APOD: The Belt of Venus Over Mercedes... (2012 Feb 07)

[Chris Peterson]

I think that the answer is that green is a primary color to our eyes, which means that we will only see green if there is a strong peak in the middle of the spectrum. This is obviously not the case with any part of the twilight sky, which is a mixture of many wavelengths.

I think you've got part of the answer here, with respect to the mixture of wavelengths. Not so sure about the notion of green being a primary color in our visual system, though. In fact, two of our color sensors peak in the green- the M cones ("green") and the L cones ("red"). In addition, the rods also peak in the green.

I think what is going on is that we have an overall blue sky created by scatter off of air molecules (you can see the gradient in intensity from the top of the image all the way to the horizon). Then, you have a zone just above the Earth's shadow that is scattered red light from the opposite sunset (red because much of the blue has been scattered away). So there is no mechanism to isolate green light, and we therefore don't see green- but it doesn't depend on the primaries our eyes use.

The reason why green is a primary color to our eyes may have to do with survival. Plants on the Earth are almost always green, and where there is vegetation, there is probably water. Humanity undoubtedly spent hundreds of thousands of years surviving on the savanna, where our ability to detect the green color of plants was of critical importance.

Maybe. But many animals that are dependent on vegetation lack color vision, or lack the ability to specifically detect green hues. We almost certainly started with a single retinal pigment- something probably not very different from the rhodopsin found in our rod cells to this day, and which is found in all animal visual systems. Its peak is closely matched to the green maximum of the solar spectrum. The development of color vision must be a combination of evolutionary pressure and practical chemistry.

[orin stepanek]

Nice job Luis; I really like today's picture!

[Ann]

Chris, I don't doubt for a moment that various evolutionary processes have led to the color vision that humans have today. I still believe that our three-color vision, where green is a primary color, has had an important survival value for humanity.

But there is another reason why we don't ever see the sky as green (apart from auroras, which is another matter). The "transition sky", which is neither red, yellow nor blue, must be similar enough to the "unfiltered" light of the Sun to appear to be the same color of the same, namely white.

Consider. Why is it that we never see stars as green? It is because their light is similar enough to the light of the Sun that they appear white to us. Stars whose light output peaks in the orange or red part of the spectrum look (mildly) yellow. Stars whose light output peaks in the infrared part of the spectrum look yellow, yellow-orange, or, in extremely rare cases (Mu Cephei!) orangeish. Stars which have "polluted" their own atmospheres with large amounts of carbon, so called carbon stars, may block so much of their own shorter wavelengths that the stars in fact look red. I have seen only one such star, V Aquilae.

Stars which are much hotter than the Sun can indeed look bluish, although their color is never saturated, and very few people can detect their color without a telescope. If you have a telescope and look at a hot and relatively unreddened star, however, you can often detect a bluish cast to its hue.

But stars never look green.

The Sun can be described as a green star, since its energy output peaks in the green part of the spectrum. But we don't see sunlight as green. We see it as white. On a mildly overcast day, when we can't see the blue of the sky, daylight is neither yellowish nor greenish. The color of daylight is still white, or a neutral gray.

Freshly fallen snow, which reflects most of the light that hits it equally, never looks green, even though it may reflect more green light than any other color. Snow either looks white to us, or, on a sunny day, yellow-white in the sun and blue in the shadows. (Or else it looks dirty and brownish.) It just never looks green. If you see a spot of green in the snow, it means some grass is peeking through.

Green auroras have an extremely strong peak in the green or yellow-green part of the spectrum, which is why we can easily see their color. (In fact, they may contain only green light.) But it is impossible for us to see with our own eyes that the light of the Sun peaks in the green part of the spectrum, so that sunlight and daylight is "really" green. It never looks that way to us. And no part of the twilight sky looks green, either.

Ann

[Ann]

Speaking of green stars, there is one that is supposedly green, namely Zubeneschamali, Beta Librae. Well, I don't believe in the supposed green color of that star at all. Beta Librae is a normal hot star, a main sequence star of spectral class B8V, with a temperature of about 12,000 Kelvin, double that of the Sun. The measured B-V index of Zubeneschamali is -0.071 ± 0.004, totally unremarkable for a star of spectral class B8V.

