Colors of Twilight vs. Sunset/Sunrise

[Peter87]

Hello all --
I'm trying to understand colors of the sky at twilight. I'm not in the field of physics, but I've tried to grasp some concepts, which I first briefly note, and then pose some questions. Any feedback would be greatly appreciated.

1. Attempt at basic understanding. The blue light waves of sunlight are nearly the smallest in size, about 400 nm. Red is nearly the largest, about 700 nm. For sunlight to reach a horizonal point, it travels a longer distance (as opposed to a shorter distance at noon when the sun is more directly overhead); more of the earth's atmospheric particles are then encountered in this denser, thicker atmosphere, the smaller blue color is scattered out by the greater number of particles but larger sized red, orange, and yellow light waves remain and are predominant at sunset/sunrise.

2. Questions. When civil twilight begins and the sun disk is just below the horizon, is some of the light that can be observed direct light, mainly the purple color in the upper sky (at the top of the picture (which doesn't seem to be visible))?

Why is the light close to the horizon red, orange, and yellow, the same color of sunset/sunrise, when the sun sun disk is below the horizon?

Source: https://www.123rf.com/photo_105348161_c ... round.html

Thank you all, I would be very grateful for any explanations.
Kind regards,
Peter

[Peter87]

Thanks to all of you who looked at my post! Please let me know -- no one wants to reply to my question because:
1. It is a such difficult question, impossible, I've asked a question that has stumped all viewers! (unlikely!)
2. On the contrary, my question is quite idiotic and incomprehensible, the question itself reveals that I have absolutely no understanding of the physics of light, and I am unworthy of being given an answer -- your failure to respond is a passive-aggressiveness that means "We don't want you on this website."
3. It is not important to you to divert resources (time, mental energy) to help someone new to this website, who signed up on this website with great hopes of getting an answer.
4. ________________________________________________________________________________.
5. ________________________________________________________________________________.
Please let me know, thanks!

[Ann]

Thanks to all of you who looked at my post! Please let me know -- no one wants to reply to my question because:
1. It is a such difficult question, impossible, I've asked a question that has stumped all viewers! (unlikely!)
2. On the contrary, my question is quite idiotic and incomprehensible, the question itself reveals that I have absolutely no understanding of the physics of light, and I am unworthy of being given an answer -- your failure to respond is a passive-aggressiveness that means "We don't want you on this website."
3. It is not important to you to divert resources (time, mental energy) to help someone new to this website, who signed up on this website with great hopes of getting an answer.
4. ________________________________________________________________________________.
5. ________________________________________________________________________________.
Please let me know, thanks!

Sorry, Peter. I, the Color Commentator, am also a math idiot, and math formulas just make my brain freeze.

I do know that the blue color of the sky is due to Rayleigh scattering. But I'd hate even trying to explain it to you!

Wikipedia wrote:

Rayleigh scattering (/ˈreɪli/ RAY-lee), named after the 19th-century British physicist Lord Rayleigh (John William Strutt),[1] is the predominantly elastic scattering of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering medium (normal dispersion regime), the amount of scattering is inversely proportional to the fourth power of the wavelength, e.g., a blue color is scattered much more than a red color as light propagates through air.

I don't know about you, but I didn't get much of that. But let's read on:

Rayleigh scattering of sunlight in Earth's atmosphere causes diffuse sky radiation, which is the reason for the blue color of the daytime and twilight sky, as well as the yellowish to reddish hue of the low Sun.

Okay. It says here that Rayleigh scattering is responsible for the color of the sky, but it doesn't say how it works (or it explains it in a way that my poor brain can make sense of). So I'm going to tell you what my Mom, a sixth grade teacher, told me: Sunset skies get red or yellow near the horizon because the light there has passed through so much air that all the blue light has been scattered away, and only the red and yellow light remains. Yes, but higher up in the sky the suset sky remains blue, although it's getting rapidly darker.

Let's look at a picture of the effect of Rayleigh scattering:

Original uploader was User:Dragons flight at en.Wikipeda. derivative work:KES47 (talk) (converted to SVG). - File:Rayleigh sunlight scattering.png

---

As for why we see yellow and red light on the horizon when the Sun has already set, I'm sure there is a nice mathematical formula to explain it. But they way I understand it, light is refracted by the atmosphere in such a way that we still see the Sun a few minutes after it has actually set (well, how long we can see it after the Sun has actually set probably depends on our latitude, so that we can see it for longer time at higher latitudes and for a shorter time at lower latitudes, as the Sun seems to sink much faster near the equator).

