Why Does Our Sun Appear Yellow Instead of White?

[Drakkith]

If we're trying to be scientific about this, we have to be precise and have a possible measurement system - not just to say what it 'looks like'.

I thought we were being pretty precise when we said the Sun radiates like a blackbody at about 5800 kelvin and looking at the Sun through a small portion of the atmosphere overwhelms your color receptors, making the Sun look about as white as you can get even though some of the light is scattered out.

 

[sophiecentaur]

I thought we were being pretty precise when we said the Sun radiates like a blackbody at about 5800 kelvin and looking at the Sun through a small portion of the atmosphere overwhelms your color receptors, making the Sun look about as white as you can get even though some of the light is scattered out.

But that doesn't correspond to D65 (there are a number of other standards), which is the 'white' for an illuminant. That white hot disc up in the sky is not the only thing that illuminates things on the ground. If you took an equivalent filament lamp, producing D65 illumination, it would not 'look' the same colour as the Sun up in the sky (assuming you used a perfect ND filter to view them through).
I think you would agree that overloaded retinal receptors are not appropriate instruments for any assessment of colour.
So:
Sun + Blue sky = D65 standard filament lamp (when reflected by a totally reflecting matt surface)
Therefore
Sun = D65 lamp - blue sky
Can they both be your 'perfect white' then?.

 

[voko]

So:
Sun + Blue sky = D65 standard filament lamp (when reflected by a totally reflecting matt surface)
Therefore
Sun = D65 lamp - blue sky

This is unobservable from the surface of the Earth, while I think the subject question was in the terrestrial setup.

 

[Drakkith]

Can they both be your 'perfect white' then?.

I don't know and I don't care. Per the OP's original question, the Sun is white, it looks about as close to white as you can get when high in the sky, turning to yellow-orange as it approaches the horizon thanks to scattering.

 

[sophiecentaur]

This is unobservable from the surface of the Earth, while I think the subject question was in the terrestrial setup.

?
I am describing exactly what you see from Earth here. You 'see' the familiar old Sun ( through the atmosphere of course) and you see the blue sky up there. Are you not following this?

 

[sophiecentaur]

I don't know and I don't care. Per the OP's original question, the Sun is white, it looks about as close to white as you can get when high in the sky, turning to yellow-orange as it approaches the horizon thanks to scattering.

And when was the last time you observed the Sun in the sky, well enough to assess its colour? In most PF discussions, it would be expected that conclusions would be based on measurement or firm observations - not just assertions. What's different about this one?

 

[voko]

?
I am describing exactly what you see from Earth here. You 'see' the familiar old Sun ( through the atmosphere of course) and you see the blue sky up there. Are you not following this?

This: "Sun = D65 lamp - blue sky" is what you do not see. Because you see the Sun through the blue sky.

 

[Drakkith]

Sophie, I still have no idea what you're trying to get at here. If you could boil down your argument to the most basic thing your trying to get across, please do. Because I'm half lost here. We've gone from Sunlight filtering through the atmosphere to measurement systems to referencing something with something else. I have a nagging feeling that we're arguing over nothing here.

 

[sophiecentaur]

This: "Sun = D65 lamp - blue sky" is what you do not see. Because you see the Sun through the blue sky.

You see the Sun - yes. but illuminant D65 is what falls on a matt surface and what you see from that surface is direct Sunlight plus what comes from a hemisphere of blue sky. l'm afraid I can't see what there is not to understand here. There are two separate issues. There is the colour seen when you look at a neutral matt reflector (which is the result of an illuminant) and the colour you see when you look directly at the Sun through a ND filter. D65 in no way refers to observing the Sun directly (we can't do it conveniently). It refers to what matches for a white surface outside in sunlight (including blue sky contribution) and a black body at 6504K (a filament). We don't look at the sun to tell us what's a 'standard' white and neither do we shine sunlight through a narrow tube; we see sunlight, reflected off a white card.
Of course our Sun (when viewed directly) is Whiteish and so are most other stars we can see. But, as the diagrams show, there is a range of temperatures - hence a range of 'whites'. In order to compare stars, viewed from above and below the atmosphere, you need to compensate for the atmospheric effect, when appropriate.

Afaiaa, standard white is only defined in terms of an illuminant because that is the majority application of the notion of 'whiteness' (i.e. in printing, TV and photography). I wish I hadn't started on this now but it really is relevant. Stars are just assigned a Temperature.

 

[voko]

You see the Sun - yes. but illuminant D65 is what falls on a matt surface and what you see from that surface is direct Sunlight plus what comes from a hemisphere of blue sky. l'm afraid I can't see what there is not to understand here.

And I cannot see the relevance of this argument with respect to the question "why is our Sun yellow". This question is about our visual perception of the Sun, not of its light that we observe indirectly.

The Sun ranges from reddish when close to the horizon, to blazing white when at the zenith, especially at places where humidity is low, and so must be yellow somewhere in between. There is nothing to debate about these facts, I would presume.

The question is "why", and I think the answer is "it is the atmosphere".

 

[sophiecentaur]

And I cannot see the relevance of this argument with respect to the question "why is our Sun yellow". This question is about our visual perception of the Sun, not of its light that we observe indirectly.

