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?

I thought the same thing.

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Freedom For Fission A breath of fresh Iodine-131

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

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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:

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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?"

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Eroica.

Ye, I wonder where Sagan got that from then?

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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?

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.

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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.

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)

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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?

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.

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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.

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.

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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.

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.

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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?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

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.

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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?!

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.

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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.

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