Anyway, if you check the exact time when the Sun rises and sets during the equinox, you can see that the Sun appears to be above the horizon for more than 12 hours. That's because the atmosphere reflects (refracts???) the light from the Sun upwards above the horizon, so that we still see it when it has actually set.

The same thing is true for the red colors of sunset. The light from the reddened Sun is reflected (or refracted, whatever) upwards, so that we see the sunset colors for a relatively short time after the Sun has sunk so far below the horizon that we really can't see the disk of it any more. Soon, however, the red light disappears and the blue colors remain, but they keep getting deeper and darker until the night is pitch dark (if you find yourself in a place where there is no light pollution). And if there is a full Moon, then the night sky is actually blue, in the same way that the daytime cloudless sky is blue. Of course, the night sky is a much, much darker shade of blue than the daytime sky.

I think you asked about purple skies. Personally I've never seen purple skies, and I don't think they exist. The reason would be that the purple light doesn't get sufficiently separated from the various shades of blue to create a true purple effect. Also, the Sun produces less purple than blue light.

The blackbody curve of the Sun, showing the amount of light (black line) that the Sun produces at various wavelengths. There is not enough purple light produced by the Sun relatively to the other colors for the sky to ever take on a truly purple hue.

---

One event that really separates the colors of the Sun is the green flash phenomenon. The "underside" of the Sun's disk is red, but the "top side" is increasingly green. You can sometimes see a small blue flash as well, and, very very rarely, an extremely faint purple flash. I very much recommend this APOD, where you can see a green flash evolve and turn into a small blue flash and a fleetingly faint purple flash.

Sorry! This is the best I can do. And, unfortunately, I don't have more time now, so I've got to go.

Ann

[Ann]

Thanks to all of you who looked at my post! Please let me know -- no one wants to reply to my question because:
1. It is a such difficult question, impossible, I've asked a question that has stumped all viewers! (unlikely!)
2. On the contrary, my question is quite idiotic and incomprehensible, the question itself reveals that I have absolutely no understanding of the physics of light, and I am unworthy of being given an answer -- your failure to respond is a passive-aggressiveness that means "We don't want you on this website."
3. It is not important to you to divert resources (time, mental energy) to help someone new to this website, who signed up on this website with great hopes of getting an answer.
4. ________________________________________________________________________________.
5. ________________________________________________________________________________.
Please let me know, thanks!

Another reason why no one answered you before is, unfortunately, that our community has shrunk, and few of us even visit this part of the forum (the Asterisk Café).

The person who would be able to give you the best answers to your questions is probably Chris Peterson, but I think he rarely visits the Asterisk Café. Chris is a real astronomer and a teacher. The answer he would give you (if he chose to answer you, which I can't guarantee - he is busy), would be absolutely correct, but he may or may not elaborate on his answer to make it easier to understand.

Ann

[Chris Peterson]

Thanks to all of you who looked at my post! Please let me know -- no one wants to reply to my question because:
1. It is a such difficult question, impossible, I've asked a question that has stumped all viewers! (unlikely!)
2. On the contrary, my question is quite idiotic and incomprehensible, the question itself reveals that I have absolutely no understanding of the physics of light, and I am unworthy of being given an answer -- your failure to respond is a passive-aggressiveness that means "We don't want you on this website."
3. It is not important to you to divert resources (time, mental energy) to help someone new to this website, who signed up on this website with great hopes of getting an answer.
4. ________________________________________________________________________________.
5. ________________________________________________________________________________.
Please let me know, thanks!

Another reason why no one answered you before is, unfortunately, that our community has shrunk, and few of us even visit this part of the forum (the Asterisk Café).

The person who would be able to give you the best answers to your questions is probably Chris Peterson, but I think he rarely visits the Asterisk Café. Chris is a real astronomer and a teacher. The answer he would give you (if he chose to answer you, which I can't guarantee - he is busy), would be absolutely correct, but he may or may not elaborate on his answer to make it easier to understand.

Ann

Thanks, Ann. I definitely follow this forum. I didn't answer because I don't really have much more to say. The basic idea has already been expressed... by both you and the OP. Scattered light is blue, light that has had most of its blue scattered away is red. Beyond that, the actual colors we see represent some combination of those processes. Clouds may remain in direct sunlight at sunset because they are further west and higher, in which case they will likely be red/orange/yellow. Or if they're just into shadow, they may still pick up the red light reflected from clouds even further west. Or they may be in shadow, in which case they are likely to look blue (which some people see as purple) because they are reflecting blue sky. With countless variations as different cloud patterns reflect around different parts of the sky because of their location and height. And all of that is modified by the presence or absence of other particulates like smoke, dust, and pollen.