On a very subjective, arm waving, level, of course the Sun is white and so are a lot of other light emitting objects we see, from reddish, through yellowish, through neutral, right out to blueish. But the only definition of neutral white is in the context of an illuminant (for reasons I have given several times already). This is because no one actually looks at the Sun; it is strictly a minority occupation (do you actually do it for more than a second?). The one rock solid definition of a white, involving the Sun refers to a white card in sunlight. Can you quote any reference that tells us otherwise? Look at the CIE Colour diagram with the positions of the various 'standard' illuminants / white points marked on it. This is not just irrelevant raving on my part. It's of great importance to people whose business is Colour. Things have got a lot better these days but still, if you look at a wall of different TVs in a shop, you will see a whole range of whites across all the sets. If we were not sensitive to white point, we wouldn't notice this spread in performance and colour TV would be easier to engineer.

The sunlight we stand out in is composed of light directly from the Sun and also light from the blue sky. I cannot understand how you seem not understand that. This must lead to the conclusion that the Sun itself is not supplying white light to your white card. Ergo the Sun is supplying light that is yellowish and mixes with blue to produce white, when diffused. Two colours can't be identical if one of them had other wavelengths added to it - can they?

 

[voko]

This is because no one actually looks at the Sun; it is strictly a minority occupation (do you actually do it for more than a second?).

I am sorry, but this sentence is a bunch of contradictory arguments. I believe you should really decide for yourself whether the Sun is or is not looked at and then stand by your position coherently.

The one rock solid definition of a white, involving the Sun refers to a white card in sunlight.

This definition is wholly irrelevant in the context of this question.

If there is no good definition that would be applicable to the question - which would be a pity - there is still no reason to insist on using something irrelevant.

The sunlight we stand out in is composed of light directly from the Sun and also light from the blue sky. I cannot understand how you seem not understand that.

I am not sure why you keep repeating your own misunderstanding. The question is not about why daylight scattered off some surface is white or yellow or something else, but about our direct visual perception of our Sun.

 

[Vanadium 50]

Enough is enough. You've been going around in circles now and it's time to stop it.

This is about perception, not physics, and the sun (at zenith) itself is bright enough to saturate the color receptors, so it appears white. Not yellow. White.

Now, one can ask a whole host of related questions: what color is reflected sunlight, what color would a dimmer sun (or filtered sun) appear, whether these answers would change outside the atmosphere, etc. But these are other questions. You've all picked different ones, and are now arguing about why the answers are different.

This will go nowhere.

 

[sophiecentaur]

I am sorry, but this sentence is a bunch of contradictory arguments. I believe you should really decide for yourself whether the Sun is or is not looked at and then stand by your position coherently.



This definition is wholly irrelevant in the context of this question.

If there is no good definition that would be applicable to the question - which would be a pity - there is still no reason to insist on using something irrelevant.



I am not sure why you keep repeating your own misunderstanding. The question is not about why daylight scattered off some surface is white or yellow or something else, but about our direct visual perception of our Sun.

I think you should read my last post more carefully. It is quite self-consistent and I can only repeat the message that the disc of the Sun never viewed directly for long enough to come to any proper conclusion. Instead, we use Sunlight to define a white illuminant. If you do not know what that means then I suggest you read about some basics of colourimetry. (This thread is about colour, after all)
OK. Then perhaps you could give me just one reference in which the spectrum of the sun's disc, as viewed from the surface of the Earth is used as a Standard White.
If you use the word 'white' then you have to define your term. There many 'whites' - as the Dulux. Crown and Albany colour charts will verify and as the list of standard illuminants also shows. The situation where white is defined in terms of the sensation you get in your eye just before you have to close your eyes or go blind is hardly ideal for defining any phycho-visual measure.

 

[Vanadium 50]

Let me repeat, for those who missed it. Enough is enough.

 

[sophiecentaur]

Enough is enough. You've been going around in circles now and it's time to stop it.

This is about perception, not physics, and the sun (at zenith) itself is bright enough to saturate the color receptors, so it appears white. Not yellow. White.

Now, one can ask a whole host of related questions: what color is reflected sunlight, what color would a dimmer sun (or filtered sun) appear, whether these answers would change outside the atmosphere, etc. But these are other questions. You've all picked different ones, and are now arguing about why the answers are different.

This will go nowhere.

Come on now. You can't claim that there is a valid measure or assessment of colour when the receptors all go into overload. Look what happens when some electronic sensors go into overload and try to tell me that you can get any reasonable measurement under those conditions. Just try looking briefly at the sun and then say honestly whether you saw white. Personally, I get so much flare and discomfort that I would say that, if anything, the disc looks very non-uniform and any colours I could identify are constantly changing - just before I have to look away. You may as well ask someone to identify the musical note that was involved when they are having the sound played at them at a level near the threshold of pain. The pitch would be the last thing they were aware of.
It really isn't justifiable to discuss the subjective effect of any sensation under those sorts of conditions. What is it about 'Colour' that makes so many, normally objective people make the wild statements that they do? PF threads on colour are really bizarre at times.

 

[Vanadium 50]

And that's that.