Why the Sun appears yellow

[robin]

I finally got the Book yesterday (I know, slap my wrists for taking so long). I read the section where Phil says we still don't know why the Sun appears yellow.

I looked up at the ceiling to think about it, and I noticed that the light bulb looked yellow. It was giving white light, because it made white things look white. But the bulb itself appeared yellow.

I then looked away, and saw why: there was a beautiful yellow after-image on my retina, that then faded through several different colours before disappearing.

Could this be why the Sun appears yellow? Because white lights produce yellow after-images on our retinas? And, with a light as bright as the Sun, the after-image would form more or less immediately. I've tried to observe what happens when I glance at the Sun (indirectly, of course), and I think it see it as white for an instant before it turns yellow; but, as Phil points out, it's hard to be objective about these things.

Do you think there's anything in this? Or have I had an "Against the mainstream" moment?

[Glom]

I thought the same thing.

[Eroica]

Could this be why the Sun appears yellow? Because white lights produce yellow after-images on our retinas?

That's the best explanation I've heard so far.

[browndwarf]

I've always thought that our sun put out a little more juice in the "yellow" wavelenghts.
Isn't this the case?

I've never measured this myself, I've just swallowed what Sagan told me ops:

[Eroica]

I've always thought that our sun put out a little more juice in the "yellow" wavelenghts.
Isn't this the case?

You've forgotten what Phil wrote on page 46 of Bad Astronomy:

"of all the colours of the rainbow, which color does the Sun produce the most? ... The answer is green. Surprise! So why doesn't the Sun look green?"

[browndwarf]

Eroica.

Ye, I wonder where Sagan got that from then?

[fusion]

hello.
I must admit I've not read the book, but isn't the reason the sun appears yellow just a result of the Rayleigh scattering of the blue light...? Or have i completely missed the point here?

[Eroica]

but isn't the reason the sun appears yellow just a result of the Rayleigh scattering of the blue light...?

It's not as simple as that. In the book, Phil says that the amount of blue light scattered out of the white sunlight isn't nearly enough to make the Sun appear yellow. His final verdict is that we just don't know why the Sun is perceived as yellow.

By the way, welcome to the board.

[Visitor]

You forget that the eye does not detect all wavelengths with the same accuracy. This is the only link in english i have at hand, sorry. Here on this site you can find a diagramm of the cone cells' absorbance against wavelenght. As you can see, the absorbance is relatively low in the green range, but it's quite high in the "yellow vicinity". Maybe this can account as an explanation.

[Eroica]

Good point, Visitor, and thanks for the link. But your argument has a fatal flaw in it. Clouds shine by reflecting sunlight, and they reflect all wavelengths equally. But clouds look pure white! If the absorbency of the cones was what makes us perceive the Sun as yellow, we should also perceive clouds as yellow - not to mention the Moon. Phil makes the same point in Bad Astronomy (page 45)

[Visitor]

I found an article loosely related to that topic in : Weischet, W.: Einführung in die Allgemeine Klimatologie. G.B. Teubner, Stuttgart (1979) (german language, sorry, I don't have anything in english on this topic). There a diagram of sun's spectral energy distribution is given which shows that towards longer wavelenghts (from the "green" maximum) more energy is emitted than towards shorter ("bluer") wavelenghts. This can, together with the construction of our optical sensors, explain the yellow appearence. On the other hand, the EM absorption of water molecules is stronger at longer wavelenghts, so maybe the two effects just even themselves to produce the clouds' white appearance. I don't know about the absorbance/reflection properties of moon dust, Mr. Plait should have better grasp to hard data on that than me. Well, if the BA doesn't have a satisfying explanation, why should I come up with the ultimate answer?

[Eroica]

I'm still not convinced. The absorbance of water is not really relevant to clouds. My principal source on this matter is the excellent book Color and Light in Nature, by David K Lynch and William Livingston. They explain why clouds are white as follows: "because practically no light is absorbed and all colors are scattered equally." They go on to remark that while water has a faint bluish colour, this hue plays no role in a cloud because the light travels only small distances through water droplets when it passes through a cloud.

Of course, I'm no more an expert in these matters than you are.

[Visitor]

The absorbance of water is not really relevant to clouds.

Water vapour absorbs light above our visibility range (UV) and from about 480 nm wavlength into the infrared part of the spectrum. That could comply with my suggested explanation. As usual, i have nothing written in english at hand, you might ask a physical chemist in your vicinity about that. As a cloud's reflection rate is quite high, it likely can have a significant absorption rate. The absorption is not complete, as a cloud does not consist of a layer of pure water vapor (in which case it would look blue), but of about 1 to 5 vol.% water vapor (about 100% relative humidity), plus the water drops. I'm not an expert for clouds, the athmosphere or the like, but from my spectroscopy courses (I dropped out halfway from a study of chemistry) I remember that water absorbs quite effectively even if present only in a small amount.