[Ann]

Hello all --
I'm trying to understand colors of the sky at twilight. I'm not in the field of physics, but I've tried to grasp some concepts, which I first briefly note, and then pose some questions. Any feedback would be greatly appreciated.

1. Attempt at basic understanding. The blue light waves of sunlight are nearly the smallest in size, about 400 nm. Red is nearly the largest, about 700 nm. For sunlight to reach a horizonal point, it travels a longer distance (as opposed to a shorter distance at noon when the sun is more directly overhead); more of the earth's atmospheric particles are then encountered in this denser, thicker atmosphere, the smaller blue color is scattered out by the greater number of particles but larger sized red, orange, and yellow light waves remain and are predominant at sunset/sunrise.

2. Questions. When civil twilight begins and the sun disk is just below the horizon, is some of the light that can be observed direct light, mainly the purple color in the upper sky (at the top of the picture (which doesn't seem to be visible))?

Why is the light close to the horizon red, orange, and yellow, the same color of sunset/sunrise, when the sun sun disk is below the horizon?

Source: https://www.123rf.com/photo_105348161_c ... round.html

Thank you all, I would be very grateful for any explanations.
Kind regards,
Peter

Peter, you wrote about the purple color of the upper sky. As I said, I have never observed purple color in the sky, except on those rare occasions when pink clouds at sunset or sunrise momentarily seem to take on a purplish-pink hue. Otherwise, I've never seen it.

I would like to warn you against judging colors of the sky from photographs. The reason is that different cameras, filters etcetera will show colors slightly differently. To show you what I mean, take a look at two pictures of a purplish-bluish flower called hepatica:

I took this picture of a hepatica a few days ago.

I took this picture of a hepatica last year.
I was unable to get any closer, because the hepatica grew
at the foot of a bush with lots of branches.

Hepatica. The picture is from an 'official' Swedish site. Photo: Lars-Åke Janzon

---

Can you see that "my'" hepaticas are just a tad bluer than the "official" portrait of this flower? Well, they are. I took the picture with my mobile camera, and I have been told that my mobile phone has a blue screen.

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Ann

[Chris Peterson]

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

[Ann]

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

I'd love to do the red cabbage experiment, but I don't much like cabbage, particularly not if it's been boiled. So to do the experiment, I would have to buy a head of cabbage, boil it and then throw it away. Feels like food waste. Maybe I'll do it anyway.

And guess what, I just revisited that bush with all the branches with a tiny number of hepaticas growing at the foot of it. (There are very few places where I live where hepaticas grow, so I revisit the sites.)

Anyway. I fought the branches and took a new photo. Afterwards, I carefully compared the color of the flowers themselves with the color that my camera gave them. Guess what? They were very noticeably bluer in my photo than in real life.

Ann

[Chris Peterson]

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

I'd love to do the red cabbage experiment, but I don't much like cabbage, particularly not if it's been boiled. So to do the experiment, I would have to buy a head of cabbage, boil it and then throw it away. Feels like food waste. Maybe I'll do it anyway.

The solution is goats. Ours will happily gobble up vegetable remnants we don't eat ourselves. I'm sure your neighbors would be fine with a few goats around the house, no?
_

[Ann]

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

I'd love to do the red cabbage experiment, but I don't much like cabbage, particularly not if it's been boiled. So to do the experiment, I would have to buy a head of cabbage, boil it and then throw it away. Feels like food waste. Maybe I'll do it anyway.

The solution is goats. Ours will happily gobble up vegetable remnants we don't eat ourselves. I'm sure your neighbors would be fine with a few goats around the house, no?
_

Click to view full size image

Thanks for the suggestion, Chris, I didn't think of that!

That's a lovely goat, by the way. What's her name? She looks very feminine to me!

Ann

[Chris Peterson]

I'd love to do the red cabbage experiment, but I don't much like cabbage, particularly not if it's been boiled. So to do the experiment, I would have to buy a head of cabbage, boil it and then throw it away. Feels like food waste. Maybe I'll do it anyway.

The solution is goats. Ours will happily gobble up vegetable remnants we don't eat ourselves. I'm sure your neighbors would be fine with a few goats around the house, no?
_

Click to view full size image

Thanks for the suggestion, Chris, I didn't think of that!

