Colors of Twilight vs. Sunset/Sunrise

The cosmos at our fingertips.
Peter87
Ensign
Posts: 15
Joined: Sun Mar 03, 2024 6:20 pm

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Peter87 » Sat Apr 20, 2024 5:01 pm

Ann wrote: Tue Apr 16, 2024 5:01 am 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).

Ann

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?

User avatar
Ann
4725 Å
Posts: 13476
Joined: Sat May 29, 2010 5:33 am

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Ann » Sat Apr 20, 2024 7:53 pm

Peter87 wrote: Sat Apr 20, 2024 5:01 pm
Ann wrote: Tue Apr 16, 2024 5:01 am 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).

Ann

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?
I'm not sure that the picture taken by Eiji Ogura was taken during the blue hour. It might have been. I think it is more likely that the other picture was. The clouds reflected the colors of the reddened Sun after it had sunk below the horizon.

Wikipedia shows us this cloudless picture of the blue hour:


My guess is that the b value of the sky during the blue hour would be 255.


I found an interesting chart showing the hues, hex codes and rgb values for several shades of purple. There is always more b than r in all these shades of purple, and the b value is at or close to 255 in all the shades of purple except the darkest ones.



Ann
Color Commentator

User avatar
Chris Peterson
Abominable Snowman
Posts: 18261
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Chris Peterson » Sat Apr 27, 2024 3:13 pm

Ann wrote: Fri Mar 29, 2024 1:48 pm
Chris Peterson wrote: Fri Mar 29, 2024 1:15 pm
Ann wrote: Fri Mar 29, 2024 5:36 am

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

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

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
PXL_20240419_165020635p.jpg
You do not have the required permissions to view the files attached to this post.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

User avatar
Ann
4725 Å
Posts: 13476
Joined: Sat May 29, 2010 5:33 am

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Ann » Sat Apr 27, 2024 5:20 pm

Chris Peterson wrote: Sat Apr 27, 2024 3:13 pm
Ann wrote: Fri Mar 29, 2024 1:48 pm
Chris Peterson wrote: Fri Mar 29, 2024 1:15 pm

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

Oh no no no, I can't stand the cuteness of this baby! 🥰 😍 ❤️ Is the little one a boy or a girl?

Couldn't find a good goat emoji, so I guess a unicorn is the closest thing! :D 🦄

Ann
Color Commentator

User avatar
Ann
4725 Å
Posts: 13476
Joined: Sat May 29, 2010 5:33 am

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Ann » Mon May 13, 2024 6:09 pm

Chris Peterson wrote: Wed Apr 10, 2024 5:27 pm
Ann wrote: Wed Apr 10, 2024 5:14 pm
Chris Peterson wrote: Tue Mar 26, 2024 2:17 pm

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
Try it with a little (chlorine) bleach.

Well, Chris, I finally got around to trying this experiment again (even having bought a humongous bottle of bleach just for this purpose - I couldn't get anything smaller).

The failure was spectacular!!! 😮 🤣

Red cabbage water to be mixed with vinegar and bleach May 13 2024.jpg
That looks okay...


Red cabbage water after being mixed with vinegar and bleach May 13 2024.jpg
That doesn't!!!

Frankly, Chris, I'm done with the cabbage experiment now! :doh: :facepalm:
And, hey, if anyone is interested in buying a barrel of bleach...

Ann
You do not have the required permissions to view the files attached to this post.
Color Commentator

Peter87
Ensign
Posts: 15
Joined: Sun Mar 03, 2024 6:20 pm

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Peter87 » Tue May 14, 2024 5:53 pm

I wonder if this Wikipedia article on Sky brightness makes sense -- it seems to that there is both direct and indirect sunlight in the sky at twilight --

"Indirectly scattered sunlight comes from ... the atmosphere itself .... the Sun has just set but still illuminates the upper atmosphere directly." https://en.wikipedia.org/wiki/Sky_brigh ... atmosphere.

I don't know if that makes sense in one way of distinguishing the colors of twilight and sunset/sunrise -- at sunset/sunrise, all or most of the visible sunlight in the sky is direct, at twilight, both direct and indirect?

