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robin
2003-Sep-17, 07:31 PM
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
2003-Sep-17, 07:35 PM
I thought the same thing.

Eroica
2003-Sep-25, 03:17 PM
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
2003-Sep-25, 03:27 PM
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 :oops:

Eroica
2003-Sep-25, 03:41 PM
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
2003-Sep-25, 05:53 PM
Eroica.

Ye, I wonder where Sagan got that from then?

fusion
2003-Sep-26, 08:11 AM
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
2003-Sep-26, 04:09 PM
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
2003-Sep-26, 07:10 PM
You forget that the eye does not detect all wavelengths with the same accuracy. This (http://science.howstuffworks.com/eye.htm) is the only link in english i have at hand, sorry. Here (http://science.howstuffworks.com/eye3.htm) 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
2003-Sep-27, 09:13 AM
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
2003-Sep-27, 03:07 PM
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
2003-Sep-27, 04:25 PM
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
2003-Sep-27, 05:54 PM
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
2003-Sep-28, 08:24 AM
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
2003-Sep-28, 09:13 AM
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
2003-Sep-28, 10:07 AM
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
2003-Sep-29, 01:10 AM
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. :D

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 (http://phoenix.physast.uga.edu/~yeti/PAPERS/NextGen/img63.gif)<<<.

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
2003-Sep-29, 08:02 AM
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 (http://phoenix.physast.uga.edu/~yeti/PAPERS/NextGen/img63.gif)<<<

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
2003-Sep-29, 10:01 AM
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
2003-Sep-29, 10:11 AM
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
2003-Sep-29, 03:57 PM
This might help >>> eye response (http://acept.la.asu.edu/PiN/rdg/color/color.shtml)<<<

(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
2003-Sep-29, 08:08 PM
Shucks! Here is another >>> eye response 2 (http://www.creativepro.com/story/feature/13036-2.html) <<<.

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
2003-Sep-29, 09:57 PM
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
2003-Oct-01, 06:10 PM
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 (http://www.colorado.edu/physics/phys1230/phys1230_fa01/topic50.html) <<<.

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
2003-Oct-01, 07:13 PM
On a german site (http://www.angewandte-geologie.geol.uni-erlangen.de/klima1.htm) discussing the "catastrophic climate change" i found this graph (http://www.angewandte-geologie.geol.uni-erlangen.de/klimaf8.jpg).
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
2003-Oct-02, 01:35 PM
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
2003-Oct-02, 09:46 PM
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
2003-Oct-13, 11:00 PM
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
2003-Oct-14, 10:59 AM
=D> :lol: Ho! Ho!

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

Swift
2003-Oct-14, 09:31 PM
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?

informant
2003-Nov-27, 06:52 PM
It never looked yellow to me. I always thought that was just a cultural stereotype. :-?

George
2003-Nov-28, 05:25 PM
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?

One guess might be that there are more atmospheric ice crystals which would cause more Mie Scattering and reflection causing more of a whitish sky.

If the sky is less of a deep blue, it may cause the sun to look less yellow but I don't know if this is even a good guess.

I'm still convinced that the sun is white around noon then, later in the day, yellow then orange and then red. Here is high tech supportive evidence - I saw the sun reflected in my car window late this morning and noticed it was all white looking. 8) I tried to look up at it but it was too bright to see color easily although it seemed white.

Alex W.
2003-Dec-03, 10:41 PM
Easy peasey. The sun emits white light. Without an atmosphere, all this would reach our eyes, and it would look white. But some of the blue photons are scattered away (giving the sky its colour) so the sunlight isn't as blue as it originally was- so it's yellow.

The sky acts as a sort of yellow filter, absorbing the blue from the light and reimmiting it towards us from all different directions.

Glom
2003-Dec-03, 11:44 PM
Doesn't work! If that was the case, then we would be living under a yellow lamp, but in fact, white looks white under sunlight.

George
2003-Dec-04, 12:15 AM
On Earth, the Sun's light spectrum is very flat throughout the visible spectrum, as I recall. This should help it look white as no color is favored. A simple test might be to compare a white sheet of paper under a flourescent lamp and one adjacent with sunlight on it at noon, midday and setting.

However, the Sun's spectrum is not flat outside our atmosphere and it favors green and blue.

I have googled myself half silly trying to find information related to the eye's color cone's limits. The Sun may look white, as astronauts have said, because the flux has saturated the cones. I'd really like to know what the real color of the sun is, assuming I'm right. You would think someone by now would have either seen it through Cassini or another "true color" satellite or duplicated the Sun's real spectrum in a lab (at an unsaturated intensity level). I suppose asking for a satelite view of the sun would be like asking someone to take a sip of water from a fire hydrant. :)

I did find one site which gave the color cone's lower limit which equates to being able to see the color of the sun as far away as about 3500 AU. The rods take over and white is all you get.

Shucks...What Color is The Sun!!! ](*,)

Alex W.
2003-Dec-04, 12:24 AM
Doesn't work! If that was the case, then we would be living under a yellow lamp, but in fact, white looks white under sunlight.

The blue like scattering in from the sides should balance things out, shouldn't it? The light that's hitting everything is white, it's just that when you try to look at the sun directly, it's apparently giving the yellow light and the sky's giving the blue light, which combine to form white.

Eroica
2003-Dec-04, 08:37 AM
The sun emits white light. Without an atmosphere, all this would reach our eyes, and it would look white. But some of the blue photons are scattered away (giving the sky its colour) so the sunlight isn't as blue as it originally was- so it's yellow.
[-X Someone hasn't read the BA's book. He addresses this problem and when you do the math, it turns out that not nearly enough blue light is scattered out of sunlight to make the Sun look yellow.

Also, according to George's site the Sun's real colour is not white: it's pink! (Actually I would call it salmon, or maybe apricot.)

However, that site does concede (near the bottom of the page) that there is a subjective element in colour, so that what's pure white to one person might appear off-white to another.

AstroSmurf
2003-Dec-04, 11:25 AM
Has it been considered that it might simply be an illusion? Put a (near-)white light in a blue sky, and it will look yellow by comparison?

Indirect sunlight seems to be much whiter than the direct kind, though...

I suppose a test would be to snap photos of some objects being lit by lightsources of different temperatures, and compare with a pic using sunlight. Colour is a subjective thing anyway...

Eroica
2003-Dec-04, 12:39 PM
Has it been considered that it might simply be an illusion? Put a (near-)white light in a blue sky, and it will look yellow by comparison?
Yes. here's what the BA says about that in his book:

Another common idea is that the Sun looks yellow because we are comparing it to the blue sky ... However, if this why we see the sun as yellow, clouds would look yellow, too, so this can't be right either.

Glom
2003-Dec-04, 03:30 PM
I think the reason the sun looks yellow is because a filament lamp looks yellow, even though it's white. It burns our eyes and leaves yellow marks behind.

And if any optometrist scum say it's because of colour defects, I'll kill them. Each and everyone of them. Born again Hitlers!

George
2003-Dec-04, 04:11 PM
Someone should do poll as to what the Sun's color appears to be at around noon and midday.

It's 10am here in San Antonio and the sun is white. I noticed reflections off curved chrome and they are also white. I do not see yellow.

I suspect strongly that our color cones are too overloaded to get a handle on yellow until later in the day.

The BA makes a great point, as Eroica pointed out, that the yellow may be an illusion due to the surronding blue. It should also be noted that the sky (overall)is, likely, bluer when the sun is lower in the sky. This is because Mie scattering is much much less overhead which whitens the sky and the sky near the horizon is always whiter (of course, no mountains here). This may contribute to the pyschological effect the BA points out. The bluer the sky, the more yellowish the Sun. Also, the dimmer the sun the, likely, greater ability we have to see more blue sky nearer to it. (ignore my grammar)

George
2003-Dec-04, 05:35 PM
...Here (http://science.howstuffworks.com/eye3.htm) 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.

This is still the best plot I've seen and I wanted to thank you for it.

The y-axis appears to be a true energy plot and not a derivative as in irradiance. The truth is I don't understand irradiance and why it is used. Does this graph mention irradiance? [My grandmother was German but I did not learn the language].

In regards to getting hard data on our eye's color cone limits, I am still here..... ](*,)

Alex W.
2003-Dec-04, 08:32 PM
Y'arr, I knew I should've remembered the book the last time I moved...

George
2003-Dec-04, 11:27 PM
Ok. "Let me explain...No, let me sum up" - Inigo Montoya- Princess Bride.

As a result of all the input from all those wiser above....

S(lambda) = f(CR) + f(**) + f(Alt) + f(L) + f(Refr) + f(FM) + f(Rt)

Terms:
S(lambda) - Strength of the color wavelength perceived by homo sapiens
........This is what we think we see and tend to tell others without thinking.

f(CR) - Crayon experience factor.
......... How many times did we draw the Sun and we never used White as a crayon! (Did they even have white?) [thanks Eroica for the crayon idea - wow] We wouldn't use Blue for the sun because Blue on Blue makes no sense. Green color clashes with Blue (so my wife keeps reminding me) and, besides, the trees get Green. Orange and Red are reserved for warmer sunsets and sunrises not normal sun shine.

f(**) - No, actually it stands for Blue Sky factor
.........The more blue sky, the more Yellow we think we see.

f(Alt) - Altitude of the Sun
......... It will actually look Yellow at a certain altitude.

f(L) - Logic
......... Related to f(alt). If the setting is Red, then Orange, then what's next.....Yellow of course but there is also the logical aspect that if it's not Red or Orange (and not Blue or Green) it has to be the color Yellow. It can't be White as White is not a color - right?

f(Refr) - Refraction in clouds around the Sun
......... Often the clouds around the sun are such that there is a Yellowish, and a little Orangeish fringe around the clouds that augment the idea that the Sun is Yellow.

f(LE) - Lighting Experience
......... Bright lights are often Yellowish. Candles, incandescent bulbs, etc.

f(Rt) - Retina factor
........ The retina leaves a Yellowish after glow (as per Robin)

Thank you Eroica.... =D>
Thank you Robin... =D>
Thank you one and all who have contributed to this monumental task! =D>

Problem solved...next problem please..Oh yeah, I remember....What color is the Sun really (above the atmosphere) ](*,) ](*,)

Alex W.
2003-Dec-05, 01:13 AM
I was going to ask that the other day, actually... all the classy NASA anims are red, but that didn't make any sense to me...

George
2003-Dec-05, 04:06 AM
It is hard to believe NASA may have no true color solar images. You think the sun would make a good color reference out there but maybe the visible flux is too great for the instrumentation. However, the Sun is very stable in the visible portion of the spectrum.

It also seems hard to believe no one has duplicated the Sun's spectrum in a lab or home. Would it really be that hard to duplicate? I would like to know as, shoot, maybe I'll do it myself.

For astronomy to be so advanced and still not be able to say what color our star is, well, it's sad. :cry:

Alex W.
2003-Dec-05, 05:30 AM
Take the spectrum from sunlight, run it through a computer, and get it to provide a picture on a particularly good monitor?

The Bad Astronomer
2003-Dec-05, 06:22 AM
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!


Well, it's blue-green. The peak is around 480 nm as can be seen in that graph linked in the first page of this thread (I looked for a solar spectrum a couple of years ago and found that very graph; in my book I say the Sun peaks in the green). The basic complication here is that the Sun is not only not a blackbody, but it has those complicated absorption features.

Those kinds of things can have funny effects. A cool star is redder than a hot star, right? Not always. As you look at cooler and cooler brown dwarfs, the color at one point stops getting redder and actually gets bluer. There are absorption lines that pop up (or would that be down?) at cooler temperatures that absorb the redder light, leaving the bluer light to get through (and this is all out in the near IR as I recall, so by bluer I mean nearer the "real" red in the optical bands. Still with me?).

Unfortunately, nature rarely gives us something as nice and neat as a laboratory/textbook example of an object.

Glom
2003-Dec-05, 11:48 AM
Well all I can say is it appears your perception of colour isn't as good as you think it is!

George
2003-Dec-05, 04:52 PM
Unfortunately, nature rarely gives us something as nice and neat as a laboratory/textbook example of an object.

Well, one thing is clear - it isn't clear what color the sun really is! :)

The computer modeling based on a blacbody curve done here (http://casa.colorado.edu/~ajsh/colour/Tspectrum.html) is doubtful. "Peach pinkish" they call it. Is it a girl star??? They even end their page with.. "So maybe the Sun is really white?".

The absorption line problem you mention certainly has to be a factor.

However, how hard would it really be to duplicate the (pre-atmosphere) radiation? #-o

I would think multiple lighting with filters for the major aborsption lines would yield a true rendering.

This particular topic I think is important since the Sun impacts all of us. The general public would have to stop and realize that what we see is not what is real. Of course, there are other lessons (i.e. magic tricks, gravity house). But these do not point our minds skyward toward Good Astronomy.

poorleno
2003-Dec-27, 06:51 PM
Here is where the first poster blabbered something semi-coherent about retinas.
;)

No, really i must disagree; if it were an eyesight illusion the photos of sun should look pure white (or green or w00tever) according to your story.

Which they don't. Nah, I instead think it's divine intervention - sort of a way for God to tell us - haha, screw you and your "laws" of physics.
:)

Glom
2003-Dec-27, 07:54 PM
What photos show the sun to not be white? It's always an overexposed blob.

The Supreme Canuck
2003-Dec-27, 08:02 PM
Ones taken through filters.

George
2003-Dec-30, 05:31 PM
What photos show the sun to not be white? It's always an overexposed blob.

I believe this really is the heart of the whole issue. Not only will film show it as white, but so will our eyes/brains as each color cone is fully saturated. If blue or green is dominant over the other colors it will not make any difference at this intensity level. However, I can not find techincal evidence to support this hypothesis.

If this is true, there should be a significant range of distance from the sun where the color cones can see the proper proportion of colors and give us the "true color" of the sun.

I have found some technical data on the low end threshold of the color cones. The cones should stay active for over 3,000 au's. This may not be accurate as there is a range of intensity where the rods and cones both function and might distort the true color.

Of course, I am talking about the sun as it appears before our atmosphere which has significantly (2x) more blue and green intensity.

As a result, it may not be "peachy pink". :)

George
2003-Dec-30, 10:26 PM
Since the thread topic is what we see below the atmosphere, I should state that I believe the prior post applies to this case as well. The sun is too intense for our color cones. [At least, that is what I think. There should be considerable information available to confirm or contradict me but I can't seem to find it.]

However, this is not true at sunrise/sunset where the sunlight must travel through more than twice as many molecules to reach us as compared to when it is at 60 degrees. [Draw two concentric circles and let the outer represent the atmosphere, then, draw lines from the top of the inner circle outward to represent sunlight angles to reveal how low angles travel long in the low atmosphere even though the distance through the entire atmosphere is only about 50% more between a 7 deg. angle and a 60 deg. angle.]

If we assume the sunlight to be at 1/4 the intensity near the horizon and we can see color at this intensity, then maybe, the true color of the sun would reveal itself to our sensitive color cones at around 2 a.u.'s or so. Not so far away. 8)

Therefore, on your next trip to Mars, go out a little further and take a look at the sun for me, you might just discover it's true color. Be sure to take you sunglasses off. 8) :lol:

If anyone can find data, I'd appreciate it.

George
2004-Jan-01, 09:04 PM
The idea that the Sun is white because it's intensity is too great for us to see it's color, can be tested. What if we make a strobe (a slit in a rotating disk) to attenuate the total light received, then, maybe it will look yellow. Naturally, the sky is 100% overcast.