[Eroica]

I'm no expert in these matters either, but Lynch and Livingston are, and they say that clouds absorb practically no light at all.

Before we can account for why the Sun appears yellow to us, two "facts" have to be accepted:

1: When the Sun is high in the sky, it is white.
2: When the Sun is high in the sky, humans perceive it as yellow.

From these premises, two things follow:

1: The Sun's yellow appearance must have something to do with human perception or psychology (since the Sun isn't actually yellow)

2: The Sun's yellow appearance must also have something to do with the extreme brightness of the Sun (otherwise clouds, pieces of white paper, and other things which reflect all wavelengths of sunlight equally would look yellow).

So far, the best explanation I have come across that takes both of these into account is robin's observation at the top of this thread: namely, when you look at a bright white light for a few seconds, it leaves a yellow after-image on your retina. I've tried this for myself and it's true. Look at the Sun, and for several seconds afterwards you will have a bright yellow blob in front of your eyes.

[Visitor]

I won't discuss against two supreme experts, but I'd like to say that i think it's a not-so-good idea to look directly into the sun when it's high up in the sky. Just believe me and don't do it again!
Maybe I should get that book, and somewhen I might come up here with a few thoughts about it.
BTW, IIRC the reason for the green afterimage is that when all types of cones are "overloaded", our brain interpretes the resulting equal "firing" of the nerve cells as green image. Remember, we actually have no sensors for the green light, this colour is interpolated from the values for blue and orange.

[Eroica]

Thanks for the tip. Actually Mr Plait addresses the question of looking directly at the Sun with the naked eye in his book, Bad Astronomy. He discovered that "There is copious evidence that little or no long-term damage results from observing the uneclipsed Sun [with the naked eye]." But you're right: there's no point in taking chances. Whenever I observe the Sun, I always use AstroSolar filters.

Keep posting. You've got some great ideas.

[George]

It's a little funny we can explain an "Einstein iron cross" as four views of a galaxy 8 billion light years away but the color of a star 8 light minutes away ...well that's another matter.

There was an interesting article on this in Astronomy earlier this year. The author, as I recall, claimed the sun was greenish (outside our atmosphere). If you want, I'll dig it up.

The atmosphere must play a crticial role in the color we perceive as it is white at noon, then yellow, orange and redish.

The sun appears to me to peak around 450 nm - Blue. Here, I hope, is the Sun's light curve >>> Graph <<<.

Others say it is more the sumation of all the wavelengths that gives the final color appearance. This, of course, is tied to our eye's reception. BTW thanks for that link Visitor.

Since the Sun is so bright in space, it is not seen as anything other than white as far as I know. Color cones are overmodulated with the Suns light. (Not the best word to describe it but I just like saying it.)

There ought to be some way to take solar data, lower the intesity so the color cones are not flooded (overmodulated) and "see" how it looks. Or how about a distant satelite near Jupiter taking a quick look in the visual spectrum....oops too late. But, maybe Galileo did. Anyone know?

[Eroica]

The atmosphere must play a crticial role in the color we perceive as it is white at noon, then yellow, orange and redish.

The sun appears to me to peak around 450 nm - Blue. Here, I hope, is the Sun's light curve >>> Graph <<<

Thanks for the graph. I found a similar one in a book and it too puts the Sun's lambda-max at about 450 nm, which is really annoying because the BA says the Sun's light curve peaks in the green (500-550 nm). Grrrhh!

Some people avoid the problem altogether by claiming that when the Sun is high in the sky it looks white, as it ought to. But if that's the case, why is it that even children automatically colour the Sun yellow in their pictures?

I think Visitor's ideas are along the right lines. It must have something to do with the way our cones process photons.

[fusion]

I've had time to think about this further since my last post. The Sun certainly peaks in the blue-green area of the spectrum. With the peak around 480nm "Solar Irradiance Data" <http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html>
At the surface it can be seen that the irradiance is approximately equal for each color. The idea that it's green is most likely based on the theoretical blackbody curve for a star of a certain temperature (I'm not sure about this.) Although it may only be coincidental Red + Green light gives you yellow, although you've said that there isn't enough blue light scattered from the path, i don't know about this, i'll certainly ask my Optics Professor. It must be remembered the "yellow" of the sun really isn't that "yellow." i've projected the disk onto paper and its not /that/ apparent. It would seem most likely at this time that it is a physiological effect. As for the children painting the sun yellow? What is this supposed to mean? Kids look at the sun, or they're born with the idea of a yellow sun?!

[kucharek]

I'd simply say, the sun is yellow for us because we can only look at it when the sun is pretty low and then yellow or yellow-red.
And when the sun is low, it also often shines a nice yellow light over the landscape, especially when you've low clouds and just a gap at the horizon.