User avatar
Chris Peterson
Abominable Snowman
Posts: 18261
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Chris Peterson » Tue May 14, 2024 6:00 pm

Peter87 wrote: Tue May 14, 2024 5:53 pm I wonder if this Wikipedia article on Sky brightness makes sense -- it seems to that there is both direct and indirect sunlight in the sky at twilight --

"Indirectly scattered sunlight comes from ... the atmosphere itself .... the Sun has just set but still illuminates the upper atmosphere directly." https://en.wikipedia.org/wiki/Sky_brigh ... atmosphere.

I don't know if that makes sense in one way of distinguishing the colors of twilight and sunset/sunrise -- at sunset/sunrise, all or most of the visible sunlight in the sky is direct, at twilight, both direct and indirect?
The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

Peter87
Ensign
Posts: 15
Joined: Sun Mar 03, 2024 6:20 pm

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Peter87 » Tue May 14, 2024 6:53 pm

Chris Peterson wrote: Tue May 14, 2024 6:00 pm
The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.
Regarding the lower blue part, which is the Earth's shadow, I wonder if the blue color is not a function of Rayleigh scattering but instead Chappuis absorption ("when solor rays graze the stratosphere the Chappuis band of ozone extinguishes the longer wavelengths, leaving only blue" https://adsabs.harvard.edu/full/2005JBAA..115..247L)?

User avatar
Chris Peterson
Abominable Snowman
Posts: 18261
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Chris Peterson » Tue May 14, 2024 6:59 pm

Peter87 wrote: Tue May 14, 2024 6:53 pm
Chris Peterson wrote: Tue May 14, 2024 6:00 pm
The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.
Regarding the lower blue part, which is the Earth's shadow, I wonder if the blue color is not a function of Rayleigh scattering but instead Chappuis absorption ("when solor rays graze the stratosphere the Chappuis band of ozone extinguishes the longer wavelengths, leaving only blue" https://adsabs.harvard.edu/full/2005JBAA..115..247L)?
It doesn't look ozone blue to me. I'd say it's dominated by Rayleigh scattered sunlight, which itself is scattered off the illuminated part of the sky.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

Peter87
Ensign
Posts: 15
Joined: Sun Mar 03, 2024 6:20 pm

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Peter87 » Tue May 14, 2024 7:16 pm

Chris Peterson wrote: Tue May 14, 2024 6:59 pm It doesn't look ozone blue to me. I'd say it's dominated by Rayleigh scattered sunlight, which itself is scattered off the illuminated part of the sky.
I see, perhaps because the Belt of Venus is visible shortly before sunrise or after sunset, during civil twilight, and ozone blue occurs closer to nautical twilight and blue hour?

Peter87
Ensign
Posts: 15
Joined: Sun Mar 03, 2024 6:20 pm

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Peter87 » Sat May 18, 2024 4:04 pm

Chris Peterson wrote: Tue May 14, 2024 6:00 pm The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.
When I think of Descartes on clear and distinct ideas, where “distinct” perceptions are "sharply separated from all other perceptions" –- then I struggle to distinguish between the upper pink part of the Belt of Venus, which is directly lit by the Sun, from the lower blue part, which is only scattering light from other parts of the sky.

The lower blue part, which is only scattering light from other parts of the sky, means that the source of this light is indirect, it hits the upper atmosphere, bounces off, and then cascades downward, so to speak?





(My apologies, I regret having to pose this question that has such a self-evident answer. It reveals my poor background in astronomy.)

User avatar
Chris Peterson
Abominable Snowman
Posts: 18261
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: Colors of Twilight vs. Sunset/Sunrise

Post by Chris Peterson » Sat May 18, 2024 4:17 pm

Peter87 wrote: Sat May 18, 2024 4:04 pm
Chris Peterson wrote: Tue May 14, 2024 6:00 pm The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.
When I think of Descartes on clear and distinct ideas, where “distinct” perceptions are "sharply separated from all other perceptions" –- then I struggle to distinguish between the upper pink part of the Belt of Venus, which is directly lit by the Sun, from the lower blue part, which is only scattering light from other parts of the sky.

The lower blue part, which is only scattering light from other parts of the sky, means that the source of this light is indirect, it hits the upper atmosphere, bounces off, and then cascades downward, so to speak?





(My apologies, I regret having to pose this question that has such a self-evident answer. It reveals my poor background in astronomy.)
The lower blue part of the Belt is just the twilight sky rising (or setting). In the evening I sometimes call it "nightrise". So yeah, it is only lit indirectly, by light that has scattered from air still lit by the Sun directly.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com