My bet is it will still look white as the spectrum (post atmosphere) suggests.

George
2004-Jan-05, 12:51 AM
No sun yet, only clouds.... :cry:

Glom
2004-Jan-05, 10:48 AM
Tell me about it. It's been either intrument meteorological conditions or borderline IMC for the past week.

ZaphodBeeblebrox
2004-Jan-05, 11:19 AM
Tell me about it. It's been either intrument meteorological conditions or borderline IMC for the past week.

I know, I know.

Next thing you're going to tell us, is The Fog is Pea-Soup Green.

Or, is that only in London? 8-[

George
2004-Jan-05, 03:51 PM
Tell me about it. It's been either intrument meteorological conditions or borderline IMC for the past week.


It's Gethen's fault....

Just a warning to all local star gazers out there. My solar filter came today and it fits the scope perfectly. This of course means that anyone within a hundred mile radius will not see the sun until at least spring. The arrival of the scope itself did the same for the night sky, so I am expecting the worst.

:) :) :(

It's ok. I have now advanced the strobe device to a much greater effective level. The strobe has been painted black and the variable speed shaft has greater support ...(I glued the pencil to the paper plate). :)

George
2004-Jan-07, 01:51 PM
The clouds are now thicker. I'd settle for a silver lining at this point.

The SAD (Sunlight Attenuator Device) is now fully operational. I've called Houston and they have given me a "go". (ok - it was really "go home" but let's think positive).

The initial test was on a 60 watt clear light bulb. The inner markings and filament reflection off the glass were easily seen. Then I turned it on....(just kidding).

I still think the Sun will look white because of the irradiance curve is so flat down here. Maybe NASA will take it and give it to an astronaut to see what he sees. Blue-green still has a fightin chance considering the irradiance curve the Sun radiates.

George
2004-Jan-09, 05:11 AM
I am now in Orlando with full sunshine but no S.A.D.! [I''l forego the pun] I did not bring it as I heard the weather was bad. [Not to mention the concern I have for it getting out cf calibration during the flight.] :)

SAD2 will be complete by tomorrow. The SAD2 strobe disk is an aluminum pie pan and the rotating shaft is a tube from a paper towel roll. The S.A.P. (SAD Activating Person) is prepared to operate.

I hope this does not distract you from your interest in other great events such as our landing on Mars. :)

George
2004-Jan-09, 06:34 PM
Another update.

Today the sun peeked through and I conducted the normal preliminary tests in my lab (bedroom).

FAILURE. The main drive shaft (and only shaft) to the strobe plate de-coupled! Apparently, Elmer's glue is not suitable for aluminum. I had feard this but did not want you t o worry too much.

I am repairing. Should be ready just after sundown, probably.

Eroica
2004-Jan-09, 09:09 PM
The tension is unbearable! ](*,)

George
2004-Jan-10, 03:17 AM
The tension is unbearable! ](*,)

Hey....your name's not George!

Oh, sorry, Eroica. The solitude of a lonely inventor can be stressful. :)



Update....di-dt, di-dit, di-dit...

Rain. :cry:

However. I did test SAD2 (semi-repaired) on the hotel lamp. I saw darkened bands. After climbing high enough to see the bulb.....sure enough....2 little flourscent bulbs. You sawie it coming too, right?

George
2004-Jan-10, 03:38 AM
S.A.D. 3 will be electric powered. A hand fan or battery powered drill will drive the strobe plate.

I wanted everyone to know in advance to thwart all the anticipated media questioning of future SAD development. Now that they beat Pres Bush down to spilling everything about Moon and Mars trips, I figured they'd be here next. :)

George
2004-Jan-11, 04:12 AM
Left SAD 2 behind as the aluminum pie pan would probably look like a land mine to the airport radar security. Hope the Smithsonian will forgive me.

Back in San Antone..

Yeah...Sunlight! And it isn't even Spring. :)

I was the S.A.P. and the Sun's image was.......WHITE. No surprise.

The hand spinning was non-uniform. I went ahead and upgraded SAD into SAD 3. An old variable speed drill held the pencil shaft nicely. The image was reasonably steady at the higher rpm (up to 1200 rpm).

The slot was about .5 mm wide (1 cm long). It was 7 cm from center of strobe plate. This reduced the net sunlight by almost 99.9%! Still the sunlight was very intense but I could see the disk, barely. (Solar angle above horizion was about 45 deg.)

I did not test it closer to the horizion. But will see how the color compares with the color changes near the horizion tomorrow, hopefully.

Try to get some rest, all. You deserve it. :)

George
2004-Jan-14, 01:03 AM
Update....di-dit, di-dit, di-dit

Finally got a sunset to watch.

No solar hue variation with SAD3. The sun's setting and changing colors seem to match what was seen through SAD3.

This seems to confirm the conventional wisdom that the color changes of the Sun near the horizon are atmospheric related only and not a due to the eye's color cones thresholds possibly being exceeded [when it looks white].

It also may confirm the Sun's color, as seen on Earth, to be white.

Now we need a SAD4 to go up with an astronaut where the Sun's irradiance is nearly twice as intense in the blue and green range as opposed to down here. Reducing the Sun's intensity might make all the difference.

Any advice? Any astronauts want to be a S.A.P.?

Eroica
2004-Jan-14, 08:38 AM
=D> Well done, George! A positive contribution to the science of astronomy. Give that man an award! :D

So, if the sunlight is reduced to about 0.1% of its normal intensity, it looks white - is that your finding?

I don't suppose you could reduce it to, say, 0.01%, or even 0.001%? Baader AstroSolar Safety Film reduces the intensity of sunlight by 99.999%. But it does not reduce it equally across the spectrum - blue wavelengths are attenuated much more than red - so it's not much use to us here.

Swift
2004-Jan-14, 04:35 PM
This is a serious comment, not a joke.... you should publish this. No thoughts as to where.

George
2004-Jan-14, 08:27 PM
Well done, George! A positive contribution to the science of astronomy. Give that man an award! So, if the sunlight is reduced to about 0.1% of its normal intensity, it looks white - is that your finding?

Yes. White. Essentially the hue that you see with a quick and analytical glance of the sun is the same hue of color you would see when the intensity is lowered to 99.9%. It is white during most the day. As the sun looks yellowish near the horizon, so also does the view through SAD3.



I don't suppose you could reduce it to, say, 0.01%, or even 0.001%? Baader AstroSolar Safety Film reduces the intensity of sunlight by 99.999%. But it does not reduce it equally across the spectrum - blue wavelengths are attenuated much more than red - so it's not much use to us here.

I can clearly understand why they have to cut out 99% more than I did...it is still very, very bright even through SAD. So, if you add a mirror for light gathering, it would be imperative to cut it down.

I assumed any kind of filter or even glass might alter "true color" viewing, so, the strobe idea makes more sense as it is "pure" sunlight (albeit atmosphericly bleached).

It was interesting to note that the image was becoming quite weak as the sun neared the horizon due to the amount of reduction through SAD3. I tried SAD on Sirius, as it was high and bright, but it was not visible. The moon was visible through SAD.

The quality of the solar viewing was not clean enough to note detail such as sunspots, although, a future SAD might improve enough to see them. A filter works easier in this case, however.


This is a serious comment, not a joke.... you should publish this. No thoughts as to where.

Thanks much! That comment makes it all worth the "colorful" effort.

The real value of SAD will be whether or not it makes sense to send it up with an astronaut who will take the few minutes to give it a try. This, I presume, would mean convincing someone with pull that the hypothesis makes sense. Specifically, the hypothesis being that the true color of the sun can not be determined until the observer is outside the atmosphere and with the ability to attenuate the solar visible light intensity to a more reasonable level for the human eye.

Since the irradiance of the sun in the blue and green range above the atmosphere is almost twice that as the irradiance below the atmosphere, there is a fair chance that we might have a blueish or greenish or blue-greenish or something other than white star. I think humanity is ready for this super advanced information. It would also be cool to see the BA add it to his future, hopefully, new book.

If you can reword this to make it "more better", let me know.

Any way I can show you the S.A.D. 3? I can easily get a jpg on it but don't know what to do after that as I do not have a working website at this time. Of course, you know what to expect as it is fairly "sad". :)

George
2004-Jan-15, 06:35 PM
The SAD3 in space would be nice. However, what if Hubble took a look at the "Sun's Twin" - 18 Scorpii. This star is very close in temperature to the Sun. If it's absorption lines match then it may be another way to get a handle on the Sun's color.

Another possibility, but likely weak in true color accuracy, would be to heat an object close to a blackbody to the Sun's 5700 deg. C temperature. The Sun is not too far off from a blackbody radiation curve in the visible portion. I can't imagine what we would heat-up that would be accurate. I still don't understand the hole in the furnace trick for that matter. It is also possible the Sun's absorption lines would make a difference in the true color afterall.

Therefore, a S.A.D. and a S.A.P. might be the best method afterall.

Swift
2004-Jan-15, 09:16 PM
Another possibility, but likely weak in true color accuracy, would be to heat an object close to a blackbody to the Sun's 5700 deg. C temperature.
As someone who regularly heats things to 1200 to 1400C, heating something to 5700C would be no easy thing to do.

George
2004-Jan-16, 12:50 AM
Another possibility, but likely weak in true color accuracy, would be to heat an object close to a blackbody to the Sun's 5700 deg. C temperature.
As someone who regularly heats things to 1200 to 1400C, heating something to 5700C would be no easy thing to do.

We've got a plasma unit that gets close but I do not think it would be worth the effort due to the amount of effort and absorption within the spectrum problems.

It would be nice to hear..."Oh,yeah, we do this all the time and whatever we heat to this looks blue". :) Not likely, heh?

George
2004-Jan-18, 04:47 AM
I found an interesting site that shows the Sun's color to be blueish.

This site (http://www.physics.sfasu.edu/astro/color.html) displays color rendering based on blackbody curves.

I do not know why the "peachy pink" site varys with this result.

Naturally, I favor the blue sun site over the yellow or peachy pink sites. It also uses Fortran which I always liked (except for the careless "do loops"). :)

Since the Sun's spectrum is so strong in blue and green, this analysis makes more sense to me.

It's possible other sites are post atmosphere "bleaching" in their color rendering.

So the quest continues!! :) 8) \:D/

Has Hubble, Cassini or others taken any "natural" or "true color" shots of G class stars?


[The report that 18 Scorpii (our supposed solar twin) was yellow-orange did not thrill me. I wonder what evidence they use to make that claim.]

If I listed other G2 stars, would anyone check them out in their scopes???

JohnOwens
2004-Mar-03, 08:05 PM
The idea that the Sun is white because it's intensity is too great for us to see it's color, can be tested. What if we make a strobe (a slit in a rotating disk) to attenuate the total light received, then, maybe it will look yellow. Naturally, the sky is 100% overcast.

My bet is it will still look white as the spectrum (post atmosphere) suggests.

Or, you could make a different device (you could even use the same materials!), by putting a pinhole in the pie tin (or a piece of paper, cardboard, etc.) and projecting the Sun onto a white surface (but you'd better hope it's an honest white!) like you do for looking at sunspots or eclipses. That would give you variable attenuation too, by changing the distance between the two; you could make it as bright as you can stand to (hopefully) get the best cone response.

-----------------------

Going back a bit, during the sky & cloud color discussion, I was wondering: what about the green color of the clouds you see just before hail and/or tornados? What brings that about? I've found that far odder than the blue sky or a perceived yellow Sun.

AstroSmurf
2004-Mar-07, 01:26 PM
I had an idea recently. While the sky's blue colour is due to Rayleigh scattering, shorter-wavelength light is also scattered, as can be seen near sunset and sunrise. However, this very same process is still happening during the day, but since the length of atmosphere traversed is less, the "scattering angle" is similarly less.

The end result of this is that the sky will look yellow, but only very close to the sun, giving us the impression that the sun itself is yellow.

Comments?

George
2004-Mar-08, 11:48 PM
The idea that the Sun is white because it's intensity is too great for us to see it's color, can be tested. What if we make a strobe (a slit in a rotating disk) to attenuate the total light received, then, maybe it will look yellow. Naturally, the sky is 100% overcast.

My bet is it will still look white as the spectrum (post atmosphere) suggests.

Or, you could make a different device (you could even use the same materials!), by putting a pinhole in the pie tin (or a piece of paper, cardboard, etc.) and projecting the Sun onto a white surface (but you'd better hope it's an honest white!) like you do for looking at sunspots or eclipses. That would give you variable attenuation too, by changing the distance between the two; you could make it as bright as you can stand to (hopefully) get the best cone response.

This is actually a good idea. I understand that Kodak makes "gray" paper, or some other name, that reflects visible light evenly.

However, this will cost more than the 19 cents allocated for the project. :wink:




Going back a bit, during the sky & cloud color discussion, I was wondering: what about the green color of the clouds you see just before hail and/or tornados? What brings that about? I've found that far odder than the blue sky or a perceived yellow Sun.

I do not know. I have always assumed it was the presence of hail stone which added some sort of refractive factor to give green the advantage. However, I have been in these storms and no sign of hail.

My guess would be that this green cloud condition ocurrs near sunset so much of the blue has already scattered away which makes green the winner when enhanced by the cloud properties. It would be easier to make this claim if the rest of the sky was green.

I am still reading up on scattering and have a long way to go. My 19 cent SAD may be worth more than what I actually know. :) 8-[

George
2004-Mar-09, 12:10 AM
I had an idea recently. While the sky's blue colour is due to Rayleigh scattering, shorter-wavelength light is also scattered, as can be seen near sunset and sunrise. However, this very same process is still happening during the day, but since the length of atmosphere traversed is less, the "scattering angle" is similarly less.

The end result of this is that the sky will look yellow, but only very close to the sun, giving us the impression that the sun itself is yellow.

Comments?

I am curious if you actually see the sun as yellow when it is near noonday. I see it as white.

The main factor to keep in mind is that Rayleigh scattering varies as the 4th power of the light's frequency. Blue light is about 10 times more effective at Rayleigh scattering than red. [Violet even more so but the Sun does not radiate much violet and, I think, our color cones are not so receptive to violet (but I once heard otherwise).]

Particle size is another major factor. Nitrogen and Oxygen, apparently, do a great job of scattering by themselves. Particles close to or greater than the wavelength of the light will not experience Rayleigh scattering.

Scattering angle is also important. Rayleigh scattering is found at all angles. Mie scattering is strong at narrower angles which tends to whiten the sky. This is why the sky is so blue overhead since the angle is around 90 deg.

At sunset, the blue sunlight is so scattered that you don't see it, rather, you see what is left.

Oddly, Mars exhibits nice blue sunset's near the sun. This might be due to the larger particles which do not allow much blue to scatter but might cause the other colors to skedaddle. [see "Martian Sky" thread for more]

darkdev
2004-Apr-23, 03:59 AM
I didn't read all four pages, so I apologize if this has been suggested...

Can we just video tape the Sun with a device with equal RGB absorbance, then watch it on a properly calibrated monitor?

If it was actually more yellow, the screen would show it. Otherwise it is only an effect for us.

George
2004-May-24, 11:12 PM
I didn't read all four pages, so I apologize if this has been suggested...

Can we just video tape the Sun with a device with equal RGB absorbance, then watch it on a properly calibrated monitor?

If it was actually more yellow, the screen would show it. Otherwise it is only an effect for us.

