When an astronomical image is taken in multiple wavelengths, the usual convention is that the shortest wavelength is rendered in blue, the next shortest in green and the longest in red. I’m not sure what they do for 4-wavelength images, but I expect the second longest would be in yellow. In practice, this may mean that an actual red or yellow is rendered blue and two IR wavelengths are in green and red.
As for artificial light, the temperature of an incandescent light filament is somewhere around 3000 K (give or take a few hundred depending on wattage), so those are definitely yellower than the Sun. Fluorescent tubes are usually rated by their equivalent blackbody temp on the package. They range from about 3500 K to 5500 K or so.
As I stated there, I strongly suspect the reason that we perceive the sun as yellow is that, as the sun descends and atmospheric scattering starts to shift its color towards the red you see when the sun is at the horizon, the sun begins to color yellow and stays that way for most of its descent. Sometimes it never does go to orange or red at the horizon, but stays at yellow all the way down, evidently depending on the amount of scattering material in the atmosphere (although the air molecules themselves will cause the 1/(wavelength) ^4 scattering).
I’ve done an extensive search on this, and haven’t found any definitive studies on the topic. The closest I found is a brief treatment by Phil Plaitt, The Bad Astronomer, in his first book. In this, he frankly says that he doesn’t know.
I seriously doubt that the sun is viewed as yellow “by contrast with” the blue sky. There is a connection, because the atmospheric scatter does extract blue from the white sunlight, leaving yellow, but it’s not the contrast with blue – if you allow sunlight directly into a room near sunset and let it fall on a white surface, the light appears yellow, even though there’s no blue sky around it. It’s not simply a “convention”, either, since “yellow sun” is a pretty universal given all over the world.
I wrote an article about this, by the way, for Optics and Photonics News last year. It’s online. I’d link to it, but you have to be an OSA member to open it.
During most of the day the scattering is insufficient to change the color of the sun – you can’t explain the apparent yellowness of the sun by appealing directly to Rayleigh scatter. Besides, as Plaitt points out, if the sun really had so much light scattered out that it appeared yellow, then the clouds would be yellow, as well.
The whiteness of clouds also argues against the idea that “the sun is yellow by contrast with the blue sky”. If contrast with the blue sky made white things appear yellow, then clouds would be yellow, too. They aren’t.
Sure, but some conventions are universal. What percentage of a head does the face cover? What color is fire? What color are tree trunks? Most of humanity gets these wrong, too.
Falcotron, you’re actually looking at it from a more sophisticated point of view than I realized; sorry for that.
There’s a big difference between an ideal blackbody and a real blackbody. (I mean where “big” is from the point of view of a quantitative scientist.) We can measure how the Sun deviates from Wein’s law, and we find out that it has more yellow-red and violet-blue light, and slightly less green light than an ideal blackbody would.
If you’re asking what processes cause the light to deviate from blackbody in the way that they do, I can easily explain the absorption lines, but, like all the other astronomers you cite, I can only give you my best guess as to why the spectrum looks like it does, not a definitive scientific conclusion.
If you want my best guess, it’d have to do with the fact that the Sunlight we see comes from ultra-high energy photons created at the core of the Sun needing to reach the surface, and taking anywhere from about 10,000 to 170,000 years to do it, and the respective number of collisions with atoms that result from that. The ones that take less time, obviously, will have (on average) higher energies, and the ones that take longer will have lower energies, and my hunch is that produces the “real” blackbody we see (more blue, more red, less green) over the ideal blackbody that a uniform-temperature radiator will produce. (And if you think the Sun really is a uniform temperature, you should check out a sunspot.)
And – although I hope this answers your question as best as I’m able – I’m surprised nobody linked to the image of the Planckian Locus in chromaticity space: File:PlanckianLocus.png - Wikipedia
Where is the Sun, a real blackbody, relative to the Planckian Locus (for ideal blackbodies)? I’d be curious to know, too!
Anyway, I now understand that (a) it’s not just a shifted blackbody spectrum, it’s completely off, and (b) the reason sites like Phil Plaitt’s just say things like “complicated processes” has nothing to do with not wanting to bother explaining it to laymen, but rather nobody knowing the answer.
But your hunch is interesting. I was thinking that it must have something to do with all that solar atmosphere above the surface of last scattering that defines the blackbody within the sun, but you’re saying it’s not even close to a blackbody at that layer, because of different travel times from the interior.
That’s something we should be able to test just by watching the sun carefully for 170000 years as it starts to turn red in a few billion years, right? I was kind of hoping for an answer sooner than that, but I’m patient.
Damn. Everyone had me so convinced that it was either scattering, or scattering plus a bit of contrast.
The clouds definitely blow the contrast theory out of the water. But I’m not 100% convinced about scattering.
Clouds don’t just reflect the yellow direct sunlight; they also reflect the blue scattered sunlight from the sky, right? Maybe there’s too little of this to make a difference (how much of the ambient light is direct, and how much scattered, anyway?), but I’d guess that (especially if you’re looking at clouds from underneath, as we usually are) there could be enough to have some contribution.
Also, don’t clouds do some scattering themselves? I vaguely remember something about water droplets being so close to the wavelength of visible light that Rayleigh scattering doesn’t apply, but clouds are chock full of other molecules of the appropriate sizes in suspension, just like air. (In fact, IIRC, sulfates scatter so much better than most other particles in the air that skies are noticeably blur when sulfate levels are high, and sulfates can concentrate in clouds.)
Anyway, I’m off to go read the other thread you linked to before I toss out even more ideas that have already been shot down.
