The water gradually took on a "dirty" color, since it was a mix of all the colors you had used. Probably we are seeing much the same thing here: the sky is being colored by a lot of colors simultaneously, so that no one color is allowed to dominate. The sky is light-colored, but not white.

In the picture you can see a rather large, pinkish "halo" around the Sun. I have no idea what that is, and I can't remember seeing this particular effect before.

Anyway, the "dirty" color of the sky then changes into blue. First a light and somewhat attenuated and maybe cyan-tinged blue, then a more saturated but also darker blue.

I found another picture which I like even better:

Image

In this picture, the "indeterminate", "dirty-looking" light part of the sky is very slowly "bleeding" from a faint peachy-pink hue over to blue. At no stage of this "bleeding" does the sky look green.

It could be like you said, Henning, that it is impossible to make green light by mixing light of various colors. Only a primary green will look green. Clearly, in a sunset light of very many different colors get scattered and mixed, never producing a clear layer of green in the sky.

Ann
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Re: Why are earthly sunsets never green?

Post by Helio George » Tue Jul 06, 2010 1:32 pm

Ann wrote:At the horizon there appears to be some clouds, nevertheless. Hugging the horizon is a thin orange-to-red layer. It is easy to imagine that here light has had to pass through so much atmosphere that even some of the red wavelengths are being scattered, though only horizontally (where light has to pass through the thickest layers of atmosphere), coloring the horizon orange-red. (Some orange and a bit of yellow light is being scattered here, too, but it seems probable that red light dominates.)
Yes, all the colours scatter some but it is the blue end of the spectrum that scatters the most. Rayleigh Scattering shows that the scattering varies inversely with the fourth power of the wavelength. Thus the blues, say at 460nm, will scatter 7 times more than the reds using 750nm. The lesser amount of violet that the Sun emits will scatter much more easily, which are the two reasons why we don't see violet.

Another important factor in the colours seen, especially at sunset, is the amount of dust, aerisols, pollens, and other particles that get lofted upward due to daytime convection. These tiny particles along with up to about 40 atmospheres of air, when the Sun's disk is on the horizon, can cause so much scattering and absorption that there are times when it is not uncomfortable to look at the Sun's disk directly as it glides over the horizon. [Unfortunately, because of the amount of infrared the Sun emits and the limited amount of scattering caused by our atmosphere of the longer wavelengths of IR, such direct observations should not include using binoculars or telescopes, which increase the amount of IR into the eye. ]
In the picture you can see a rather large, pinkish "halo" around the Sun. I have no idea what that is, and I can't remember seeing this particular effect before.

Anyway, the "dirty" color of the sky then changes into blue. First a light and somewhat attenuated and maybe cyan-tinged blue, then a more saturated but also darker blue.
These are common transitional colors. The pink from the higher cloud is logical since sunlight is being reflected so the red end of the spectrum isn't passing directly into space but is deflected by the cloud.

You might enjoy looking at others images of the Belt of Venus, which is the anti-solar effects.

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Re: Why are earthly sunsets never green?

Post by jman » Tue Jul 06, 2010 3:31 pm

Ann wrote: you can see a rather large, pinkish "halo" around the Sun. I have no idea what that is
Me neither. The halo is less pronounced and darker colour in the second photo. It could be lens flare inside the camera.

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Re: Why are earthly sunsets never green?

Post by Chris Peterson » Tue Jul 06, 2010 4:08 pm

Ann wrote:I've chased down a few more sunset pictures that I find revealing...
Don't overlook the fact that the gamut of cameras is different from that of the eye, and in images with "unusual" color combinations, such as sunset images, cameras generally produce colors somewhat different from what the eye sees. I take a lot of sunset pictures, and they never quite look the same as what I see. In addition, when you have the Sun in an image, you typically have internal reflections from the lens, and these are highly color distorted by the anti-reflection coatings on each element. If the Sun is centered in the frame, the internal reflections will generally manifest as a halo, which might explain the odd pinkish region you mention.

While images like this are quite useful for understanding many of the scattering and absorption mechanisms that produce sunset effects, it's also possible to overanalyze them.
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Re: Why are earthly sunsets never green?

Post by Helio George » Tue Jul 06, 2010 4:38 pm

Chris Peterson wrote:
Ann wrote:I've chased down a few more sunset pictures that I find revealing...
Don't overlook the fact that the gamut of cameras is different from that of the eye, and in images with "unusual" color combinations, such as sunset images, cameras generally produce colors somewhat different from what the eye sees.
I suspect you might be able to help me with something I have assumed is correct. When attempting to get colors to match that which we see in daylight, I have set the manual color setting to about 5850K. Is this about the best we can do in camera settings to get colour accuracy of, especially, celestial objects?
If the Sun is centered in the frame, the internal reflections will generally manifest as a halo, which might explain the odd pinkish region you mention.
Yes, that seems logical since the pink is smeared out beyond the pink cloud. The pinkish hue of the cloud is a typical observation, but the pink to the left of it is likely the result of the camera issue you state.