A video or camera observing the sun on the ground should show it as white if they truly simulate eye response. These devices were not designed, however, to observe something so bright.

This problem is even worse when trying to observe the sun from space as it is even brighter.

Filters would help but might corrupt the observation. However, I am not a filter pro. A simple strobe would seem to be the best alternative and a good camera might do the trick (attached to the strobe) but might not as cameras are designed to give reasonable true color based on conditions down under the atmosphere with less UV, etc. I just don't know.

[Sorry, I just noticed your post.]

John Dlugosz
2004-Jun-23, 06:43 PM
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.

In a book on lighting for digital scenes, it teaches that sunlight is yellowish, compared to shade which is bluish. Setting virtual lights of the right colors from proper directions does indeed make a rendered scene look more real. I'm sure the advice is based on studio lighting skills; for example making something look like it's outside when it's really in a soundstage.


A video or camera observing the sun on the ground should show it as white if they truly simulate eye response. These devices were not designed, however, to observe something so bright.

I could shoot a photographer's Grey Card which is designed to be pure in color, and compare the R,G,B values recorded (with the camera set identically, auto-whiteballance turned off) for the card shot in sunlight and in shade.

Point is, I disagree with the proposed idea that the disk of the sun looks yellow but actually produces white light, because artists always portray sunlight as yellow. Exagurated yellow cast gives the impression of sunny.

My conjecture:

If you measure the spectrum of direct sunlight at noon, factor that curve by each of the three response curves of the human R,G, and B sensors, then plot the resulting tristimulus value on the CIE diagrams, you'll find it is right at the spot labeled "white". By definition! The experiment was a tautalogy.

But, the visual perception doesn't work with absolute values. If you take a film-based photograph inside using artificial light, the slide (or uncorrected print) will look orange. A spectrograph would tell you that it is indeed an accurate photo. But we didn't perceve the room as orange when in it!

The vision is calebrated based on overall ambiant light. So, what is the overall lighting when outside? The diffuse ambiant light is the scattered blue light from the sky. We adjust our vision so the white paper looks white to us, under this slightly blue light. Now, with our internal white-ballance set up that way, pure sunlight looks yellow, and pure diffuse light looks blue. (R+G makes yellow). That is, R,G,B has been factored into (R+G),(B) from two sources.

That doesn't explain why a sunset goes from a yellow disk to a red disk without passing through green. update: what was I thinking? I shouldn't post anything requiring thought while skimming boards during a quick 'fresher at work. I see now that after taking the B out and starting to remove some G, it will be more and more R.

tjm220
2004-Jun-23, 09:56 PM
...That doesn't explain why a sunset goes from a yellow disk to a red disk without passing through green.

Because green isn't between yellow and red in the EM spectrum? :-k

George
2004-Jun-23, 10:09 PM
In a book on lighting for digital scenes, it teaches that sunlight is yellowish, compared to shade which is bluish. Setting virtual lights of the right colors from proper directions does indeed make a rendered scene look more real. I'm sure the advice is based on studio lighting skills; for example making something look like it's outside when it's really in a soundstage.
Bluish light from shade makes sense as it would be reflecting blue sky lighting (assuming a non-cloudy day). Yellow light from the sun, IMO, is limited to mornings and evenings. Earlier in the thread I made a S.A.D. (really cheap strobe) and we observed the Sun. Not very high from either horizion, it truly looks white with no hint of any color.




A video or camera observing the sun on the ground should show it as white if they truly simulate eye response. These devices were not designed, however, to observe something so bright.
I could shoot a photographer's Grey Card which is designed to be pure in color, and compare the R,G,B values recorded (with the camera set identically, auto-whiteballance turned off) for the card shot in sunlight and in shade.

I elected to just use my 19 cent strobe. :) I hope you will try the Grey Card and let us know your findings.


Point is, I disagree with the proposed idea that the disk of the sun looks yellow but actually produces white light, because artists always portray sunlight as yellow. Exagurated yellow cast gives the impression of sunny.

My conjecture:

If you measure the spectrum of direct sunlight at noon, factor that curve by each of the three response curves of the human R,G, and B sensors, then plot the resulting tristimulus value on the CIE diagrams, you'll find it is right at the spot labeled "white". By definition! The experiment was a tautalogy.

Here is a computer rendition that is colorful..... Peachy Pink (http://casa.colorado.edu/~ajsh/colour/Tspectrum.html)!! (Have mercy :) )

Here is an irradiance plot of sunlight above and below the atmosphere... here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

Notice how flat the spectrum is from blue to red on the ground. My interest is what color would the eye see above the atmosphere with the much greater blue component. The intensity would first need to be reduced with a strobe or, possibly, use the "Grey card" reflection, or both.
We just might have a bluish star afterall. 8)


But, the visual perception doesn't work with absolute values. If you take a film-based photograph inside using artificial light, the slide (or uncorrected print) will look orange. A spectrograph would tell you that it is indeed an accurate photo. But we didn't perceve the room as orange when in it!

The vision is calebrated based on overall ambiant light. So, what is the overall lighting when outside? The diffuse ambiant light is the scattered blue light from the sky. We adjust our vision so the white paper looks white to us, under this slightly blue light. Now, with our internal white-ballance set up that way, pure sunlight looks yellow, and pure diffuse light looks blue. (R+G makes yellow). That is, R,G,B has been factored into (R+G),(B) from two sources.

That doesn't explain why a sunset goes from a yellow disk to a red disk without passing through green.

Near as I understand, Rayleigh scattering is pulling blue light out first, then green, etc. By the time the narrower green becomes dominant over blue, the yellow, orange and red become more of a factor. Also, I suspect the green is somewhat added to the red, as you noted above, to give yellow the edge to our visiual interpretation. It's probably not quite that simple as other scattering effects are also part of the equation, too.

[edit: "Peachy Pink" link is now correct - (Thanks John)]

centsworth_II
2004-Jul-12, 01:02 PM
Hi all,

I found your discussion very interesting, especially George's contribution. Experiment trumps theory, don't you think? Anyway, I was involved in a similar discussion in a painting forum and suggested that they take a look here. I thought some of you might like to see it from the artists' perspective, so here's the link http://www.wetcanvas.com/forums/showthread.php?t=199848&page=1&pp=15. By the way, I'm DuhVinci over there.

George
2004-Jul-12, 01:25 PM
Hi all,

I found your discussion very interesting, especially George's contribution. Experiment trumps theory, don't you think? Anyway, I was involved in a similar discussion in a painting forum and suggested that they take a look here. I thought some of you might like to see it from the artists' perspective, so here's the link http://www.wetcanvas.com/forums/showthread.php?t=199848&page=1&pp=15. By the way, I'm DuhVinci over there.

Welcome to the board, centsworth_II. =D>

The BA's book addresses the misconception of the Sun's color. Your link is interesting. A couple of posts brought up the idea of seeing a color as a result of another color's influence. This can be quite dramatic. It may play a role when it comes to certain shadows, etc. However, I suspect the color of the sun would not suffer from much contrasting colors since it is up in the sky. The strobe I used should be a true representation of it's apparent color. I painted the strobe plat flat black to help avoid any psychological coloring effects.

centsworth_II
2004-Jul-12, 02:55 PM
Thanks for the welcome, George.

Color is a fascinating topic since it involves the physics of light as well as the biology of the human eye and the psychology of human perception.
I'll remember your experiment when I think of how we see the sun. Once the sunlight hits the objects around us, things really start to get complicated!

George
2004-Jul-14, 10:10 PM
If you find any detail specifications on the eye's performance, let me know. I would be curious to find out how many photons at different wavelengths the color cones can handle till they are "maxed-out". My hope is to answer a question I have, namely, if all three cones each receive enough photons each, does this cause "white" to be the color regardless of actual ratios? It should be an easy - yes.

Hope all goes well for you and that your studies, well, are colorful. :wink:

StormSeeker
2004-Jul-21, 03:09 AM
You know, something flashed in my mind early this morning about why we don't see the sun as green, but it kind of slipped away before I could grab hold of it. Something about why chlorophyll is green, and the absorption of red light, and something about if we could see what color the inside of our eyes are. It made sense at the time.

George
2004-Aug-31, 11:29 PM
Ya'll have gotten soft, dag nab it! :evil: This polemic problem is still unsolved.

From Ask the Space Scientist #35 (http://image.gsfc.nasa.gov/poetry/ask/asun.html)


Sunlight is not 'white' at all, but is only described that way as an approximation. The sun is an honest to god yellow star by DIRECT observation (watch your eyes), and any combination of colors that matches its color will appear yellow...but we call this particular combination 'white' for reasons that seem as much based on tradition as on poor observation.

"All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program."

He didn't capitalize "god" so I knew he wasn't absolutely, positevely right on this. :)

I'd say "yellow" is the tradition, not "white". I'd send him a S.A.D. (if he'd take it) but the postage is more expensive than the instrument. :)

Note: This is probably an old answer. He may be reading the BA's book as we speak, whereupon, he will edit his answer, right? :)

George
2004-Aug-31, 11:34 PM
I should state that extensive research into this issue is being conducted on a highly limited scale. :roll:

I am finding numerous sites from the lighting industry indicating that a solar temperature should produce a blue light. This is very encouraging.

Sometimes industry gets a better perspective. For instance, oil geologist, reportedly, understood crust movement before the theory had much accpetance.

samsara15
2004-Oct-08, 05:04 PM
What color do sun-like stars appear to be when viewed in a telescope?

George
2004-Oct-08, 07:14 PM
What color do sun-like stars appear to be when viewed in a telescope?

General star color can be found at this site.... star colors (http://www.vendian.org/mncharity/dir3/starcolor/)

Your question is a good one. If "sun-like" stars look blue or yellow, then ureika. Maybe the Sloan Survey will help or Hubble pictures. However, the problems in getting "true color" are several....

1) Telescopes on Earth are hendered by bleaching caused by atmospheric effects (scattering, absorption, etc.). This causes the sun (therefore, "sun-like" stars) to look white.

2) The "color" represented by telescope imaging are not true "naked-eye" color as scientific coloring yields signifcantly more information. I suspect some images are close such as from Hubble and Cassini.

3) Like the sun, they may be too bright. Just because they are far away does not mean they will look less bright. The sun provides us with 1000 watts/sq. meter at ground level. If the Earth were twice as far away (2 a.u.), the power would be 250 w/m^2. However, it would be 1/4 in apparent size viewed from 2 a.u. This means the surface brightness will appear the same. Nevertheless, it may be that once you are far enough away so the sun is a point and not a disc, then maybe it's color is revealed.

4) To compute the color that we would see involves a great deal. The star color site has determined our sun is "pinkish peach". I am not convinced this is valid.

Now that we have SpaceShipOne, maybe they will take a strobe up with them and settle this issue. Hmmmm....I did buy a computer with Microsoft software recently...ya think they.... :roll: :)

George
2004-Oct-26, 04:07 AM
Had another idea ( another s.a.d. kind) :)

What if we made a template in the shape of a solar irradiance curve (almost a Planck blackbody one), proportion it's size to match the eminating rays refracted from a prism, place it on the prism, allow a rectangular region of sunlight into the prism, recombine the adjusted colors via prism, and see what color emerges. 8)

The light into the prism would be sunlight itself. Sunlight is very flat across the spectrum after it has been bleached by our atmosphere. Therefore, it should be a great light source.

Here is an example of both irradiance curves (above and below the atm.) irradiance (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

Anyone give this a chance? 8-[

[Edit: all my ideas seem to have an oops. Oops, additional work would be needed to even the distribution of color as the template would allow more blue and green on the end (or ends depending on template design). Hopefully, the principal, hopefully, is sound.]

Eroica
2004-Oct-28, 08:00 AM
What if we made a template in the shape of a solar irradiance curve (almost a Planck blackbody one), proportion it's size to match the emanating rays refracted from a prism, place it on the prism, allow a rectangular region of sunlight into the prism, recombine the adjusted colors via prism, and see what color emerges.
Let me see if I follow.
We take pure white light Split it up into its constituent wavelengths using a prism Intercept these waves with a template of the Sun's light-curve so that the light which gets past the template has the same intensity-versus-wavelength distribution as Sunlight Recombine these wavelengths using a second prism to produce our own artificial Sunlight (of whatever intensity we choose) See what colour we get


The light into the prism would be sunlight itself. Sunlight is very flat across the spectrum after it has been bleached by our atmosphere. Therefore, it should be a great light source.
I don't quite get this. Surely we should start with pure white light of much less intensity than Sunlight. Isn't the fundamental problem with trying to determine the Sun's colour the fact that Sunlight is so intense it overpowers the cones, maxing them out?


[Edit: all my ideas seem to have an oops. Oops, additional work would be needed to even the distribution of color as the template would allow more blue and green on the end (or ends depending on template design).
Again, I don't quite follow. I thought the whole idea of the template was to ensure that more blue and green would go into the mix, since we're trying to create our own artificial Sunlight - or have I misunderstood the whole experiment?

George
2004-Oct-28, 01:24 PM
We take pure white light Split it up into its constituent wavelengths using a prism Intercept these waves with a template of the Sun's light-curve so that the light which gets past the template has the same intensity-versus-wavelength distribution as Sunlight Recombine these wavelengths using a second prism to produce our own artificial Sunlight (of whatever intensity we choose) See what colour we get
Yes. The irradiance curve is a geometric representation of energy distribution as a function of wavelength. The y-axis is a derivative, so, integrating yields the total energy radiated (per unit area), which is simply the area under the curve. Pretty neat, huh. Your understanding of this is probably better than mine, but I believe I am not mistaken.

Therefore, if we have a pure flat spectrum source, we can simply cut out a template that is identical to the above-the-atmosphere irradiance curve for the sun, resize it proportionately to match the refracted spectrum from a prism to obtain the same identical amount of light for each wavelength (as the Sun radiates in space), recombine the light into a homogenous area which is reflected off of a "gray" card (which reflects all colors evenly), then observe the resulting net color.

The template could be placed onto the incident side of the second prism as this would be the spectrum's largest point.

A converging 3rd lens may be needed to help the homogenization.




The light into the prism would be sunlight itself. Sunlight is very flat across the spectrum after it has been bleached by our atmosphere. Therefore, it should be a great light source.
I don't quite get this. Surely we should start with pure white light of much less intensity than Sunlight. Isn't the fundamental problem with trying to determine the Sun's colour the fact that Sunlight is so intense it overpowers the cones, maxing them out?
The light is greatly diminished since it is enlarged as it hits the "gray" card, and we only see a very small portion of it (inverse square law, of course). We can also adjust the light intensity by simply moving the "gray" card since the light from the final lens will be converging to diverging.

However, the sun's spectrum at sea level is not perfectly flat. If you know of a "pure" white light source it would be wonderful. Nevertheless, the template can be cut to allow for the variations in it's sea level spectrum. Take the mean of this post atmosphere spectrum, then, add and subtract from the pre-atmosphere spectrum for each wavelength. This would allow sunlight to be used if no better source is available.



[Edit: all my ideas seem to have an oops. Oops, additional work would be needed to even the distribution of color as the template would allow more blue and green on the end (or ends depending on template design).
Again, I don't quite follow. I thought the whole idea of the template was to ensure that more blue and green would go into the mix, since we're trying to create our own artificial Sunlight - or have I misunderstood the whole experiment?
If, for instance, a thin horizontal beam is refracted through the prism, the template (at least the one described) would allow more blue on one side than red, for instance. Recombination with a second prism would not integrate this extra blue into the middle, so one edge of the final appearance might be blue but, probably, not the middle of the image.
A small round beam could be used, but, it will be harder to make the template and the problem would still exist.

