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Thread: Why the Sun appears yellow

  1. #181
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    Quote Originally Posted by PhantomWolf
    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.
    We know time flies, we just can't see its wings.

  2. #182
    sometimes the Sun appear blue.

  3. #183
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    Quote Originally Posted by bronzeman
    sometimes the Sun appear blue.
    When do you see it as blue?
    We know time flies, we just can't see its wings.

  4. #184
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    Quote Originally Posted by George
    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.

  5. #185
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    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/O...erception.html

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

  6. #186
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    Quote Originally Posted by ozark1 View Post
    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/O...erception.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.
    We know time flies, we just can't see its wings.

  7. #187
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    Quote Originally Posted by George View Post
    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.

  8. #188
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    Quote Originally Posted by pghnative View Post
    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.
    We know time flies, we just can't see its wings.

  9. #189
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    Quote Originally Posted by George View Post
    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.

  10. #190
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    That tool sounds handy. I don't have Photo Editor, though.

    Here is my favorite illusion.
    We know time flies, we just can't see its wings.

  11. #191
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    Quote Originally Posted by George View Post
    That tool sounds handy. I don't have Photo Editor, though.

    Here is my favorite illusion.
    Here's a small collection a friend pointed me to years ago.

  12. #192
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    Excellent! There were many I had not seen before. The two triangles took me a minute.
    We know time flies, we just can't see its wings.

  13. #193
    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?

  14. #194
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    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.
    We know time flies, we just can't see its wings.

  15. #195
    I'm not willing to concede the sun is yellow. I challenge my students to look at it. It's white!

  16. #196
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    Quote Originally Posted by Michael Huster View Post
    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 regarding how others see the sun's color which you might enjoy. Please place your vote, too.

    Here 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.
    We know time flies, we just can't see its wings.

  17. #197

    a more challenging question.

    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? I'm a little confused.

  18. #198
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    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?
    We know time flies, we just can't see its wings.

  19. #199

    Lightbulb hmm. good point

    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.

  20. #200

    I think I figured it out.

    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. also, I need help with finding out how to post a new thread. could somebody please help?

  21. #201
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    Quote Originally Posted by Question View Post
    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.
    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.]
    We know time flies, we just can't see its wings.

  22. #202
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    Quote Originally Posted by Question View Post
    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.]
    We know time flies, we just can't see its wings.

  23. #203
    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.

  24. #204
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    Quote Originally Posted by schrodingers-dog View Post
    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.
    We know time flies, we just can't see its wings.

  25. #205
    Quote Originally Posted by Eroica View Post
    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.

    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.
    Last edited by PooprScooper; 2008-Aug-01 at 04:39 AM.

  26. #206
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    Welcome to BAUT Pooprscooper.

  27. #207
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    Quote Originally Posted by PooprScooper View Post
    My first post here, hi guys.
    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. It is white.
    We know time flies, we just can't see its wings.

  28. #208
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    Quote Originally Posted by George View Post
    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.

  29. #209
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    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. ]
    We know time flies, we just can't see its wings.

  30. #210
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    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/askasc...0/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.

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