[George]

This might help >>> eye response <<<

(see lower graph)

The eye's color cones really like green and yellow light but not blue. Even the red reception is greater than blue. Therefore, the comment of Fusion regarding red and green adding to yellow may play a critical role in the yellow look. That plus the strong yellow irradiance and some blue Rayliegh scattering makes some sense in why the sun looks yellow.

A graph that would plot reduced irradiance proportional to the eye's receptivness would be interesting to see.

[George]

Shucks! Here is another >>> eye response 2 <<<.

It appears very contradictory as the blue response is twice that of the other two. Hmmmmmmm. (It is the 1931 standard which might be a factor).

[Visitor]

Thanks for the link, fusion.
Some of the blue light "scattered out" in the athmosphere comes as indirect light. This maybe has to be considered when explaining the white clouds. The more-green-to-red light directly from the sun and the more-green-to-blue light that is scattered around in the athmosphere might be mixed-up to some extent. I insist on that the clouds do not reflect all wavelenghts equally, light reflection on Ice crystalls or water drops must have some absorption as side-effect, and that absorption will be more in the "red" range than in the "blue" range of the reflected light. The expression given in the book Eroica mentioned (Color and Light in Nature) is at least unprecise.

[George]

Yellow appears to have a distinct advantage as a color. The "green" and "red" color cones overlap greatly and the red is only somewhat more receptive in the red. Apparently, not only will yellow light register as yellow but a source with green and red light will also register yellowish. The sun, of course, does this. More info >>> here <<<.

I would like to see more info on how much of the blue light is taken out by our atmosphere. As I look out my window there is a lot of blue light from the sky here in Texas. Therefore, the sun should be more yellow in Texas as our skies are bigger down here (removing more blue)....(ok, lighten-up [oops, worse - a pun])

[Visitor]

On a german site discussing the "catastrophic climate change" i found this graph.
It gives the intensity of the sunlight with the sun at 35° over the horizon.
The red (upper) graph gives the intensity of the sunlight above the athmosphere and the orange (middle) graph gives sunlight intensity on the ground resulting from directly incoming sunlight. The blue (lowest) graph shows the intensity of the "scattered around" light that comes in at the surface. The area with the green headline is the visible range of EM waves.
BTW: One should expect the sunlight to lose less blue intesity over texas than over places that are at greater latitudes, because the sunlight comes in at a greater angle than at my location, for example. This means that the light passes a smaller distance through the athmosphere to reach texas than to reach germany, so there is less scattering.

[George]

BTW: One should expect the sunlight to lose less blue intesity over texas than over places that are at greater latitudes, because the sunlight comes in at a greater angle than at my location, for example. This means that the light passes a smaller distance through the athmosphere to reach texas than to reach germany, so there is less scattering.

Logical. However, don't forget to factor in all our dust!

Thanks for that graph, Visitor.

[George]

BTW: One should expect the sunlight to lose less blue intesity over texas than over places that are at greater latitudes, because the sunlight comes in at a greater angle than at my location, for example. This means that the light passes a smaller distance through the athmosphere to reach texas than to reach germany, so there is less scattering.

Actually, it's not that simple even though the logic is there. There are other scatterin effects which come into play that work against us seeing blue. For instance, is your blue sky more rich on the coast or at the top of the Alps even though there is more air to scatter light at the coast?

The dust works against us in Texas (more so than most) due to Mie scatterin. Rayleigh Scatterin is due to particles smaller than the wavelength of the light and it favors shorter wavelength by the fourth power of the wavelength. The result of Mie scatterin is whiter sky and it works off larger particles (fine dust). Another example of this, and without the mountain climb, is to notice the color of smoke from a cigarette (blueish- Rayleigh scatterin) vs. the color of the same smoke exhaled (whiteish - Mie scatterin). Exhaled smoke particles are larger than smoke directly off tobacco.

I suppose if the dust exists far away and only in the direction of the Sun we would have bluer skies overhead as the Mie Scatterin would transfer more light elsewhere generating more Rayleigh Scatterin.

[Why the Sun looks yellow has not been nailed down yet here in this thread. Scattering effects obviously play a roll.]

So how do I brag on Texas....hmmm....ok...since our view of the blue sky is about 180 deg. the total amount of blue light viewable is greater than anywhere else. If not quality than quantity! =D>

I hope the wiser ones (not "wise-crackers" like me) will jump in here as I want to learn more. My hope is to determine if accretion disks can be seen as colorful once a star starts fusion (assuming enough aperature would gather enough light to excite the color cones).

[Snydley]

But if that's the case, why is it that even children automatically colour the Sun yellow in their pictures?

Maybe the same reason they color aliens green?

[Eroica]

=D> Ho! Ho!

But they've never seen aliens before, whereas they have seen the Sun!

[Swift]

Ok, I'll take this interesting discussion off in a slightly different direction.

I've always wondered why the ambient light in winter is generally more "white" than other times of the year, when the light is more "yellow-white"?

Any ideas?