There are alternatives for this minor problem. Instead of clipping off the edge of the red light, for instance, with the template, we could keep the sides even but place tiny obstructions evenly across the width of the beam for each respective color. However, the 3rd converging lens might eliminate the need for the extra work.

As you can see, I am trying to build from the tremendous success established by the S.A.D. experiments. :roll: :wink: But, there is still a chance I am a "few cards short of a full deck" on this idea. If it is so easy, why hasn't anyone done it (or have they?)? #-o

[Edit: Here is the location for the irradiance curves.... here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

George
2004-Oct-29, 04:13 PM
The following represents the basics of the idea....


http://img50.exs.cx/img50/3146/SPACCSunsColor.jpg


A - Inlet slit to allow sunlight (ground level light) into SPACC.

B - White light (for simplicity, assume flat spectrum)

C - Prism 1

D - Typical dispersion

E - Template: This is a pre-atmosphere irradiance curve cut into a dark plate. The outlet light is now of the same proportions as sunlight above our atmosphere.

F - Prism 2 (to converge the light)

G - Converging light. Note that the blue light, for instance, is wider than the other colors. This presents a minor problem as all the light must be hongenized to correctly represent the Sun's "true" color.

H - Converging lens to homogenize the light. Something a little fanzier may be required. A simple convex lens will help but complete homogenization would be only at a tiny point, I think.

I - "Gray" card or something to reveal the - Suns True Color. I wonder what it is but I am hoping it is cyan or blue just to stir things up. :)

X - "marks the spot"!

1 - This is the above and below (sea level) irradicance plot representation. (The upper curve is the intensity of light for each color radiated by the Sun as observed in space, the lower is what we see down below after the atmosphere has "bleached" it.)

2 - If the upper curve can be used to "cut-out" light from a pure flat spectrum by the amount the sun does not emit, then the light seen will be a true representation of the color of the sun since each color will be in proportion to the suns actual color spectrum.

3- Template. The net light that passes through from a pure flat spectrum source, that is aligned in wavelength with this template, should be a true representation of the light radiated from the Sun. This light, when recombined equally into one area, will reveal the true color of our star, hopefully.

This is my thinking. If it is valid, there are some details to work out as we won't likely have a pure light source. Using existing sea-level sunlight should work if we compensate for the variations, thus producing a modified upper curve. The key point is to have the same proportions of all colors, as that of the sun, for the light that is allowed through the template.

The template will likely be placed onto the surface of Prism 2 as this would be the largest area to workwith and provides an easy "glue" point.

Well.....whata ya think? There probably is no big rush to discover the Sun's color. Afterall, it has been about 2 or 3 million years since the Sun was first discovered by man. :) Hmmmm, was it man's first discovery?

Regardless, wouldn't it be great if the BA could go live and announce to the world the Sun's true color debunking all the "yellow" sun nonsense. Teachers could finally give kids the proper crayon to use to color the sun and, 15 years later, they would be that much better prepared for astrophysics and, maybe, sun bathing. :)

George
2004-Oct-31, 12:19 AM
I felt it would be best to move this to GA... here (http://www.badastronomy.com/phpBB/search.php?search_author=George)

Please post there.

Maddad
2005-Feb-08, 05:59 PM
In combining two ideas mentioned before on this thead, water absorbs more UV light than other frequencies. That should mean that the more water vapor the light travels through in getting to your eye, the further the color should shift away from true green toward red since UV is on the other side of the color spectrum. When the sun is high in the sky, appearing white, the light passes through the least atmosphere in reaching your eye. As the sun sinks to the horizon the light passes through more and more atmosphere and therefore water vapor. We would expect the color to shift toward red, which is what we observe.

This leaves open the issue of why we preceive white light from a green source. White is the presence of all colors in equal proportions. The equal proportions part though is probably wrong. We evolved on this planet to take advantage of the light that was available. We should see this light, whatever actual color it has, as white.

George
2005-Feb-19, 04:36 AM
This leaves open the issue of why we preceive white light from a green source.
A "green" source can be tricky. If the true color of the sun is green, it will still look white to us, as you have said. However, a green laser or led will never look white since the other colors do not exist in the light.


White is the presence of all colors in equal proportions. Not necessarily in equal proportions. If our three color cones are "maxed out", then white is the color our brains says it is. This happens when we look at the sun from space or when it is high overhead.


We evolved on this planet to take advantage of the light that was available. We should see this light, whatever actual color it has, as white. Other species see it different (pun time :) ).

White-tail deer, for instance, are dichromatic (two color cones). Univ. of Ga. and a northern univ. conducted a study I read. These deer can see extremely well in blue and, possibly, some in the UV band. Not only is their range excellent but also their sensitivity level. Of course, evolution helps them here, too. They can see all the dear hunters clearly as they sneak into their deer blinds just before dawn. The blue and violet light overhead (due to Rayleigh Scattering) is strong enough for them, but too weak for the hunters to know it. :)

BTW, the other color they see is yellow. So, when you see hunters saying the deer can't see their red or orange outfits and they think they are invisible - they're wrong. The deer will see them as wearing solid black against a rather bright background. :) [Still, the others can see them and not shoot them, hopefully]

fosley
2005-Feb-24, 06:09 AM
You all seem to be doing a very good job on the experimental side of the house, but here's a couple thoughts that may (or may not) be of use:

I look at the Sun and see white.

I look away from the Sun (or any other bright, white light) and see a spot that rapidly changes colors, like my eyes can't decide what color it is.

From a design standpoint (whether by evolution or special creation or aliens-created-us or whatever), I would think that the eye would be set up so that we should be able to define "white" by "the average color of the Sun".

The fact that everything doesn't look blue seems to support this:
- Blue light is much more energetic, right?
- So our eyes don't notice it as well
- The end result is that during most of the day everything looks like it's being illuminated by white.

And it would be defined by the average color of the Sun from our perspective on Earth, since we obviously don't live in space.

As far as children painting the sun yellow, I like the idea that it's the *best* choice, but not necessarily the *correct* choice.

And I think it's probably influenced by other artwork where the Sun is yellow.

I always tried to paint the Sun red, orange and yellow because those are the colors of fire, and the Sun is fiery, right?

Although I always thought that the Sun should be blue, since it's really hot, then found out that maybe it should be white. But I've seen green flames, so temperature isn't the only determination of what color the fire is. . .

So I got confused and started making it yellow.

But I've always thought of it as white, and color it so if I can. It's just hard to denote that "this circle is the sun" without actually painting the blue sky, and a black circle looks kind of stupid.

At least those were *my* thoughts as a kid (seriously, I can remember those "fiery" thoughts as far back as 4 years old, and the "white-hot" thoughts as far back as 9 years old).

Now, it seems to me that a true test of the Sun's color would be to test how many photons from each of the basic parts of the spectrum there are (and I don't know enough about such things to know if we can even determine that, although I'm pretty sure the answer is 'yes').

But, since our eyes are biased in how they see differenct wavelengths of light, and the percieved color is formulated from some arcane equation that only our brain knows, but is somehow related to the various types of receptors and the proximity of one receptor to the other, etc., the number of photons doesn't help us determine the percieved color, just the actual color.

Anyhow, my two cents worth.

George
2005-Feb-24, 09:31 PM
Now, it seems to me that a true test of the Sun's color would be to test how many photons from each of the basic parts of the spectrum there are (and I don't know enough about such things to know if we can even determine that, although I'm pretty sure the answer is 'yes').

I'm hopeful the answer is "yes", too. :)

You are correct. Color is what we see due to the no. of photons/sec, for any given wavelength, our color cones receive. The wavelength is imporant and the intensity at each wavelength is important. A plot is called spectral irradicance.

There is much you can find regarding how our eyes work. We have three color cones (often called red, green and blue cones). It is complicated because, for instance, the green cone can see yellow and some red. The green is more sensitive than red which is more sensitive than the blue.

These cones work similar to the color dots on a tv. The amount of excitation of each of a group of 3 dots (red,green and blue) will produce one of millions of different hues (colors).

White is the result of fairly even intesity for each color, I presume.

Also, if the intensity is greater than the eye can handle, white will also be a result. This will prevent the true color from being revealed. The sun's true color is not known, apparently, for this reason. Astronauts see the sun as bright white, yet, the blue and green photons are much stronger than the other colors. Down here on the surface, the intesity has been affected by our atmosphere enough to make each color intesity level about the same for all colors. Therefore, it appears white (but not when it gets near the horizon, of course).

Eroica
2005-Mar-24, 12:41 PM
16 Cygni A and 85 Pegasi are both G2 V, the same spectral type and luminosity class as the Sun. I bet they look white and colourless to us!

George
2005-Apr-07, 11:14 PM
16 Cygni A and 85 Pegasi are both G2 V, the same spectral type and luminosity class as the Sun. I bet they look white and colourless to us!
But what color are they when imaged through a properly color calibrated Hubble? !!!

[Added]
The color idea boils down to the difference in the two plots on the following graph..... here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

[use 400nm to 700nm for vision response range.]

The intensity of colors in blue and green above our atmosphere are just too much more than what we see below our atmosphere. The lower line, the light we see down here, is fairly flat so that a white sun makes sense. But in space, the blue and green kick in hard and a net color just might be there. I just know more in the coming weeks.

genebujold
2005-Aug-17, 05:10 AM
Consider the following website:

http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

Even though his technical capacity for black-body radiation is beyond mine, I'm sorry to say that he missed the mark on this one.

I'm a semi-pro photographer and have worked with many types of photography mediums for years, so if I may...

The peak frequency would not, by itself, determine the sun's color. Rather, it's the sum of all the frequencies over the entire spectrum (more appropriately, the medium of the area beneath the curve) which determines the sun's initial "color" as measured before it hits the eye.

But that's not quite all there is to, it...

The human eye sees some frequencies better than others. Thus, if the frequency curve were flat (same intensity across all frequencies in the visible spectrum), the color we saw as a result would tend to be closer to the frequencies in which our eyes are more sensitive.

Thus, it's not a just a matter of computing the median under the curve. First, we have to assess the impact of each frequency against our eye's proportional ability to see that frequency.

The most appropriate way to do that is to normalize both curves, making the sum of the area beneath them equal to one, then multiply them along all points, thereby obtaining a curve representing the ability of our eyes to perceive the sun's frequency curve.

Then you find the median of the area under the curve, and that's the "color" of the sun as we humans perceive it.

You know - yellow. Or actually, during noon-day, more like a very bright off-white with a slight tinge of yellow to it.

Alternatively, you can obtain a dark, chromatically neutral filter such as we use in photography, and just look at the noon-day sun for a second through that filter.

Yep - it's yellow, but not school-bus yellow. It's more like a very sun-faded manila folder yellow.

Of course the only problem with the technique I espoused above is the fact that the frequency response curves for our eye's ability to perceive colors differs from person to person. Generally speaking, however, we learn around that, so everyone knows the difference between yellow, green, and blue, for example.

George
2005-Aug-23, 12:45 AM
The peak frequency would not, by itself, determine the sun's color.
I did not get the impression they were claiming peak wavelength would do so.



So CIE stds. do not work well with blackbody spectrums?


[quote]Alternatively, you can obtain a dark, chromatically neutral filter such as we use in photography, and just look at the noon-day sun for a second through that filter.

Yep - it's yellow, but not school-bus yellow. It's more like a very sun-faded manila folder yellow.
I made a strobe (S.A.D.) and got white, but this only reduced it by 99.9% roughly. How flat is your filters transmissivity spectral curve?

ceritus
2005-Aug-26, 07:56 AM
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.

I have tried this and all I get is a bright purple blob its like dark pruple almost blue and it sticks with me for about a minute or so and fades.

Tanalia
2005-Nov-13, 12:01 PM
Shucks! Here is another >>> eye response 2 (http://www.creativepro.com/story/feature/13036-2.html) <<<.

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).

Actually it agrees, because it's a graph of something different; it is showing the required amounts of three imaginary primary colors needed to represent the spectrum perceived at equal brightness. It is because of the reduced eye response to blue that you have to "turn up" the blue contribution -- in effect it is an inverse of the response graph.

The description with the graph is somewhat vague, and the "(akin to cone response)" phrase is rather misleading; it's related to cone response, but it's not like it.

George
2005-Nov-14, 01:04 AM
Actually it agrees, because it's a graph of something different; it is showing the required amounts of three imaginary primary colors needed to represent the spectrum perceived at equal brightness. It is because of the reduced eye response to blue that you have to "turn up" the blue contribution -- in effect it is an inverse of the response graph.

The description with the graph is somewhat vague, and the "(akin to cone response)" phrase is rather misleading; it's related to cone response, but it's not like it.
Welcome, Tanalia. :clap:

This is an area I need considerable help. [I even skipped biology in school].

I don't really understand what is being said. There are only about 2% as many 'blue" color cones compared to the other two. Yet, I understand the overall processing bumps the blue up to the other levels. Thus, the net color response is about the same.

Or, is it normalized in graphs to show only the relative wavelenth to sensitivity value for each cone? [This is the graph where they are all set to 1]

I am guessing you are saying the latter. Also, if I understand you, the last graph shows that we need to receive about twice as much blue to generate a final processed response equal to the others. Is this close?

Tanalia
2005-Nov-20, 09:21 AM
Welcome, Tanalia. :clap:
Thanks. I've been lurking for a while. I work in computer graphics, so I am familiar with colors and the CIE model, and figured I could, um, shed some light on the subject :razz:

This is an area I need considerable help. [I even skipped biology in school].

I don't really understand what is being said. There are only about 2% as many 'blue" color cones compared to the other two. Yet, I understand the overall processing bumps the blue up to the other levels. Thus, the net color response is about the same.

Or, is it normalized in graphs to show only the relative wavelenth to sensitivity value for each cone? [This is the graph where they are all set to 1]

I am guessing you are saying the latter. Also, if I understand you, the last graph shows that we need to receive about twice as much blue to generate a final processed response equal to the others. Is this close?

Yes, that is the case, the one graph is normalized, and you need more blue to see the same apparent brightness.

http://www.handprint.com/ has quite a bit on color, if you follow the color wheel (http://www.handprint.com/HP/WCL/water.html) to color vision (http://www.handprint.com/HP/WCL/wcolor.html), you can find interesting topics like light and the eye (http://www.handprint.com/HP/WCL/color1.html) and opponent color processing (http://www.handprint.com/HP/WCL/color2.html).

The first has graphs like the first ones you were looking at (plus some interesting stuff about color and direct/indirect sunlight), the second has a variant form of the CIE graph.

George
2005-Nov-21, 12:22 AM
Thanks, Tanalia. Your links contain some of the best info I've seen.

The complexity of color processing led me to attempt the making of an instrument which would reclaim the portion of the spectra lost by the atmosphere. SPACC (http://www.bautforum.com/showthread.php?t=17237) was born. It does not fully recombine all the colors, but it comes close. It is in the hands of a college physics dep't. which plan to work with it next semester.

The results convince me our sun's natural color is not yellow (if we could see it from space and at a lesser intensity).

Thanatos
2005-Nov-29, 11:20 AM
I like the peak spectrum concept - the sun radiates most intensely at it's kinematic temperature. It is not the least bit odd to me that our retina is most sensitive at these wavelengths.