Except that clouds are larger, in angular size, than the Sun. No point on the surface of the Sun is ever more than a quarter of a degree from sky, or maybe two or three degrees if you don’t count the glare surrounding the disk itself, so comparison with the surrounding blue is inevitable. A cloud can easily be ten or twenty, or 180, degrees across, though, so there’s much more cloud which isn’t directly adjacent to blue.
First, the discussion between you and billfish678 apparently got derailed once he got angry and a bit rude and you stopped answering him, which is a pity. I’ll try to avoid making the same mistake.
Second, your suggestion is that the sun and sunlight really are the same color all the time–but we only look at the sun when it’s more yellow, while we see ambient sunlight all day long, including when it’s white.
But normally, our concepts aren’t based on averaging out perceptions like that. If we think the sun is yellower than the sunlit day, that’s most likely to be based on perceiving a contrast between the two of them at the same time. And, in fact, even in late afternoon, when we can look at the sun and see it looking yellow, the sunlit day still doesn’t look yellow to us.
Finally, billfish brought up basically the same cloud explanation as me, and you replied:
I just tested this. Here’s the setting: My living room has 4 large windows, and it’s pretty bright. The sun is shining directly through 1 of those windows. I have no interior lights on except the MacBook I’m typing on. And I have two white bedsheets that just came out of the laundry. So, I hung one in front of the window with the sun shining in, and one in front of the window on the far end of the same wall. And I opened both windows.
It’s too hard to tell whether one sheet is more yellowish than the other (although one is very clearly brighter). But what’s very easy to tell is that, in the window the sun is shining it, the brighter part near the center of the sheet is also yellower than the rest.
This seems to show that:
My contrast idea is completely and utterly wrong, just as you argued.
Direct sunlight really is yellower than ambient sunlight. (Unless my sheets are fluorescing yellow or something, I can’t see what other explanation there could be.)
The “clouds are white because they reflect both yellow sunlight and blue sky” idea that both I and billfish suggested isn’t proven, but it’s still plausible.
I think billfish’s digital camera experiment also sounds pretty interesting, but he left out something important: How did he take a picture of “direct sunlight reflected onto a white piece of paper (but not exposed to the blue of the sky)”? I can imagine ways to do it, but it would have been nice to know how he actually did it.
Anyway, I think the key question is, how much of the light that reaches us is scattered blue sky light? If it really is 10% or more (as billfish claims), that’s more than enough to explain why clouds are white. (The sky is a lot bluer than the sun is yellow, so it wouldn’t have to be anywhere near 50/50.)
2.) The same argument works for the edges of white buildings. Or white street signs.
3.) take a mirror and reflect a portion of a white light or something toward your eye against the blue sky. It won’t look yellow.
Much less than scattered out. You can’t claim that light gets scattered out, then perfectly scattered back in to perfectly correct the loss. In that case nothing would ever get colored by scattering.
Part if it may be that most of the times when we can look at the Sun without it being too painful is during dusk and dawn, when the light has to pass through more of the atmosphere. So our mental imageof the sun is more red/yellow than it actually is.
Sure, but it doesn’t have to be anywhere near perfect.
Since I don’t know the numbers, I’m going to pull some out of my ass.
Let’s say that 33% of the blue light from the sun is lost to scattering. But 33% of that scattered light reaches us, and therefore becomes part of the ambient lighting.
So, the direct image of the sun is missing 33% of its blue, but ambient sunlight is missing only 22% of its blue. Since ambient sunlight is what we define as white, and direct sunlight has less blue, the sun is yellow.
And likewise, if 33% of that scattered light is reflected off clouds, they end up the exact same color as ambient sunlight, so they’re white.
Now, if you tell me that the numbers are more like 33% and 0.001% instead of 33% and 33%, that might be a different story. But some decent amount of that scattered light must reach us, or wouldn’t the sky would look black, not blue?
It’s been a while since I’ve done it, but I’m pretty certain the loss is a lot less than 33%, and the sun viewed directly has (for practical purposes) no blue scattered out. That’s why people like Phil Plaitt in his book say that sunlight isn’t yellow because of light scattered out of it.
I’m not sure what you mean by “ambient sunlight” and “direct sunlight”, but the clouds scatter essentially direct sunlight. If there was enough scatter from the atmosphere to make direct sunlight appear yellow, then that’s the light that would hit the clouds and scatter. And, as I say, that would make the clouds appear yellow, not white.
You know, my father gave the exact same answer. Since I’m as much a child of the 80s as he is a child of the 60s, I had no choice but to reply, “I learned it from you, dad! I learned it from watching you!” It took him a moment to get the sun/son reference, after which he said, “I see what you did. Bread and Circuses.”
Sorry; I don’t know the right terms. By “ambient sunlight” I mean all of the light reaching our eyes on a sunny day (directly, scattered through the sky, reflected off surfaces, and anything else I’m forgetting). By “direct sunlight” I mean just the light that reaches our eyes more-or-less straight from the sun.
So you’re saying the clouds neither reflect any significant amount of light from below, nor re-scatter any light that’s already been scattered by the air?
And meanwhile, this is probably an irrelevant side question (but I’m not sure): Why doesn’t the clouds’ scattering change the color of the light that reaches us from them? Do they scatter all wavelengths equally, or happen to preferentially scatter exactly the right wavelengths? Is this related to that bit about water droplets being too close in size to visible wavelengths for Rayleigh scattering to work?
Ok, so does anyone actually have a regular old color photograph of the sun from space? Or are scientist always using xrays and radio waves to look at it?