That's a lovely goat, by the way. What's her name? She looks very feminine to me!

Ann

Gretel. She'll be kidding in a couple of weeks.

[Ann]

The solution is goats. Ours will happily gobble up vegetable remnants we don't eat ourselves. I'm sure your neighbors would be fine with a few goats around the house, no?
_

Click to view full size image

Thanks for the suggestion, Chris, I didn't think of that!

That's a lovely goat, by the way. What's her name? She looks very feminine to me!

Ann

Gretel. She'll be kidding in a couple of weeks.

Thanks for teaching me a new word, Chris! I only knew one meaning of the word "kidding".

I'm sure Gretel's kids will be adorable!

Ann

[Fred the Cat]

Thanks for the suggestion, Chris, I didn't think of that!

That's a lovely goat, by the way. What's her name? She looks very feminine to me!

Ann

Gretel. She'll be kidding in a couple of weeks.

Thanks for teaching me a new word, Chris! I only knew one meaning of the word "kidding".

I'm sure Gretel's kids will be adorable!

Ann

I'd pay a thousand dollars so, one day, I could have Gretel's grandkids. Just kidding but it beats a new lawnmower - hoofs down.

[Peter87]

Peter, you wrote about the purple color of the upper sky. As I said, I have never observed purple color in the sky, except on those rare occasions when pink clouds at sunset or sunrise momentarily seem to take on a purplish-pink hue. Otherwise, I've never seen it.

Ann, thanks for putting me in the right direction, probably what I am thinking about is antitwilight / so-called "Belt of Venus," which if I understand it, occurs during civil twilight when the sun is not far below the horizon. Because the sun is near the horizon, the angle of the sunlight projected from below the horizon is small, and so sunlight is still traveling though a dense particle-heavy atmosphere. The sunlight is scattered (or backscattered), and because the atmosphere is dense, blue is scattered out and, for some reason, there are various color/shades/hues observable -- yellow, orange, pink, perhaps others? -- near the horizon.

[Chris Peterson]

Peter, you wrote about the purple color of the upper sky. As I said, I have never observed purple color in the sky, except on those rare occasions when pink clouds at sunset or sunrise momentarily seem to take on a purplish-pink hue. Otherwise, I've never seen it.

Ann, thanks for putting me in the right direction, probably what I am thinking about is antitwilight / so-called "Belt of Venus," which if I understand it, occurs during civil twilight when the sun is not far below the horizon. Because the sun is near the horizon, the angle of the sunlight projected from below the horizon is small, and so sunlight is still traveling though a dense particle-heavy atmosphere. The sunlight is scattered (or backscattered), and because the atmosphere is dense, blue is scattered out and, for some reason, there are various color/shades/hues observable -- yellow, orange, pink, perhaps others? -- near the horizon.

With the Belt of Venus the colors are almost always dominated by pink and blue. Pink above where the Sun is still illuminating the air (and because that light, which is reflected and scattered back at us, has passed through so much air, it lacks short wavelengths), and blue below where the air is in Earth's shadow, so lit primarily by scattered light, which is rich in shorter wavelengths and impoverished of longer ones.

[Ann]

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

Well, Chris, I did the experiment. I got a very small piece of red cabbage from my friend, boiled it and then poured the purple water into two glasses.

I then put baking soda into the glass at left and vinegar into the glass at right. The liquid in the glass at right instantly changed color to a bright red hue. But the liquid in the glass at left, where I had put baking soda, remained very dark, almost black-looking.

So I decided that I had to dilute the cabbage water into which I had put baking soda. I poured water into a smaller glass and added just a bit of the dark-colored cabbage water that contained baking soda. Now this liquid became clearly blue.

You can see the poor cabbage leaves on the small plate in the background.

All in all, Chris, I guess you can say that the experiment worked.

Ann

[Ann]

Peter, you wrote about the purple color of the upper sky. As I said, I have never observed purple color in the sky, except on those rare occasions when pink clouds at sunset or sunrise momentarily seem to take on a purplish-pink hue. Otherwise, I've never seen it.

Ann, thanks for putting me in the right direction, probably what I am thinking about is antitwilight / so-called "Belt of Venus," which if I understand it, occurs during civil twilight when the sun is not far below the horizon. Because the sun is near the horizon, the angle of the sunlight projected from below the horizon is small, and so sunlight is still traveling though a dense particle-heavy atmosphere. The sunlight is scattered (or backscattered), and because the atmosphere is dense, blue is scattered out and, for some reason, there are various color/shades/hues observable -- yellow, orange, pink, perhaps others? -- near the horizon.