George
2005-Nov-29, 03:23 PM
I like the peak spectrum concept - the sun radiates most intensely at it's kinematic temperature. It is not the least bit odd to me that our retina is most sensitive at these wavelengths.
Yes. What a coincidence? ;) :) It is interesting to see that not all animals have this spectral respone. White tail deer, for instance, have two cones (blue and yellow). The blue response is quite strong and goes into the uv. They likely see red as black, I guess, which makes some sense when we consider chromatic aberation would be a problem for it. Their strong blue/uv response, I suspect, gives them even better evolutionary advantage as they have the ability to see things, and movement, much earlier, pre-dawn, than those with our eyes. In addition, their eyes are larger, and, I think, they have less UV filtering. It is a little humorous to think how likely it is for them to easily watch hunters climb into their blinds in "total darkness".:)

wayneee
2005-Dec-04, 08:29 AM
Im out of my depth here , but I have been reading with some intrest. Color exists as a preception of eyes. Eyes are restricted and confined in thier process of color due to amounts cone cells . Are we realy asking what Empirical Color the Sun is. It was my understanding that the color of somthing is determined by it Molecular structure and Atomic configuration. The Sky is Blue Because Nitogen is Blue , Plants are green beause Chorophl is a combination elements Blue and Yellow. What are we seeing when we look at the surface of the SunWhat element is present?

George
2005-Dec-04, 09:15 PM
Are we realy asking what Empirical Color the Sun is?
You may be asking for its natural color if we could see it under normal conditions, which would not allow its true color to be hidden. I call this the Sun's intrinsic color. Is this your meaning?


It was my understanding that the color of somthing is determined by it Molecular structure and Atomic configuration. The Sky is Blue Because Nitogen is Blue , Plants are green beause Chorophl is a combination elements Blue and Yellow. What are we seeing when we look at the surface of the SunWhat element is present?
The question of the color of atoms is one I would like to see answered. I believe the first atom ever actually observed was a gold atom, and it looked gold, too.

However, I believe, atoms are very limited in what visible colors they can emit due to quantum limitations. Hydrogen can emit red, but it can look other colors too, depending on circumstances.

The sky looks blue because nitrogen and oxygen (and other extremely small particles) cause blue light to scatter towards us more than the longer wavelength light. If the sunlight were stronger in violet intensity, and our cones were more sensitive to violet, you would see a much more violet sky. Going the other direction, a sunsetis red, not necessarily because certain molecules are red (though dust can be), but because the blue has scattered away from you leaving the longer wavelength colors to finally reach you.

Clogs
2005-Dec-07, 12:44 PM
Maybe the Sun is made of sulfur! :clap:

Grand_Lunar
2005-Dec-17, 05:07 PM
I never realized the sun's color was such an issue!

Wouldn't a filter to tone down the brightness to a level we can look at it safely provide a clue? I think I've heard of such filters; ones that maintain the natueral color of an object.

This brings up a point; what would we see in our sky if a blue star or a red star was in place of the sun? (aside from the fact a blue star would overheat the Earth and a red one would not provide enough heat for us!)

Grand_Lunar
2005-Dec-17, 05:09 PM
Maybe the Sun is made of sulfur! :clap:

But wouldn't it look like this: http://www.nineplanets.org/io.html

Then we'd call it the Pizza Star!

George
2005-Dec-19, 02:35 PM
Wouldn't a filter to tone down the brightness to a level we can look at it safely provide a clue? I think I've heard of such filters; ones that maintain the natueral color of an object.
Let me know if you find one. I think the Baader filter comes close.

I chose another method....a strobe ( SAD - Solar Attenuation Device (http://www.bautforum.com/showpost.php?p=165255&postcount=61) :) ). It indicates the sun at midday is white, at least for me.


This brings up a point; what would we see in our sky if a blue star or a red star was in place of the sun? (aside from the fact a blue star would overheat the Earth and a red one would not provide enough heat for us!) That is an interesting question. A slightly hotter sun would push the peak radiation further into the blue. [Its peak now is blue-green (cyan).] This might make a difference, but I doubt it. Scattering would increase and diminish the blue effect. If we use an even hotter, bluer, star, the color contrasts will not improve, I think. Therefore, it may still look white after the atmosphere "bleaches" its light.

A red star is different, however. It should look more red than it really is because the scattering of the blues, greens and yellows will produce a redder image.

jon.clovis
2006-Jan-19, 10:21 AM
Descriptions of stellar types often say things like "yellow".

> > Those 'yellow' stars are white, and what are usually termed 'red' stars are
> > yellow to yellow-orange. Stellar types are defined using for calibration the
> > blue star Vega, which provides an extremely simple spectrum at the '0'
> > magnitude. The colors assigned stars were never intended to have any
> > relationship to reality, but simply showed the temperature relative to Vega.
> > Thus we have the seeming absurdity of our overwhelmingly, brilliantly white
> > Sun being called a 'yellow' star.

> Fascinating. What an unfortunate naming scheme.
> I wonder if it started out fully qualified ("Vega-yellow", "yellow
> relative to Vega"), and then degraded, or whether correct interpretation
> always presumed domain knowledge. At this point, judging by some of the
> colors used in stellar type tables by astronomers teaching intro
> astronomy classes, this context is sometimes forgotten even within the
> profession. And outside... what a mess.

It did [presume domain knowledge], but that 'domain' was originally
comprised of astronomers doing photometry who would presumably
understand the system taken from whatcolor is the sun web page..jon:naughty:

HenrikOlsen
2006-Jan-19, 11:10 AM
This brings up a point; what would we see in our sky if a blue star or a red star was in place of the sun? (aside from the fact a blue star would overheat the Earth and a red one would not provide enough heat for us!)
We'd see it as white, having evolved to see in that light:)

George
2006-Jan-19, 04:30 PM
It did [presume domain knowledge], but that 'domain' was originally comprised of astronomers doing photometry who would presumably understand the system taken from whatcolor is the sun web page..jon
I like the quotes you gave as they are likely correct in arguing for a white sun. I did not find the web page you mentioned, however.

This is an interesting topic since many conflicting statements are found on various web sites regarding the Sun’s color. There are several threads here which deal with the subject besides this one, including my favorite… Sun’s Intrinsic Color (http://www.bautforum.com/showthread.php?t=17237).

Here, an instrument was invented to reconstruct the actual true spectral irradiance of the sun, as seen in space, in order to allow the eye to “see” it as it really is. Its lens system is not quite perfected enough to get all the restored light homogenized, but it does strongly suggest white, or possibly slightly bluish-white, is the true color. A solar astronomy class is using it this semester; so maybe good news will come from their work.


We'd see it as white, having evolved to see in that light
No doubt, evolution is the biggest process which describes how our eye developed. There are other interesting aspects of this, too. I stumbled onto an article showing that white tail deer have very sensitive blue color cones, possibly capable of seeing slightly into the uv band. Their evolutionary trail may be better, at least for them, as they should see things before sunrise far easier than we can – hunters be advised :). Albeit, these deer can not see in the red or orange, and they have only two color cones compared to our three. They should see the sun as cyan or a little more greenish (combining blue and yellow); not that they would tell us. :) Of course, other critters have similar interesting eyesight.

jon.clovis
2006-Jan-20, 06:38 AM
The Color of the Sun



Most people (even some astronomers, but not solar astronomers ) describe the sun as yellow. It is not. Snow is a pretty good reflector of all visible wavelengths; the sun is roughly the color of snow. Then why do people think it is yellow? Perhaps because it is hard ( indeed, dangerous!) to look at the sun when it is high in the sky. We can and often do look at the sun comfortably when it is low in the sky, and we see it through a long path of air, which scatters the short wavelengths out of the beam, allowing only the long ones to pass through. Consequently the sun, when seen low or setting, generally appears yellow, orange or red. Maybe it's just tradition to regard the sun as yellow. Still, it's white. from a web page color of the sun..:wall:
the sun is white-blue..nasa knows it.
ask the people who when up there..
but its so big of a mess..it will take years to get it right.
the sun is a white star. not yellow or green.
and it does not matter ..if you have a white star. putting a little yellow
will not make it yellow. it will be white..
putting blue or green will not make blue..it will look white.

here on earth..its white..at 12.00 noon in mid summer..its white.
its temp is white it does ot matter if its 5500 k or 6000k.
its all white. our eyes have nothing to do with it..

the yellow is a working color to vega.

ask any solar astronomer..

for all that nasa does its mess up.

jon.clovis
2006-Jan-20, 06:52 AM
you know.. you keep telling people its yellow ..they will start seeing yellow.

but the people up in space will tell you its verry verry bright white-blue.

nasa needs to re vamp all of the star colors..and the o.a.f.g.k.m.
that is out of date...and look at the stars by there real colors.

re vamp the h and r charts..make new ones with the right colors..

what a mess!!!

jon.clovis
2006-Jan-20, 09:32 AM
:wall: Color of Sun

Extensive cut-and-paste replaced with link by moderator:

Color of Sun (http://www.newton.dep.anl.gov/askasci/phy00/phy00940.htm)

jon.clovis
2006-Jan-20, 09:36 AM
Massive cut-and-paste replaced with link by moderator:

Star color-details (http://www.vendian.org/mncharity/dir3/starcolor/details.html)

ToSeek
2006-Jan-20, 04:02 PM
Jon,

Please do not do massive cut-and-pastes from other websites as it's a copyright violation and could get us into legal trouble. The first one might not have been a problem as it's from a US government site, but the second one was definitely problematical - not to mention that the information on the second site is a lot easier to read when it's formatted as it is on their page, not when all the formatting is lost when it's pasted here.

George
2006-Jan-20, 04:28 PM
:wall: Color of Sun

Extensive cut-and-paste replaced with link by moderator:

Color of Sun (http://www.newton.dep.anl.gov/askasci/phy00/phy00940.htm)

There are many sites with similar answers, although the answers can vary from place to place.

The comment that the sun looks white as seen from space lacks some intersting aspects inherent in discovering the sun's true intrinsic color. The sun's intensity is so great at every color, the eye is overwhelmed and will conclude the sun must be white. If you could go into space and attenuate the flux, then the eye could ascertain its true color. An astronaut, when asked, said the sun was blinding white. Blinding is the correct term as the intensity blinds one's ability to see the true color.



[/URL]


nasa needs to re vamp all of the star colors..and the o.a.f.g.k.m.
that is out of date...and look at the stars by there real colors.

re vamp the h and r charts..make new ones with the right colors..

what a mess!!!
Color is more of a subjective term to science. I can not think of a serious scientific equation where a term requires the observed color. :)

Nevertheless, it is a mystery and I like it!

You might like my semi-serious formulae for color determination found [URL="http://www.bautforum.com/showpost.php?p=154888&postcount=44"] here (http://www.bautforum.com/showpost.php?p=154888&postcount=44).

If you will, take a quick glance at your midday sun and see if you can tell its color. Then go to a poll (http://www.bautforum.com/showthread.php?t=20208&highlight=sun) and let cast your view (pun intended). :) Thanks.

jon.clovis
2006-Jan-21, 06:15 AM
well then why call it yellow..it gos back to the past..working colors.
at noon its white..in space its white-blue.
no one can get this right. solar astromoners will tell you its white.
this why it will never change..people want to see yellow.
look up and tell me its yellow.and if you see yelow you need to have your eyes look at.this is chazy..maybe in 100 years it will change.

color is a subjective to science.

ok..science likes to call the sun yellow.
call it black and white..

George
2006-Jan-21, 09:21 PM
well then why call it yellow..it goes back to the past..working colors.
at noon its white..in space its white-blue.
no one can get this right. solar astromoners will tell you its white.
this why it will never change..people want to see yellow.
look up and tell me its yellow.and if you see yelow you need to have your eyes look at.this is chazy..maybe in 100 years it will change.
Ug. You just pointed out an obvious term I am missing in that colorful formula I referenced in the last post - the tradition of everyone calling it yellow.

There is a very cheap insturment you can make, which I call SAD (Solar Attenuation Device). :) It will allow you to reduce the intensity of direct sunlight in order to observe more accurately the sun's color. Take a paper plate, cut a very thin and clean slot about 2 cm's long, and several cm's from the outer rim. Penetrate the very center of the plate with the eraser end of a pencil without warping the plate (cutting the hole out perferably). Glue both sides of the plate and pencil junction. Spray paint the plate flat black. When glue has completely set, put the pencil in an electric drill. Aim it at the sun such that you observer the sun through the slot during fast rotation.

The SAD will help render the sun's color, but not its true intrinsic color as determined from space, and without all our atmospheric color depletions. Those that see it as yellow around midday should be surprised at how white it is instead.

jon.clovis
2006-Jan-22, 06:47 AM
its true color in space is white to-blue..
as seen by the space shuttle crews.
with there dark gray sun glasss.
an on earth its white.all you do is look at the white clouds.
or make a hole in a paper let the white light hit a white paper..it looks white.
snow is white . your sad looks white.

now thuis time at 1200 noon.
it may look a little white-yellow.

i dont see it..and my eys are 20 20.
i am not color blind.

look at the sun through a telescope.

let the light hit a white paper.it is white.

that is why it is called a white light.

solor astronomers`take photos in white light. not yellow light.
so whats the matter with people .

they will say it white on paper and then say its yellow to ther eyes.
its the same light!

what a mess!

out in space it looks blue white as most blue light it hitting our eyes .

and makes at look whiter.

the suns is blue-white. not yellow.and never has been.
it all the press..and a mess up science.
jon

jon.clovis
2006-Jan-22, 06:58 AM
i take a lot photos..
and photo people and video people,
well tell you that at 12:00 noon on june 21..at 40 dreegs
that the suns color temp..is about 5500k this is very white to blue
very white.at sea lev. at 40,000 feet its 5,600k and verry white to blue
out in space its about and heres the big pont..its temp gos from 7,500k
to 6500k. whit to blue. as the black body says.

but they cant get this right..come on what a mess!!

George
2006-Jan-22, 03:08 PM
its true color in space is white to-blue..
as seen by the space shuttle crews.
with there dark gray sun glasss.
Really! I would love to see their account. Can you link me to it?


an on earth its white.all you do is look at the white clouds.
or make a hole in a paper let the white light hit a white paper..it looks white.
snow is white . your sad looks white.
All good points. However, the atmosphere does play with us, the sky is blue then yellow, orange, and red around sunset. Some clouds are pink and some stormy ones are green at times. So, questioning the sun's color has merit.


now thuis time at 1200 noon.
it may look a little white-yellow. A slight majority around here will agree with you, based on the little poll results.


they will say it white on paper and then say its yellow to ther eyes.
its the same light!
Since paper can be modified in color appearance, it could be argued modern paper is now white because that's what people purchase, and not the color it would appear if it reflected sunlight evenly across the spectrum.

The very best way to determine the sun's color, other than a SAD used in space, is to replicate the sun's spectral irradiance and observe it. This was done last summer thanks to BAUT folks here.


what a mess! :) But it's a colorful one. ;)

jon.clovis
2006-Jan-23, 06:45 AM
ok so whats the color??in space.
and what color is the sun from earth to you.
i just want to know what you thank!

jon.clovis
2006-Jan-23, 06:57 AM
you thank its green.
ok i like green white.
as you will you never see that color.
and there are no green stars.
it will never happen.
they will go for yellow or white stars not green.
no matter ifyou can turn down the sun 99.99%
it will look white. its r g b as long as you got red blue green.
it will be white.pull out a little blue its white.
pull out a little red its white.