I believe that if a large number of observers were shown Beta Librae through a telescope and were asked to judge the color of the star without being told what star they were looking at and without being asked to look for anything unusual, chances are that not a single person would report seeing any hint of green in the color of this star. Of course, this experiment should take place at a time and a place where Beta Librae was moderately high in the sky, so as to avoid unnecessary reddening.

I googled "Beta Librae" and got this star. It doesn't look green to me.

Ann

[Flase]

Colour is a funny thing. Painters know of a thing called "atmospheric perpective", where the distant objects appear bluer when filtered through the atmosphere. For distant green hills, you need to add a lot of blue. Even stars high in the sky would have some amount of this...

What intrigues me about colour is how it might relate to acoustics, which I know a little bit more about. The way colours mix is different if it is additive or subtractive, but red and blue always make purple. If you look at a colour wheel with the rainbow on it, the indigo and violet are next to the red and the circle begins again. This is analogous to the piano keyboard where, if you double the frequency of the note, you get the octave above and the pattern repeats itself. That colour wheel also seems to repeat itself at about twice the frequency.

Many musicians have given musical pitches a colour, but not many have looked in the opposite direction. For example, in nature, a vocal cord, a twig, or any vibrating body has a sound enriched by the "harmonic series" which in its purest form is a theoretically infinite number of frequencies at the simplest of ratios.

The 1st harmonic is the frequency of the heard pitch (the standard pitch for a violin A string is 440Hz)
the 2nd harmonic is twice the frequency (an octave A), at half the amplitude
The 3rd harmonic is three times the frequency (another fifth, an E), at a third the amplitude
The 4th is two octaves
The 5th is two octaves and a third (a C#)
etc...

Do luminous bodies exhibit similar behavour?
If you create a colour image at twice or half the frequencies of normal RGB filters (UV or near IR), would we see magic?

[Chris Peterson]

Chris, I don't doubt for a moment that various evolutionary processes have led to the color vision that humans have today. I still believe that our three-color vision, where green is a primary color, has had an important survival value for humanity.

I don't doubt it. But evolution never optimizes a "design", it just takes advantage of what is already present, and pushes things in a certain direction. Our visual response is not ideal for survival in the different environments we evolved in. Our color vision is poor- it would be much better if we didn't have two green primaries and one low response blue one! If that system were designed, it would have to be considered a poor design.

Consider. Why is it that we never see stars as green?

I would say it is because their output is defined by blackbody spectra, and there is no blackbody spectrum which we can see as green.

This is different than sky color. It would only require minor changes to the atmosphere to make the sky green instead of blue.

[Ann]

Chris, I don't doubt for a moment that various evolutionary processes have led to the color vision that humans have today. I still believe that our three-color vision, where green is a primary color, has had an important survival value for humanity.

I don't doubt it. But evolution never optimizes a "design", it just takes advantage of what is already present, and pushes things in a certain direction. Our visual response is not ideal for survival in the different environments we evolved in. Our color vision is poor- it would be much better if we didn't have two green primaries and one low response blue one! If that system were designed, it would have to be considered a poor design.

Consider. Why is it that we never see stars as green?

I would say it is because their output is defined by blackbody spectra, and there is no blackbody spectrum which we can see as green.

This is different than sky color. It would only require minor changes to the atmosphere to make the sky green instead of blue.

Chris, I'm most definitely not suggesting that our color vision works the way it does because it was designed to be the way it is.

I'm interested in what you said about the atmosphere. You said it would require only minor changes to the atmosphere to make it look green. Can you give examples of changes that would actually make the atmosphere look green to our eyes?

Ann

[Flase]

I believe the reason we don't really see green in a sunset is that the spectrum is too blurred. If you spread it out further, you might see more in-between colours.

Maybe if our eyes could see near infrared, we might not see so much red either (our cellphones can see near IR, which isn't usually a good thing).