Peter, I think you are right that the anti-twilight sky phenomenon, the Belt of Venus, can make the sky take on hints of a purple color.

In this image, the full Moon is sitting in the pink (or peach-colored) Belt of Venus, whereas the Earth's shadow below it looks almost violet in color.

---

In this image, the sky appears to change color from yellow to orange to pink to purple to blue. The full Moon is sitting in the 'pink strip', which is the Belt of Venus. Credit: Kent Duryee

---

Ann

[Chris Peterson]

I think that if you see a color photo whose color balance seems surprising, chances are that it is the picture that is different, not the natural phenomenon that has taken on a strange hue.

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

Well, Chris, I did the experiment. I got a very small piece of red cabbage from my friend, boiled it and then poured the purple water into two glasses.


Red cabbage water in two glasses.jpg


I then put baking soda into the glass at left and vinegar into the glass at right. The liquid in the glass at right instantly changed color to a bright red hue. But the liquid in the glass at left, where I had put baking soda, remained very dark, almost black-looking.


Red cabbage water with baking soda and vinegar.jpg


So I decided that I had to dilute the cabbage water into which I had put baking soda. I poured water into a smaller glass and added just a bit of the dark-colored cabbage water that contained baking soda. Now this liquid became clearly blue.


Diluted red cabbage water with baking soda and cabbage water with vinegar.jpg


You can see the poor cabbage leaves on the small plate in the background.

All in all, Chris, I guess you can say that the experiment worked.

Ann

Try it with a little (chlorine) bleach.

[Ann]

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

Well, Chris, I did the experiment. I got a very small piece of red cabbage from my friend, boiled it and then poured the purple water into two glasses.


Red cabbage water in two glasses.jpg


I then put baking soda into the glass at left and vinegar into the glass at right. The liquid in the glass at right instantly changed color to a bright red hue. But the liquid in the glass at left, where I had put baking soda, remained very dark, almost black-looking.


Red cabbage water with baking soda and vinegar.jpg


So I decided that I had to dilute the cabbage water into which I had put baking soda. I poured water into a smaller glass and added just a bit of the dark-colored cabbage water that contained baking soda. Now this liquid became clearly blue.


Diluted red cabbage water with baking soda and cabbage water with vinegar.jpg


You can see the poor cabbage leaves on the small plate in the background.

All in all, Chris, I guess you can say that the experiment worked.

Ann

Try it with a little (chlorine) bleach.

Can't! I used up my cabbage!

Ann

[Peter87]

Or if they're just into shadow, they may still pick up the red light reflected from clouds even further west. Or they may be in shadow, in which case they are likely to look blue (which some people see as purple) because they are reflecting blue sky.

Chris, I don't understand to which shadow you are referring? A cloud may be "just into shadow" means in the shadow of the earth?

Then you raise a second possibility: "Or they may be in shadow" -- and now you are referring to a different type of shadow? Or it is a a temporal factor to which you are referring -- "just into shadow" ("just" meaning at that moment) as opposed to "in shadow" for a longer duration?

[Chris Peterson]

Or if they're just into shadow, they may still pick up the red light reflected from clouds even further west. Or they may be in shadow, in which case they are likely to look blue (which some people see as purple) because they are reflecting blue sky.

Chris, I don't understand to which shadow you are referring? A cloud may be "just into shadow" means in the shadow of the earth?

Then you raise a second possibility: "Or they may be in shadow" -- and now you are referring to a different type of shadow? Or it is a a temporal factor to which you are referring -- "just into shadow" ("just" meaning at that moment) as opposed to "in shadow" for a longer duration?

The lower part of the Belt of Venus- the blue- is literally the atmosphere that is in Earth's shadow. From that height, the Sun has set. The upper pink part is still in sunlight. From that height, you would look back at a setting Sun still above the horizon.

[Peter87]

Peter, I think you are right that the anti-twilight sky phenomenon, the Belt of Venus, can make the sky take on hints of a purple color.
Ann

Ann, I was following up on hints of a purple color and the anti-twilight sky phenomenon, the Belt of Venus, and I came across "afterglow," at the moment in Wikipedia, "with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light.[1][2] Purple light mainly occurs when the Sun is 2–6° below the horizon, from civil to nautical twilight, while the bright segment lasts until the end of the nautical twilight." https://en.wikipedia.org/wiki/Afterglow

Have you come across much about "afterglow" and some aspect of a regular pattern of an observable purple color?