BertL
2006-Jan-23, 07:37 AM
you thank its green.
ok i like green white.
as you will you never see that color.
and there are no green stars.
it will never happen.
they will go for yellow or white stars not green.
no matter ifyou can turn down the sun 99.99%
it will look white. its r g b as long as you got red blue green.
it will be white.pull out a little blue its white.
pull out a little red its white.
I've seen a red sun often enough to know you're completely wrong.

George
2006-Jan-23, 02:58 PM
you thank its green. I originally had hope that it was green, but after developing a device (SPACC), with the help of BAUT, to determine the sun's color, as found in this thread (http://www.bautforum.com/showthread.php?t=17237), the color is very likely white or bluish white.


no matter ifyou can turn down the sun 99.99%
it will look white. its r g b as long as you got red blue green.
it will be white.pull out a little blue its white.
pull out a little red its white.
Only if the propotions of r g b are about equal will white be seen. [Unless the r g b levels all exceed the eye's upper threshold, as it does when one is in space.] The sun in space radiates much more blue and green than red. However, out atmosphere greatly cuts down the blue and some greens, thus the balance produces a white appearance for us down here.

jon.clovis
2006-Jan-23, 08:27 PM
as i said its white to blue in space.
and a white on earth.
not yellow.
but we got to get a.
ll people to see it that way.
as they look up and see a yellow.

as you said that for most people its above ther heads.

as far as it looking red i hope its a sun set.

sorry to get so upset..but ..

it upsets me when i see it look yellow in mazs..and books.

they are telling people its that color..thats wrong.

it keeps this thing going.. i guess its hard to show it white on white paper.

my guess is they like it way.

jon.clovis
2006-Jan-23, 08:33 PM
but to the eye its white to blue..
a camara will see it white..now people
will see it white some will see a little blue.
can you turn the sun down?
to see the blue? so it gos back to white.
mybe at 4 au it might look a little blue?

i kown i am beeting a dead horse.

jon.clovis
2006-Jan-23, 08:39 PM
you know that some people say its blue to white here on earth.

i see white... may be i am see things..

George
2006-Jan-23, 08:51 PM
you know that some people say its blue to white here on earth.

i see white... may be i am see things..
White is how I see it during midday. I suppose yellow is the more poetic view of it. I do not believe anyone has provided enough evidence to nail down the sun's true intrinsic color yet.

jon.clovis
2006-Jan-23, 09:43 PM
this all comes back to working colors.
that been around for 80 years.
in astro.
the colors we have are vega blue , blue white , vega white , vega red.

the sad thing is that for 80 years its been out there..

an old school techer told me that it will take years to change to the
right color..(white)

that this info..has been out ther for years..and most solar people know it.

but can not change it. as there is to much to change.

everbody must come on board..

so it will stay the same..the crazy yellow.even if the temp is 6500k
white blue..

jon.clovis
2006-Jan-23, 09:46 PM
its been 80 years in all that time its yellow.
people like it..
in astro and out.

yelow fire red. not white blue veeey hot.

Glutomoto
2006-Jan-28, 08:23 AM
i kown i am beeting a dead horse.


This is how I like to beet my dead horse

Westphalian Sour Roasted Horse (http://www.theiling.de/recipes/sauerbraten.html)

Recipe: Creamed Beets (http://www.massrecipes.com/recipes/98/02/creamedbeets292064.html)

mmmmmmmmmmmmmmmm good.

:)

Roy Batty
2006-Jan-29, 04:39 PM
This is how I like to beet my dead horse

Westphalian Sour Roasted Horse (http://www.theiling.de/recipes/sauerbraten.html)

Recipe: Creamed Beets (http://www.massrecipes.com/recipes/98/02/creamedbeets292064.html)

mmmmmmmmmmmmmmmm good.

:)

LOL!
Hmm, question to the moderaters; should this thread now be moved to a different category? Every few days I check when it's updated only to find the same ole sun thang :)

jon.clovis
2006-Feb-27, 10:19 AM
white at noon white at 3:oopm
white at 9:ooam. white.
in space white/blue.

Roy Batty
2006-Feb-27, 02:50 PM
Heh, it lasted almost a month before being resurrected again:D

Jon.clovis, you need to replace the 'o'hs with zeros to negate the smilies:)

George
2006-Feb-27, 03:24 PM
It is still a bit of a mystery.

Some say, viewed from space, it is peachy pink (http://www.vendian.org/mncharity/dir3/starcolor/sun.html), though it is traditional to consider it yellow, as in the H-R diagrams and other color tables. The lighting industry shows bluish white for objects as hot as the sun. The limited heliochromometer (http://www.bautforum.com/showthread.php?t=17237) says it is white, possibly bluish white (as viewed in space).

The terrestrial view sees it more as yellow-white based on the BAUT poll (http://www.bautforum.com/showthread.php?t=20208&highlight=midday). BTW, please vote if you haven't....
.....I want your colorful VOTE!! [insert Uncle Sam pointing here] :)

Thomas(believer)
2006-Mar-02, 12:19 PM
When reading about this subject, I have to think about the way we perceive sound. Some tones get suppressed relative to others when you increase loudness. The reason they put that loudness button on your amplifier. Also frequenties get mixed in our ears and frequencies can be heard which are not coming from the source.
The ear is a non-linear instrument. I don't know how this exactly works for the eye. There are some links mentioned in this thread, but no time yet to read all in detail.

George
2006-Mar-02, 02:03 PM
That is a fair analogoy. The color of objects is usually a mixture of wavelengths; what we hear is a mixture, too. I would bet people will vary in their hearing perception much more than their variance in color perception, however. The dimensions within the ear may greatly affect the strength of certain wavelengths, whereas the eye would seem less susceptible.

Nevertheless, there does seem to be some differences on how one sees very intense light such as the sun. That appears to be one conclusion we can draw from the poll, as some see a white midday sun and others see more yellow.

ZaphodBeeblebrox
2006-Mar-02, 04:27 PM
That is a fair analogoy. The color of objects is usually a mixture of wavelengths; what we hear is a mixture, too. I would bet people will vary in their hearing perception much more than their variance in color perception, however. The dimensions within the ear may greatly affect the strength of certain wavelengths, whereas the eye would seem less susceptible.

Nevertheless, there does seem to be some differences on how one sees very intense light such as the sun. That appears to be one conclusion we can draw from the poll, as some see a white midday sun and others see more yellow.
What'll Be Even Weirder, Is If we Ever Discover Creatures, Who Evolved Under a Different Sun ...

In The Latest Book, In his Worldwar/Colonization Series, Harry Turtledove Reveals The Main Aliens, a.k.a. The Race or The Lizards, See TWO Colours In The Infra-Red, And Can't See Violet to Boot ...

Moreover, One of their Subject Species, The Halessi, Come From a Planet With an Even Redder Sun, And See a Further, Additional Colour in The Infra-Red, And Although it's Not Stipulated, Probably Can't Even See Indigo, Either!

eugenek
2006-Mar-02, 05:22 PM
In The Latest Book, In his Worldwar/Colonization Series, Harry Turtledove Reveals The Main Aliens, a.k.a. The Race or The Lizards, See TWO Colours In The Infra-Red, And Can't See Violet to Boot ...

Hmm, so violet leisure suits would be a good option for night time covert ops? However, in the last book I read of the series I believe they were still swapping nukes in the 1940's so I guess the leisure suit hadn't become style yet.

ZaphodBeeblebrox
2006-Mar-02, 08:14 PM
Hmm, so violet leisure suits would be a good option for night time covert ops? However, in the last book I read of the series I believe they were still swapping nukes in the 1940's so I guess the leisure suit hadn't become style yet.
Probably ...

You REALLY Should Read, The Next Four Books In The Series ...

It's Now 2031, Humans have Reached The Planets of The Star Tau Ceti, And Advances In String Theory, Are Beginning to Yield Interesting Experimental Results!

:think:

George
2006-Mar-02, 11:32 PM
What'll Be Even Weirder, Is If we Ever Discover Creatures, Who Evolved Under a Different Sun ...

In The Latest Book, In his Worldwar/Colonization Series, Harry Turtledove Reveals The Main Aliens, a.k.a. The Race or The Lizards, See TWO Colours In The Infra-Red, And Can't See Violet to Boot ...
I read where white tail deer are dichromatic (two color cones). They can see blue and slightly into the uv range with one, and, green and yellow with the other, IIRC. This plus their larger eyes, should allow them to see all the deer hunters as they sneak into their deer blinds. I don't know if these have evolved from unnatural selective pressure. :)

HenrikOlsen
2006-Mar-08, 05:22 PM
The way I see it, we've evolved to see "white" things illuminated during the day as "white".
As they are illuminated both by the sun and the blue scattered light from the rest of the sky, the color we percieve as white is the color of the sun with some blue added, which is why the sun itself looks slightly yellow.

Had we evolved under a different star, or in an atmosphere with a different composition, I believe we'd have had an intensity curve experienced as white, but it would be very different from the one here.

George
2006-Mar-08, 11:13 PM
The way I see it, we've evolved to see "white" things illuminated during the day as "white".
As they are illuminated both by the sun and the blue scattered light from the rest of the sky, the color we percieve as white is the color of the sun with some blue added, which is why the sun itself looks slightly yellow.
Evolving to a white appearnce for daylight is interesting and makes sense.

However, surprisingly, the sun radiates with peak intensity in blue. The atmosphere does take about half of the blue, not just scattering effects though. The energy level for each wavelength (spectral irradiance) is almost flat when observing the sun from the surface.

It is also interesting to see a flat curve for the sun in space if the curve plots the distribution of photons (using E=h*nu).

KingNor
2006-Jun-05, 03:13 PM
i think most kids draw the sun as yellow because when the sun starts to set, its easyer to look at, yet isn't casting crazy colors everywhere like it does when it's low enough to be red.

Even a little kid i think wouldn't beable to look at the sun at it's peak for long enough to think about what color it is, it just flat out hurts to look at it like that.

when kids see the sun lower in the sky, easyer to look at and beign turned yellow by going through the atmosphere, thats probably where they get the idea, not to mention all the "yellow" suns in marketing and their parents saying "here is a yellow crayon for drawing the sun!"

George
2006-Jun-10, 01:33 AM
I agree, the sun is much more observable near the horizon, not only because of the pain factor but because it is so hard to not see it. Sunsets are often quite attractive.

The crayon factor is also very likely.

You might enjoy the master formula (http://www.bautforum.com/showpost.php?p=154888&postcount=44). :)

Ronald Brak
2006-Jun-10, 02:09 AM
Why not check to see if children colour the sun red in Japan?

Anyways, the sun is white. At least it is where I am here in Australia. It typically it gets a bit of a yellow tinge when it goes down. But usually that's just a tinge. I don't own a yellow crayon light enough to draw it. I imagine that a lot of people in the Northen hemisphere look at the sun through a lot more air and filth than here. I know in Japan the setting sun often looked like a filthy wobbly orange thing.

George
2006-Jun-10, 03:34 AM
Why not check to see if children colour the sun red in Japan?

Interesting question. Perhaps the drawings they color only have a rising sun? :)

Anyways, the sun is white. At least it is where I am here in Australia. It typically it gets a bit of a yellow tinge when it goes down. [/quote]
I agree with the white sun appearance, though others see a midday sun as yellow-white. I would have guessed a dusty sunset would produce a orange-red sun for ya'll. A clean atmosphere is not thick enough in scattering material to produce a redish sun; dust or other material must exist in the sky, too.

Champion_Munch
2006-Jun-12, 12:03 PM
I've always seen the sun as just white.

with regards

derick
2006-Jun-12, 01:22 PM
http://home.case.edu/~sjr16/advanced/sun_ourstar.html

Try this link...

George
2006-Jun-12, 03:07 PM
http://home.case.edu/~sjr16/advanced/sun_ourstar.html

Try this link...
This site is a good example why there is much confusion as to the sun's color.


For the sun, this works out to 5014 &#197;, which is in the green part of the visual spectrum. Therefore, the sun's true color is green.
Assuming 5014 Angstroms is correct for the peak intensity of the solar spectrum, and assuming the peak color determines what color the eye observes, it still would not be green. A monochromatic beam of light at 5014 Angstroms would appear cyan in color (blue green). It is possible, that the 5014 Angstrom value is wrong, too. I get about 5,850K as the Planck curve temperature by matching the sun's known spectral irradiance (AM0) to a Planck BB curve at 5850K. If this is correct, then the peak wavelength is 4954 Angstroms (blue). The actual spectral irradiance happens to have two peaks deeper into the blue end of the spectrum.

From the Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html) solar irradiance graph, you can see the two peaks are located between 4500 and 4900 Angstroms. [I do not have an explanation for these, but they are there and are the actual peaks.]

The other assumption the article seems to make (that the color at peak intensity would produce the color the eye would behold) is likley false. [Albeit, there are times when the peak intensity does produce the color seen by the eye, but not in this case, IMO.] The eye/brain will combine all the colors it sees and then tell you the color. The other colors will blend together to produce a result. This is how a television scrren, or monitor, produces such a wide range of color - by combining the three color dots, each at different intensities. If the only color you saw was the strongest color dot illuminated, you would see everything as either red, green, or blue (using an RGB system), which we don't, of course.


However, if you look at it (which you should never do, for it can cause severe eye damage and in some cases blindness), the sun appears yellow. It only appears yellow when the sun is close to the horizion due to the "bleaching", or removal, of more of the shorter wavelengths. This means the remaining longer wavelength colors will begin to dominate your view of the sun, which is why the sun, when closer to the horizon, appears orange and, in some cases, red.


No one has yet successfully explained why the sun appears to be yellow instead of green. Not to worry, heliochromology is still colorfully ramping-up to meet the needs of the world. ;)

The very first article I read regarding the sun's true color claimed the sun was green. But, it is clear to me now, that it is not green because of the way the eye blends all the colors together, and because the peak isn't green, as I stated before.


However, an interesting fact is that our eyes are best able to see green light at approximately that wavelength.
This is correct, our eyes are more receptive to a light shade of green over any other color. You may have noticed emergency vehicles now being this color for this very reason. However, it does not cause the sun to appear green. The eye has three color cones, but they each see more than one color. The combined sensitivity of each gives light green the advantage, apparently.

cjl
2006-Jun-13, 06:32 AM
I have to say that even at the horizon, it is a rare occurence that it looks anything but white to me.

George
2006-Jun-13, 12:23 PM
Perhaps your altitude makes a difference. The change in color of the sun requires several air masses to scatter enough of the shorter wavelength colors. Dust and other small particles greatly increase scattering, too. If mountains block a normal horizon view of a setting or rising sun, this would further decrease your chances for seeing color changes.

cjl
2006-Jun-16, 05:23 AM
True - the only time I see the sun truly at the horizon is in the morning - when it is setting, the mountains block it from about 3 degrees above the actual horizon. This might make some difference in appearance.

George
2006-Jun-16, 12:27 PM
True - the only time I see the sun truly at the horizon is in the morning - when it is setting, the mountains block it from about 3 degrees above the actual horizon. This might make some difference in appearance.
Though there is less dust in the air at sunrise, I would still think you would see a yellow rising sun. Is it usually white, or yellow-white (and not yellow)?

cjl
2006-Jun-16, 09:14 PM
White, every single time I can remember (in normal conditions). It was definitely white this morning...

