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Thread: Why the Sun appears yellow

  1. #31
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    It never looked yellow to me. I always thought that was just a cultural stereotype. :-?

  2. #32
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    Re: Winter light

    Quote Originally Posted by Swift
    Ok, I'll take this interesting discussion off in a slightly different direction.

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

    Any ideas?
    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.
    We know time flies, we just can't see its wings.

  3. #33
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    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.

  4. #34
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    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.

  5. #35
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    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!!! ](*,)
    We know time flies, we just can't see its wings.

  6. #36
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    Quote Originally Posted by Glom
    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.

  7. #37
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    Quote Originally Posted by Alex W.
    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.

  8. #38
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    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...

  9. #39
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    Quote Originally Posted by AstroSmurf
    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.

  10. #40
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    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!

  11. #41
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    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)
    We know time flies, we just can't see its wings.

  12. #42
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    Quote Originally Posted by Visitor
    ...Here on this site you can find a diagramm of the cone cells' absorbance against wavelenght. As you can see, the absorbance is relatively low in the green range, but it's quite high in the "yellow vicinity". Maybe this can account as an explanation.
    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..... ](*,)
    We know time flies, we just can't see its wings.

  13. #43
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    Y'arr, I knew I should've remembered the book the last time I moved...

  14. #44
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    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) ](*,) ](*,)
    We know time flies, we just can't see its wings.

  15. #45
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    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...

  16. #46
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    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.
    We know time flies, we just can't see its wings.

  17. #47
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    Take the spectrum from sunlight, run it through a computer, and get it to provide a picture on a particularly good monitor?

  18. #48
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    Quote Originally Posted by Eroica
    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.

  19. #49
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    Well all I can say is it appears your perception of colour isn't as good as you think it is!

  20. #50
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    Quote Originally Posted by The Bad Astronomer
    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 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.
    We know time flies, we just can't see its wings.

  21. #51
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    Re: Why the Sun appears yellow

    Quote Originally Posted by robin
    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.

  22. #52
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    What photos show the sun to not be white? It's always an overexposed blob.

  23. #53
    Ones taken through filters.

  24. #54
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    Quote Originally Posted by Glom
    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".
    We know time flies, we just can't see its wings.

  25. #55
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    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)

    If anyone can find data, I'd appreciate it.
    We know time flies, we just can't see its wings.

  26. #56
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    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.
    We know time flies, we just can't see its wings.

  27. #57
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    No sun yet, only clouds....
    We know time flies, we just can't see its wings.

  28. #58
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    Tell me about it. It's been either intrument meteorological conditions or borderline IMC for the past week.

  29. #59
    Quote Originally Posted by Glom
    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-[

  30. #60
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    Quote Originally Posted by Glom
    Tell me about it. It's been either intrument meteorological conditions or borderline IMC for the past week.

    It's Gethen's fault....
    Quote Originally Posted by Gethen
    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).
    We know time flies, we just can't see its wings.

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