It gets even more intriguing here in the same article, where the purple is seen as a mix of blue and red colors in the sky:

Sunlight reaches Earth around civil twilight during golden hour intensely in its low-energy and low-frequency red component. During this part of civil twilight after sunset and before sundawn the red sunlight remains visible by scattering through particles in the air. Backscattering, possibly after being reflected off clouds or high snowfields in mountain regions, furthermore creates a reddish to pinkish light. The high-energy and high-frequency components of light towards blue are scattered out broadly, producing the broader blue light of nautical twilight before or after the reddish light of civil twilight, while in combination with the reddish light producing the purple light.[5]

But it is Wikipedia, and so may be unreliable, though I can't find much elsewhere, and footnote 5 doesn't seem to discuss this topic clearly. It would be interesting to find a clear discussion of afterglow and purple color somewhere in the literature...

[Ann]

Peter, I think you are right that the anti-twilight sky phenomenon, the Belt of Venus, can make the sky take on hints of a purple color.
Ann

Ann, I was following up on hints of a purple color and the anti-twilight sky phenomenon, the Belt of Venus, and I came across "afterglow," at the moment in Wikipedia, "with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light.[1][2] Purple light mainly occurs when the Sun is 2–6° below the horizon, from civil to nautical twilight, while the bright segment lasts until the end of the nautical twilight." https://en.wikipedia.org/wiki/Afterglow

Have you come across much about "afterglow" and some aspect of a regular pattern of an observable purple color?


It gets even more intriguing here in the same article, where the purple is seen as a mix of blue and red colors in the sky:

Sunlight reaches Earth around civil twilight during golden hour intensely in its low-energy and low-frequency red component. During this part of civil twilight after sunset and before sundawn the red sunlight remains visible by scattering through particles in the air. Backscattering, possibly after being reflected off clouds or high snowfields in mountain regions, furthermore creates a reddish to pinkish light. The high-energy and high-frequency components of light towards blue are scattered out broadly, producing the broader blue light of nautical twilight before or after the reddish light of civil twilight, while in combination with the reddish light producing the purple light.[5]

But it is Wikipedia, and so may be unreliable, though I can't find much elsewhere, and footnote 5 doesn't seem to discuss this topic clearly. It would be interesting to find a clear discussion of afterglow and purple color somewhere in the literature...

All I can say is that purple light triggers a response from both the blue-sensitive and the red-sensitive rods in our retinas. Consider this color, ███. The (rgb) values of this particular color is, r = 123, g = 0, and b = 222. So purple, at least as far as our eyes are concerned, is indeed a mixture of red and blue.

I can see that the mixture of red and blue light in the sky under certain circumstances will make parts of even in the cloudless dawn or dusk sky look purple (because the atmosphere itself will reflect light and provide some mixing of the colors). I know I have seen clouds reflecting the dawn or dusk light look purple (but mixed with other colors as well).

I have seen the sky look like this. Credit: EIJI OGURA/EYEEM/GETTY IMAGES

---

I have seen the sky look like this. Credit: I don't know.

---

Ann

[Chris Peterson]

I can see that the mixture of red and blue light in the sky under certain circumstances will make parts of even in the cloudless dawn or dusk sky look purple (because the atmosphere itself will reflect light and provide some mixing of the colors). I know I have seen clouds reflecting the dawn or dusk light look purple (but mixed with other colors as well).

Given a well understood mechanism for creating blue light in the sky, and a well understood mechanism for creating red light in the sky, and well understood mechanisms of reflection and scatter, it would be difficult to explain if we didn't regularly see purple!

[Peter87]

Given a well understood mechanism for creating blue light in the sky, and a well understood mechanism for creating red light in the sky, and well understood mechanisms of reflection and scatter, it would be difficult to explain if we didn't regularly see purple!

I think we are indebted to Edward Olson Hulburt for part of a well understood mechanism for creating blue light in the sky, that "during the day the clear sky is blue and that ozone has little effect on the color of the daytime sky. But near sunset and during twilight, ozone profoundly affects the color of the sky. For example, in the absence of ozone the zenithal sky would be a grayish-greenish blue at dusk, turning yellowish at twilight, but with ozone the zenithal sky is blue at dusk and throughout twilight (as seen), resulting in blue. at dusk approximately 1/3 to Rayleigh and 2/3 to ozone, and during twilight completely to ozone."