Can't remember ever seeing it yellow, though I can remember once (when there was a lot of smoke in the air from a forest fire) that it looked orange.

George
2006-Jun-17, 04:15 PM
That is surprising. What color is the adjacent sky, during sunrise? Are you over a mile high?

cjl
2006-Jun-18, 01:56 AM
Adjacent sky is usually blue at sunrise. I am over a mile - 5600 feet actually.

George
2006-Jun-19, 12:23 AM
Interesting. Perhaps the altitude makes the difference.

Ronald Brak
2006-Jun-19, 12:37 AM
I watched the sun raise here in South Australia and it looked deep yellow. I looked at the clouds to see what colour they were and they were tan. There was a rim of pink haze all around the horizon. It didn't take long for the sun to appear white, once it got a little higher, however. (Not that I could look at it directly however.)

George
2006-Jun-19, 12:59 AM
That sounds normal. Are your sunsets orange or yellow-orange?

Ronald Brak
2006-Jun-19, 01:11 AM
Usually yellow, although I haven't paid attention lately.

George
2006-Jun-19, 09:29 PM
Allow me to be a bit more colorful. These were taken in sequence through an 8" SCT during sunset (no filter due to clouds and haze, surprisingly). The first was when the sun was several degrees above a flat horizon. The second when about 1/2 degree (the diameter of the sun).

http://img235.imageshack.us/img235/4656/sunset13be.th.jpg (http://img235.imageshack.us/my.php?image=sunset13be.jpg)

http://img270.imageshack.us/img270/1879/sunset23cf.th.jpg (http://img270.imageshack.us/my.php?image=sunset23cf.jpg)

http://img270.imageshack.us/img270/478/sunset31mh.th.jpg (http://img270.imageshack.us/my.php?image=sunset31mh.jpg)

The colors are quite representative of what I beheld.

ebbixx
2006-Jun-20, 11:11 AM
"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?"

Ummm, because we're talking about additive color mixing here (as happens with stage lights, or any other combined light sources, where yellow+cyan=white, not green)? If we were talking about paint, or color theory as applied to reflected light, then a predominance of green would "look" green.

Since we're talking additive mixing, though, any other colors mixed in with the green will tend to shift our perception of the color closer to white. (A perfectly balanced mix, would appear white, give or take some anatomical quirks or filtering properties of the atmosphere.

http://www.yorku.ca/eye/colormx2.htm

has one simplistic illustration of the difference between additive and subtractive mixing.

George
2006-Jun-20, 11:28 PM
Ummm, because we're talking about additive color mixing here (as happens with stage lights, or any other combined light sources, where yellow+cyan=white, not green)? If we were talking about paint, or color theory as applied to reflected light, then a predominance of green would "look" green.
Yes, it is a matter of degree; a slight majority of one color may, or may not, alter the color observed.

I am not all that knowledgeable on color theory, however. You must be quite diligent to have read much of this thread, so I will assume you do know something about color theory.

It would be nice to know if the number of photons registering in our color cones is a more refined approach to color determination. Since blue has almost twice the energy of red, it would take almost twice as much intensity of blue to match red; thus, in this example, the photon flux would be the same for these two colors. Unfortunately, as logical as it sounds, the eye has very few blue receptor cones compared to the green and red ones. Also, the spectral response of each of the three color cones will overlap into other color zones. Then the brain processes all this with its own pecularities. The end result is compounded complexity. :)

Nevertheless, perhaps an even flux of photons at each wavelength would produce an optimum white. If so, it would support further the appearance of a white sun when seen beyond the horizon.

PhantomWolf
2006-Jun-26, 04:21 AM
Well it looks pretty yellow in my rear vision mirror driving from home to work and then back afterwards at the moment.

George
2006-Jun-26, 12:45 PM
Well it looks pretty yellow in my rear vision mirror driving from home to work and then back afterwards at the moment.
The additional atmosphere its light travels through scatters the blue end of the spectrum more than the red end. For dusty atmospheres, the sun can look red.

The old sailor warning, "red in the morning, sailor taking warning" would not get busted on Mythbusters.

PhantomWolf
2006-Jun-27, 01:29 AM
I already knew that, was just pointing out that when it is low, and that's the time most people notice, it is very yellow, infact the light was yellow enough that standard silver coloured aluminium siding appeared gold when I saw it last night on my way home.

George
2006-Jun-27, 09:57 PM
I already knew that, was just pointing out that when it is low, and that's the time most people notice, it is very yellow, infact the light was yellow enough that standard silver coloured aluminium siding appeared gold when I saw it last night on my way home.
That is when it is very strong in yellow, but is favored when it is reflected of objects such as aluminium siding. Often, when the sun is close to the horizon and appears yellow, a white object will still look white. Our brains have an automatic white balance control system to it (like nice cameras have).

I encourage anyone to try looking at a white object when the sun is yellow. White objects rarely look yellow.

bronzeman
2006-Jul-05, 03:46 PM
sometimes the Sun appear blue.

George
2006-Jul-05, 04:23 PM
sometimes the Sun appear blue.
When do you see it as blue?

Click Ticker
2006-Jul-12, 08:33 PM
When do you see it as blue?

When you stare at it long enough that it burns an after image on your retina. Of course then you see that blue dot everywhere you look. Or so I've heard.

ozark1
2006-Aug-17, 11:00 AM
The answer is that the Sun appears yellow because the sky is blue. If you don't believe this have a look at illusion 1 here:

http://www.echalk.co.uk/amusements/OpticalIllusions/colourPerception/colourPerception.html

The cross in the centre (which is grey) appears yellow when the surroundings are blue.

George
2006-Aug-17, 02:51 PM
The answer is that the Sun appears yellow because the sky is blue. If you don't believe this have a look at illusion 1 here:

http://www.echalk.co.uk/amusements/OpticalIllusions/colourPerception/colourPerception.html

The cross in the centre (which is grey) appears yellow when the surroundings are blue.

Nice link. The first illusion is excellent and probably does have some affect on those who claim the sun is yellow while high in the sky. A slight majority favor a yellowish-white sun based on a poll conducted here. Hmmm, maybe it's time to bump it and get more opinions.

However, notice their error in the third illusion. If you mask the two center squares, one is lighter than the other.

pghnative
2006-Aug-18, 08:53 PM
However, notice their error in the third illusion. If you mask the two center squares, one is lighter than the other.Not true. Drag a copy into photo editing software, then drag a portion of one square next to the other. They're the same, by my eyes.

George
2006-Aug-18, 10:43 PM
Not true. Drag a copy into photo editing software, then drag a portion of one square next to the other. They're the same, by my eyes.
How do you drag it?

I went ahead and made a physical mask, and still notice the lower square is distinctly lighter. This could be a monitor issue, although if they are the same, it shouldn't matter, I suppose. Either way, the color illusion is still valid, just not as vivid a difference if I am correct.

pghnative
2006-Aug-18, 11:20 PM
How do you drag it?

I went ahead and made a physical mask, and still notice the lower square is distinctly lighter. This could be a monitor issue, although if they are the same, it shouldn't matter, I suppose. Either way, the color illusion is still valid, just not as vivid a difference if I am correct.
I simply used Microsoft Photo Editor and used the cut tool. This slices a rectangle that one can drag around the screen.

Due to data compression (presumably) when I expanded the screen shot 200%, each square was a blend of at least two colors, but the mixtures looked the same when placed next to one another.

George
2006-Aug-19, 04:52 AM
That tool sounds handy. I don't have Photo Editor, though.

Here is my favorite illusion (http://www.michaelbach.de/ot/lum_adelson_check_shadow/index.html).

94z07
2006-Aug-19, 06:27 PM
That tool sounds handy. I don't have Photo Editor, though.

Here is my favorite illusion (http://www.michaelbach.de/ot/lum_adelson_check_shadow/index.html).

Here's a small collection a friend pointed me to years ago. (http://www.tekzoned.com/weird/)

George
2006-Aug-19, 06:55 PM
Here's a small collection a friend pointed me to years ago. (http://www.tekzoned.com/weird/)

Excellent! :clap: There were many I had not seen before. The two triangles took me a minute.

Aries_7
2006-Dec-08, 02:22 AM
couldnt it be just the right combination of an optical phenomena where one sees yellow whent the surroundings are blue, and also the particular wavelength that the sun emits colour in?

George
2006-Dec-08, 03:17 PM
I suspect the surrounding blue can be a factor. Also, it may be the outer edge of the sun would actually appear yellow if isolated from all other light. This is because only the upper atmosphere of the limb (outer edge) is all we can see and it the upper atmosphere of the sun is cooler. Thus, we will receive more greens, yellows, and reds than in we see in the central disk regions. The temperature difference we see at the limb is around 5000K vs. the central region of 6390K.

However, the worlds largest solar telescope (McMath-Pierce at Kitt Peak) reveals the disk to be all white. But, I need to be sure they use no filter. Another possibility is the fact they are over a mile above sea level which reduces the loss of blues due to atmospheric extinctions.

Michael Huster
2007-Feb-15, 02:40 AM
I'm not willing to concede the sun is yellow. I challenge my students to look at it. It's white!

George
2007-Feb-15, 02:22 PM
I'm not willing to concede the sun is yellow. I challenge my students to look at it. It's white!
Welcome to BAUT, Michael. You are sounding like a future heliochromologist already! :)


There is a poll (http://www.bautforum.com/showthread.php?t=20208) regarding how others see the sun's color which you might enjoy. Please place your vote, too.

Here (http://www.bautforum.com/showthread.php?p=762745) is a post that includes a picture of an unfiltered solar projection taken at the world's largest solar telescope: the McMath-Pierce at Kitt Peak. It sure looks white to me. :)

Serious Warning: The rods and cones within the eye are easily damaged by excessive direct sunlight. Using a solar projection via a pinholed sheet is the safest.

Question
2007-Apr-26, 04:21 PM
at 15, I'm pretty young to be posting a comment, but if the sun looks white when we stare up at it mid-day, and then looks yellow in a sunset, then I believe the question really is, what is different? We know the sun didn't just change colors, and we know that the atmosphere is the same everywhere. So why is it different? :eh: I'm a little confused.

George
2007-Apr-26, 05:04 PM
There's no question like a good question, and you are one. :)

Welcome to the board.

You are on the right path in finding the answer. Think a little more on what is really different. Perhaps imagining the Earth at a distance where you can see land, atmosphere, and sunlight going to an observer when the sun is overhead and when it is on the horizon for the same observer will help you see better. Is there anything you see different?

Question
2007-Apr-26, 07:52 PM
I guess I notice that there is more atmosphere that the observer is looking through when it's on the horizon, then when it's strait up. your eye path to the sun encounters less atmosphere, therefore it's a possibility that it has something to do with how much atmosphere is in the way of the sun. Hmm. that's a good point. :think:

Question
2007-Apr-26, 08:33 PM
The atmosphere is filled with minute particles called aerosols. These particles are of about the same size as the wavelength of visible light and scatter light at the short 'blue' wavelengths' a bit better than they do the longer-wavelength 'red' light. This means that as we look towards the sunset on the horizon, our path through the atmosphere intercepts lots of this aerosol material which preferentially scatters the blue component of the incoming sunlight out of our line of sight. The light is then reddened. As we look up towards the zenith over head, we are not seeing the sunlight coming directly from the Sun, but are seeing the light scattered by the aerosols at large angles to the line between the aerosol particle and the Sun. This light is blue because it contains little if any red light which is not scattered as well by the aerosols. The rate at which blue replaces red light as you go from the horizon to the zenith at sunset and sunrise can be used to determine the optical properties of the aerosols and their size distribution. I think that basically sums it up. but if there's something wrong, please let me know. :dance: also, I need help with finding out how to post a new thread. could somebody please help?

George
2007-Apr-26, 09:15 PM
I guess I notice that there is more atmosphere that the observer is looking through when it's on the horizon, then when it's strait up. your eye path to the sun encounters less atmosphere, therefore it's a possibility that it has something to do with how much atmosphere is in the way of the sun. Hmm. that's a good point. :think: Excellent logic, and that is the answer. The real color credit belongs to our atmosphere and not the Sun itself. Though I would bet most people do not know this....yet. :)

Looks like from your last post you have quickly studied this issue. How much aerosols play a role, I can not say, but the N2 and O2 molecules (representing the majority of our atmosphere) are small enough, obviously, to allow what is known as Rayleigh Scattering, or Tyndal-Rayleigh Scattering, or just elastic scattering. It was discovered that when particles are a good bit smaller than the wavelength of light, that light will take off in all different directions (scatter) without loss of energy, so they maintain their "color". Also, the amount of scattering varies as to the 4th power of the wavelength (actually an inverse relation). The wavelength of red light is about 1.5x that of blue (depending on which part of red is compared to which portion of blue). So, (1.5)^4 = 5, roughly, which is the number of times blue light will scatter more than the red light. At the extremeties of their color range, the scattering is closer to 9x, I think.

The result is as you stated, removing more of the blues and greens and leaving more of the yellows, oranges, and reds, will first produce a yellow result followed by an orange result (rarely a red result unless the atmosphere is loaded with smoke particles from fires or volcanoes).

So, we know the Sun is not yellow. Surprisingly, the true color of the Sun is not fully defined.....yet. :) [It is mostly white, but there is a chance it is a little blue in the middle portion of the disk. This assumes the observer is in space and is seeing it at a very reduced intensity to allow normal photopic vision.]

George
2007-Apr-26, 09:22 PM
also, I need help with finding out how to post a new thread. could somebody please help?
Once you are in any given forum, above the list of threads you will see the New Thread button. That will do it. [It may be hard to correct any error in the thread title, so check it twice before establishing it.]

schrodingers-dog
2007-Nov-16, 04:11 AM
This may be far more simple, perhaps we see the sun as yellow since its against a blue sky, we compair the two colours automatically, the same way a cup of tea looks darker in a white mug and seems lighter when put into a dark mug. Im guessing some would think that the same would happen to clouds but i spose clouds are a lot bigger.

This may add to the effect but in my opinion the best lead so far is the idea that we are mearly 'seeing' a yellow after image of the sun burnt into our retinas.

George
2007-Nov-16, 04:59 AM
This may be far more simple, perhaps we see the sun as yellow since its against a blue sky, we compair the two colours automatically, the same way a cup of tea looks darker in a white mug and seems lighter when put into a dark mug. Im guessing some would think that the same would happen to clouds but i spose clouds are a lot bigger.But some clouds are small and they are not yellow. We did a poll that indicated a slight majority see the Sun at midday as yellowish white. It may be due to the color contrast issue you present. I only see it as white.

You could mask-off the surrounding blue sky and see what happens. I would be a little surprised if it appeared differently.

PooprScooper
2008-Aug-01, 04:11 AM
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.

My first post here, hi guys. :hand:

I'm not sure why Phil would say he doesn't know; I thought we were pretty sure that Rayleigh scattering explains why the sky is blue and why the sun is yellow. Just like why it's red at sunset; more of the red light is scattered toward you than blue.
"After having crossed so much air, most of the blue light is scattered out, as well as most of the green. This leaves the red, yellow and orange colors free to paint their pictures of fiery sunsets and hazy moons."

The molecules don't scatter green as much do to their size compared to the wavelength but they do more so to blue and orange-red wavelengths.

And yes our eyes see green light more prominent than any other color, (yes we do have a green sensitive pigment) just like we hear the most at 1khz which is why commercials always sound so much louder than the show(because they compress it to the 1khz range).