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Re: Why are earthly sunsets never green?

Post by Chris Peterson » Tue Jul 06, 2010 5:24 pm

Helio George wrote:I suspect you might be able to help me with something I have assumed is correct. When attempting to get colors to match that which we see in daylight, I have set the manual color setting to about 5850K. Is this about the best we can do in camera settings to get colour accuracy of, especially, celestial objects?
Well... the problem is that there is no single way of mapping color temperature to the limited gamut available with an RGB filter system. So different camera manufacturers use different algorithms. A specific temperature setting on a Nikon will look different from the same setting on a Canon. And the solution in either case is an approximation. When I work with RAW images, I adjust color temperature in post processing, and to get close to natural (as I remember it) for daytime scenes, I usually adjust the temperature to somewhere between 5500K and 6500K.

For astronomical targets, the only ones that concern me as far as true color accuracy are Jupiter, Mars, the Moon, and stars in wide field images. Nothing else shows significant color, so there's really no comparison between an image and anything that could be called "true" color. For deep sky objects, it is usually necessary to selectively color correct if you don't want to distort the star colors too much- especially if you use a DSLR or other camera using a sensor with an integral color filter array. To more specifically answer your question, when I image any of the bright targets mentioned previously, I do find that setting the camera (or post processor) to "daylight" produces what seems to be the most accurate color. If you shoot RAW images, or have the ability to set the exact color temperature in the camera, something around your 5850K is probably going to be best- give or take a couple hundred K, depending on the camera, target, and lens.
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Re: Why are earthly sunsets never green?

Post by Helio George » Tue Jul 06, 2010 6:53 pm

Chris Peterson wrote:Well... the problem is that there is no single way of mapping color temperature to the limited gamut available with an RGB filter system. So different camera manufacturers use different algorithms. A specific temperature setting on a Nikon will look different from the same setting on a Canon. And the solution in either case is an approximation. When I work with RAW images, I adjust color temperature in post processing, and to get close to natural (as I remember it) for daytime scenes, I usually adjust the temperature to somewhere between 5500K and 6500K.
That's helpful, thanks. I do need to switch to RAW imaging, and I have the software (Photoshop) but have not been active enough to attempt the climb around the short learning curve. [I purchased the not too expensive Canon 50D so I could have more color control.]
For astronomical targets, the only ones that concern me as far as true color accuracy are Jupiter, Mars, the Moon, and stars in wide field images. Nothing else shows significant color, so there's really no comparison between an image and anything that could be called "true" color.
Yes, though there are a few limited nebulae that allow some direct observing of colors -- larger telescopes have a slight advantage here.

Stefan Seip and David Marlin have used progressive defocussing to create a nice colorful effect for stars. Here is Seip's Southern Cross. Of course, the color settings are still important for any hope of color accuracy, but the overexposure problem is, at least, solved.
For deep sky objects, it is usually necessary to selectively color correct if you don't want to distort the star colors too much- especially if you use a DSLR or other camera using a sensor with an integral color filter array.
Yes, I remember the Canon 20D, I think, that offered a modified (unfiltered) astromer's option to improve imaging, though this was not improving color accuracy.
To more specifically answer your question, when I image any of the bright targets mentioned previously, I do find that setting the camera (or post processor) to "daylight" produces what seems to be the most accurate color. If you shoot RAW images, or have the ability to set the exact color temperature in the camera, something around your 5850K is probably going to be best- give or take a couple hundred K, depending on the camera, target, and lens.
Thanks, I will remember this good info.

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Re: Why are earthly sunsets never green?

Post by bystander » Wed Jul 14, 2010 1:28 pm


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Re: Why are earthly sunsets never green?

Post by ifi12ican@gmail.com » Tue Mar 13, 2012 5:18 pm

Please put an attribution next to my photo, as required by my Creative Commons license. Mine is the top one under sunsets. . bright orange clouds
http://creativecommons.org/licenses/by- ... .0/deed.en
It should read "Photo by Timothy K. Hamilton"

In addition, flickr has a rule that there should be a link from the photo itself back to the original webpage of the photo on flickr.
It would be a great idea and appropriate to give credit to every photo that is on your site.

Thank you, and thanks for using my photo.

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Re: Why are earthly sunsets never green?

Post by bystander » Tue Mar 13, 2012 6:09 pm

ifi12ican@gmail.com wrote:Please put an attribution next to my photo
done
It is the policy of this board to attribute all photos and articles used.
Unfortunately, not all posters believe the policy applies to them.
BTW: It took some time and effort to identify the photo in question.
Next time, please quote the offending post or link back to your photo.
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