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Thread: Why the Sun appears yellow

  1. #211
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    Quote Originally Posted by sabianq View Post
    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?

  2. #212
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    Quote Originally Posted by sabianq View Post
    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
    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.
    We know time flies, we just can't see its wings.

  3. #213
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    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.

  4. #214
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    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

  5. #215
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    [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.
    We know time flies, we just can't see its wings.

  6. #216
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    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-sci...ys-expert.html

    http://www.bautforum.com/general-sci...go-figure.html

    Heh.

    Thanks for the corrections

  7. #217
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    Quote Originally Posted by PraedSt View Post
    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.

    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.
    Last edited by George; 2008-Oct-04 at 04:53 PM. Reason: apostrophe
    We know time flies, we just can't see its wings.

  8. #218
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    Quote Originally Posted by George View Post
    It is a trivial thing, but a colorful one.
    Groan! And lol

  9. #219
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    You'll be pleased to know that I'm not as punny as I used to be.
    We know time flies, we just can't see its wings.

  10. #220
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    Hi there!

    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.

  11. #221
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    Quote Originally Posted by Still Alive View Post
    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.
    We know time flies, we just can't see its wings.

  12. #222
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    what If our sun may be a yellow looking Object from the afterglow effects from our Van Allen radiation belt. ?

  13. #223
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    No.

  14. #224
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    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.

  15. #225
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    Quote Originally Posted by Eroica View Post
    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.

  16. #226
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    Quote Originally Posted by Fenster Karton View Post
    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.
    We know time flies, we just can't see its wings.

  17. #227
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    Quote Originally Posted by jdavisabc View Post
    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.
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