I'm confused why there is still confusion about the color of the sun. If you see pictures of the sun taken from astronauts in space, it appears white.

Whirlpool
2008-Aug-01, 04:16 AM
Welcome to BAUT Pooprscooper.

George
2008-Aug-01, 12:56 PM
My first post here, hi guys. :hand: Welcome, and you picked one of my favorite topics. :)


I'm not sure why Phil would say he doesn't know; I thought we were pretty sure that Rayleigh scattering explains why the sky is blue and why the sun is yellow. Just like why it's red at sunset; more of the red light is scattered toward you than blue.
"After having crossed so much air, most of the blue light is scattered out, as well as most of the green. This leaves the red, yellow and orange colors free to paint their pictures of fiery sunsets and hazy moons." Nice quote, where did it come from?

You are correct about atmospheric scattering, but I suspect Phil was talking about why the Sun appears yellow to many while it is overhead, which is when it is more than 1000 times as bright than when it is on the horizon.


The molecules don't scatter green as much do to their size compared to the wavelength but they do more so to blue and orange-red wavelengths. Yes. Rayleigh Scattering gives the best description of scattering behavior in our atmosphere. It shows that, in general, the scattering varies as to the inverse fourth power of their wavelength. Since blue ranges from about 460nm to 480nm and red ranges from about 630nm to 760nm, then blue photons will scatter from 3 to 7.5 times more than the reds. [Depending on circumstances, these color bands will vary a little.]


And yes our eyes see green light more prominent than any other color, (yes we do have a green sensitive pigment) just like we hear the most at 1khz which is why commercials always sound so much louder than the show(because they compress it to the 1khz range). Nice comparison. It is also why many emergency vehicles are light green.


I'm confused why there is still confusion about the color of the sun. If you see pictures of the sun taken from astronauts in space, it appears white. I assume these images only include the Sun that was in the background and not a picture for the purpose of capturing the Sun's image. Thus, it is very likely that the Sun's image was overexposed. Any bright object that has enough reds, greens, and blues will cause the overexpsoure to appear white. If the Sun were yellow, it would still appear white in the image since the Sun also emits the other colors, too.

But the Sun ain't yeller (http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/blog/the_color_of_the_sun). It is white.

hhEb09'1
2008-Aug-01, 01:22 PM
If the Sun were yellow, it would still appear white in the image since the Sun also emits the other colors, too.When I use my solar filter on my telescope, the sun is a deep yellow. Black polymer filters do that, aluminized mylar gives a bluish tint to the sun image.

George
2008-Aug-01, 02:47 PM
I think my filter is a Baader filter and it yields a very nice yellow, too. I tried to find a true neutral filter, but could never find one. It seems likely that no such filter exists, unfortunately. If one does, then we could simply stack them and hand it to the next astronaut going up and settle this color conundrum.

BTW, if you get a chance, try that link I gave and let me know what you think. [The corn there runs in fairly deep furrows. :) ]

sabianq
2008-Aug-01, 03:53 PM
if anyone is interested:
Dick Plano, Professor of Physics emeritus, Rutgers University has this to say about the color of the star Sol (our sun)
http://www.newton.dep.anl.gov/askasci/phy00/phy00940.htm


The sun as seen in outer space is quite white. It emits different
wavelengths with intensities given by the law of blackbody radiation. It
may feel a little strange to think of the sun as a black body. It just
means that the sun absorbs all the light striking it and is an equally good
radiator of electromagnetic radiation. (Good absorbers must also be good
radiators or they would get hotter than their surroundings).

The maximum intensity of light emitted by black body radiation is at a
wavelength given by the Wien Displacement Law which says: Wavelength of
maximum intensity in meters times temperature in Kelvin = 0.0029.
Since the temperature of the surface of the sun is around 5800 K, the sun
emits light of wavelength
500 nm with the greatest intensity. Since visible light extends from around
400 nm (purple) to 700 nm (red), all of which are emitted by the sun with
almost equal intensity, the sun looks white to our eyes.

In passing through our atmosphere, the shortest wavelengths (blue /purple)
are scattered most strongly. This answers one of the best known questions
in all of physics: "Daddy, why is the sky blue?" It also explains why
sunsets (and sunrises) tend to be reddish; much of the blue light is
scattered out after the sunlight has travelled through so much atmosphere
leaving a preponderance of red.


and David Kessel, Ph.D., Professor, Wayne State University, Detroit
says this:
http://www.physlink.com/Education/AskExperts/ae665.cfm


The short wavelengths (blue) of light from the sun are scattered by the atmosphere (which is why the sky appears to be blue.), leaving behind the longer (yellow-red) wavelengths.. From a high-flying airplane, or from the moon, the sun appears to be white.

so the consensus seems to be that the atmosphere acts like a filter, causing the sun to appear what ever color (red, yellow, green) it is when or where it is observed.

hhEb09'1
2008-Aug-01, 04:31 PM
so the consensus seems to be that the atmosphere acts like a filter, causing the sun to appear what ever color (red, yellow, green) it is when or where it is observed.I thought the discussion originated with the idea of children drawing yellow suns. Is that then considered an acceptable practice? Given that the scene is surface of the earth, rather than outer space, I mean?

George
2008-Aug-01, 07:33 PM
if anyone is interested:
Dick Plano, Professor of Physics emeritus, Rutgers University has this to say about the color of the star Sol (our sun)
http://www.newton.dep.anl.gov/askasci/phy00/phy00940.htm (http://www.newton.dep.anl.gov/askasci/phy00/phy00940.htm)
Allow me to tweak some of his comments. [He isn't really wrong as he is speaking in general terms.]


Since the temperature of the surface of the sun is around 5800 K, the sun emits light of wavelength 500 nm with the greatest intensity. This isn't the actual case because stars are not solid surfaces, but are gas balls. The central region of the Sun is much hotter (6390K) to our viewing as we can look deeper into the atmosphere, whereas the limb temperature is much less (5000K). Solar physicists use at least five different temperatures for the Sun, depending on what they want to study. If you take the total energy from the Sun and do a blackbody calculation you will get around 5777K, I think, but if you simply try to match a blackbody curve to the Sun's spectral irradiance profile, you will need to use a bb curve at ~5850K.

These above complications, along with some others, give a more non-univform blackbody curve. The actual peak energy wavelength is found between 451nm and about 462nm, depending on instrumentation and date. The later is probably more accurate as it comes from current instrumentation, SORCE, where the former value is from Thuilier 2000 et al and Wiehrli 85.

At 450nm, the peak color then is a blue or, possibly, a violet blue. 462nm is a deep blue. [If the light is monochromatic at these wavelengths.]


visible light extends from around 400 nm (purple) to 700 nm (red), all of which are emitted by the sun with almost equal intensity, the sun looks white to our eyes. This statement can be contradictory to any view of the Sun's spectral energy distribution (SED). It is rare to see a graph that converts the energy distrubtion per wavelength to a photon flux per wavelength. Yet, it is the latter that is important to what the eye sees, since it is photons that trigger the color cones of our eyes, as well as, electronic sensors.

As he said, the photon flux of the Sun across the visible spectrum is, essentially, flat. It is a little weak, surprisingly, in the blue end, but not much. Thus, this information alone should justify the claim that the Sun, as a whole, is white.

You might be surprised at how often other "Ask Astro" sites miss this key point. At least one site says the peak flux of the Sun is in the yellow portion of the spectrum, thus it is yellow. Wrong, although the photon flux peak is in yellow, the overall flux is still flat and, again, justifys white.



In passing through our atmosphere, the shortest wavelengths (blue /purple) are scattered most strongly. A nit here since purple is generally red + blue, and he meant violet.


This answers one of the best known questions in all of physics: "Daddy, why is the sky blue?" It also explains why sunsets (and sunrises) tend to be reddish; much of the blue light is scattered out after the sunlight has travelled through so much atmosphere leaving a preponderance of red. This is a bit misleading as it suggests a red result. It is a very rare event to see a red Sun during sunset since our atmospere usually lacks the number of scattering particles. I suspect, however, that he is correct and the largest photon flux will be in the red band.


so the consensus seems to be that the atmosphere acts like a filter, causing the sun to appear what ever color (red, yellow, green) it is when or where it is observed. Yes, the result would be the same, though a filter would not give us a blue sky. :)

Quickshift
2008-Sep-30, 08:12 PM
As an amatuer photo bug I can say that different inert gases in bulbs and the metal in the filiments have dominant color properties. If you take a picture in a room with standard flourescent bulbs like in an office building the picture is overly green. Most elements have their own spectral thumb print.
Try taking a photograph of the sun and compare it to what you see. Please avoid burning your retinas.

PraedSt
2008-Oct-02, 10:03 PM
Is this a trick thread? :)

Sorry if this answer has already been given. A lot of posts...couldn't read them all!

1. 'Yellow', like any colour, is based on perception.

2. We see the sun as yellow, because the spectral distribution of its output is weighted towards blue and green.

3. Blue and green receptors in our eyes are preferentially activated over the red ones.

4. This is interpreted in our brains as 'yellow' because of (5)

5. We do not have yellow receptors. Yellow is a secondary colour. Blue and green are primary (along with red).

AND:

6. The sun is still yellow when viewed from outside the atmosphere. Same process, same colour.

Hope this helps

George
2008-Oct-02, 10:25 PM
[I sense a hint of background "Uh-Oh's" ;)]

2) A sunset Sun is yellow because of the green to red strengths. Blues and some greens have been scattered much more than the red end of the spectrum, so these combine to give us a yellow or, with greater scattering conidtions, an orange or red Sun.

3) Why do you say that? The net peak sensitivity of our eyes is aroun 555nm, IIRC, which is in the green. Interestingly, only about 2% of the cones in our eyes are blue receptors; and none of these are in the fovea -- where we have the greatest accuity.

6) Nope. The Sun Ain't Yeller (http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_revelation).

PraedSt
2008-Oct-02, 10:47 PM
Hehe..

You're right about sunset yellow of course!

Rest of your answer: good points. I have to agree, direct sunlight is certainly whitish rather than yellow.

Especially your receptor comment. I think receptors in the fovea probably evolved to respond most strongly to those wavelengths we see/need to see. Greens and Browns? That would mean green and red receptors.

But isn't this a trick question because of the whole perception/subjective/semantic/anthropomorphic shebang? :)

AND, your link reminds me of two other posts of mine:

http://www.bautforum.com/general-science/79575-oh-no-pterosaurs-couldnt-soar-says-expert.html

http://www.bautforum.com/general-science/79610-cannabis-less-harmful-than-drinking-go-figure.html

Heh.

Thanks for the corrections :)

George
2008-Oct-04, 01:20 AM
Especially your receptor comment. I think receptors in the fovea probably evolved to respond most strongly to those wavelengths we see/need to see. Greens and Browns? That would mean green and red receptors.
That's an interesting thought as browns would be perceived by the green and red receptors in the fovea, and would be an important color to an ancient observer.

The other factor is the problem of chromatic aberration for the eye. The lens of the eye does not correct the refractive problem for the blues, though there is a film in the fovea region (macula lutea) that absorbs blue light and helps reduce distortion caused by the blues.


But isn't this a trick question because of the whole perception/subjective/semantic/anthropomorphic shebang? :) I don't consider color to be all that subjective. Red apples are red by the definition we set for color and it is very likely what you and I perceive as red, and other colors. Colors should be very close to the colors we would see if we could switch brains back and forth. :) I don't know if any proof of any sort exists for this view, however. The ability to detect color blindness, I would think would be evidence of objective reasoning behind color perception.


AND, your link reminds me of two other posts of mine:

http://www.bautforum.com/general-science/79575-oh-no-pterosaurs-couldnt-soar-says-expert.html

http://www.bautforum.com/general-science/79610-cannabis-less-harmful-than-drinking-go-figure.html

This study of Sun's color (heliochromology) has been a heck of a lot of fun and it has involved a number of folks here in BAUT over the years. That link I gave is a small dose of some of it, including a touch of the corn that was sown in most posts. :)

It is far more common to see people, including scientists, get the Sun's color wrong. It is a trivial thing, but a colorful one. ;)

PraedSt
2008-Oct-04, 06:13 AM
It is a trivial thing, but a colorful one. ;)

Groan! And lol

George
2008-Oct-04, 04:52 PM
You'll be pleased to know that I'm not as punny as I used to be. :)

Still Alive
2009-May-13, 10:52 PM
First post on forum. Looks like some fun inquisitive people here, I may stick around.

My brief thought to add to this discussion is that this also may be tied up with the fact that when you look at the sun everything surrounding it is blue.

I know you can change the perception of the color of black dots to red and green depending on the fields they are surrounded by etc. So perhaps a bright white dot in a blue field looks yellow to the eye. Just an idea.

If you want some literature on it I know there has been some in Scientific American over the years. Probably relating to optics/optical illusions.

As far as the suns overall spectral characteristics, It was my understanding from everything I have ever read or seen that it emits most strongly ON THE EARTH'S SURFACE as green. Which is why plants are green (most total photon energy). Obviously this is different than how it emits in space, and I think before it hits the atmosphere blue predominates.

George
2009-May-14, 01:40 AM
First post on forum. Welcome!


My brief thought to add to this discussion is that this also may be tied up with the fact that when you look at the sun everything surrounding it is blue.

I know you can change the perception of the color of black dots to red and green depending on the fields they are surrounded by etc. So perhaps a bright white dot in a blue field looks yellow to the eye. Just an idea. Yes, many agree with this logic, however, it seems to me that it would only apply when the Sun is not on the horizon since the horizon sky is often not blue, though the Sun is yellow. Most people do seem to see a yellowish-white Sun at higher altitudes. But the exact reason is still unknown.


If you want some literature on it I know there has been some in Scientific American over the years. Probably relating to optics/optical illusions. Any specific articles dealing with the Sun's true color would be very interesting to see.


As far as the suns overall spectral characteristics, It was my understanding from everything I have ever read or seen that it emits most strongly ON THE EARTH'S SURFACE as green. Which is why plants are green (most total photon energy). Well, now that you are here, we can fix all that you have ever read. ;) Green plants appear green because that is the color they don't absorb, assuming no metamer issues are involved.


Obviously this is different than how it emits in space, and I think before it hits the atmosphere blue predominates. Yes, from space, blue is the winner, though many don't seem to know this little fact. However, if the energy flux is converted to photon flux, which better suits how our eyes see, then blue is not the winner.

TRUTHisnotfacts
2009-May-14, 10:11 PM
what If our sun may be a yellow looking Object from the afterglow effects from our Van Allen radiation belt. ?

Musashi
2009-May-14, 11:23 PM
No.

jdavisabc
2009-Jul-28, 09:36 PM
Incandescent lamps (your 'light bulbs') actually burn yellow. I guess with all the explanations here, and the mass knowledge, it might have avoided you all that WHO CARES!

It burns yellow for me, white for you. It doesn' matter at all.

Fenster Karton
2009-Sep-09, 02:48 AM
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.

Because the white crayon looks even more unnatural.

George
2009-Sep-09, 03:02 AM
Because the white crayon looks even more unnatural. Especially on the white paper! I think you're off to a nice start here. :) Welcome, color warrior!

I too think it was the yellow crayon that made the yellow color of the Sun so axiomatic. :)

HenrikOlsen
2009-Sep-10, 10:35 PM
Incandescent lamps (your 'light bulbs') actually burn yellow.
That's because incandescent lamps are much cooler than the sun so have their peak energy shifted to the longer wavelengths.