It begs a question I've been trying to look up: what frequencies are filtered by the atmosphere?
I know UV and gamma rays are filtered out, or our skin would all have to be a lot blacker. Short wave radio is reflected by the stratosphere. What about visible wavelengths?

[hotdigittydawg]

Wow! I wasn't close at all, was I? I have a few more thoughts, though somewhat contradictory. I've seen the sky looking greenish before a tornado (very spooky), and of course someone mentioned green auroras (which I've never seen in person). But then what about chlorophyll? My understanding is it looks green because it reflects that part of the spectrum rather than absorbs it. Since the sun peaks in the green (which I've just learned here today), why do you suppose plants reflect what I assume would be the highest portion of energy available from the (green) sun away from themselves? I'd think if chlorophyll was black it would be able to absorb considerably more (free) energy from the sun (not that I'm advocating a "designed" ecology). But then maybe that's the problem - there is such a thing as too much!

[Chris Peterson]

But then what about chlorophyll? My understanding is it looks green because it reflects that part of the spectrum rather than absorbs it. Since the sun peaks in the green (which I've just learned here today), why do you suppose plants reflect what I assume would be the highest portion of energy available from the (green) sun away from themselves? I'd think if chlorophyll was black it would be able to absorb considerably more (free) energy from the sun (not that I'm advocating a "designed" ecology).

Again, the reason is because the system wasn't designed. Photosynthesis is possible because there was some chemical already around that could be hijacked by evolutionary processes to function as a pigment for energy extraction from light. The fact that chlorophyll has little variation in structure across species argues for photosynthesis having only evolved once, and the complexity of the process largely locks things down.

As previously noted, evolution doesn't drive organisms to optimal designs, only to functional ones.

[Chris Peterson]

Chris, I'm most definitely not suggesting that our color vision works the way it does because it was designed to be the way it is.

I know... and I wasn't suggesting otherwise. I was just using "designed" in the sense that unlike designed systems, those which evolve are always suboptimal. There's no progression towards optimization, so we end up with odd results that seem counterintuitive at times (like having two of our three primaries practically on top of each other in the green part of the spectrum).

I'm interested in what you said about the atmosphere. You said it would require only minor changes to the atmosphere to make it look green. Can you give examples of changes that would actually make the atmosphere look green to our eyes?

For our atmosphere, it would require having some larger molecules than O₂, O₃, and N₂ in the upper atmosphere. Something like chlorine or fluorine species would probably do, and could exist in low enough concentrations that they wouldn't make the atmosphere toxic. But really, I wasn't thinking so much of our atmosphere as of planetary atmospheres in general. A planet with an atmosphere similar to our own, but circling a K star rather than a G star should have a green sky- drop the temperature of the star by 1000 or 1500 K and you significantly reduce the ratio of blue to green spectral components. Green should be the dominant scattered "color" in the atmosphere in that case.

My real point here being that we can never see a star as green, under any circumstances (outside something like a filtering nebula around it). We can readily construct atmospheres, however, that allow for a green sky (and probably many other colors, as well).

[NoelC]

It's a beautiful shot, Luis. Thanks for sharing it with us.





-Noel

[Sam]

I have an observation about the Belt of Venus I'd like to ask about.

Since I first noticed this, I have observed it every time I've clearly observed the belt of Venus (and had the time to stick around). Namely, once the height of the belt directly opposite the sun rises to about 20° and begins to noticeably fade, a similarly pink-colored (though with more brilliant orange in place of brown/purple) patch then begins to appear on the opposite side of the sky, about 30° above the sun's azimuth. It seems to brighten as the belt of Venus fades, and also sets a little bit, 5°-10° before itself fading away into the general blue of the twilight sky. What I think is interesting is that the sunward pink-orange patch begins to become noticeable at the same time that the thick of the belt of Venus starts to fade.

My idea is that the bright patch in the west is the other side of the belt of Venus in the east; that is, people half a time zone to the west are viewing this orange-pink glow as the pink-purple belt of Venus to the east. Is this plausible?

Thanks,
--
Sam