The Yellow Sun Paradox

Color of the sun in the sky.
The Color of The Sun

Who as a child didn’t draw pictures showing the sun – as a round ball with a smiling face, surrounded by a wreath of light rays? What color was the sun in your pictures? If the answer is “yellow”, you’re like me – my suns were always yellow, too. Why is that? Is the sun really yellow?

The sun is yellow.  Or not?
Is the sun yellow?

No, the sun is not yellow or golden, but white – sunlight is even the definition of white par excellence. What color is a white sheet of paper when viewed in the light of the midday sun? White. What color are cluster clouds on a sunny afternoon? White. If the sun were yellow, clouds would be yellow too, or snow in winter. A strange sight.

Why do most people perceive the sun as yellow? This has occupied quite a few authors, as a short Google search proves. Most give a standard answer, which is correct, but in my opinion incomplete. Some explanations are wrong. And one article I found also gets to the bottom of the “yellow sun paradox” experimentally.

So What Makes the Sun Yellow?

Sunlight contains photons, or “light particles” of all wavelengths and thus of all colors. However, not all photons and thus all colors occur with equal frequency. Blue-violet and red are rarer, green most common. Nevertheless, we do not see the sun as green. This is because green lies approximately in the middle of the perceptible color spectrum: There are therefore lots of photons in sunlight with shorter and longer wavelengths. These mix together, and the mixture of all the colors from red to violet gives white.

Before we see the sunlight, it has to pass through the earth’s atmosphere. And that’s where it gets exciting (again). That’s because the photons collide with the molecules of the Earth’s air envelope, changing their direction of motion in the process. “Scattering” is the physical technical term. The probability of collision is not the same for all photons. It increases the shorter the wavelength of the light particles: blue photons are scattered most frequently, red ones least frequently.

When the sunlight finally reaches our eyes, it has traveled a more or less long way through the atmosphere, depending on the position of the sun. A more or less large part of the blue photons has been “scattered away” in the process. On the one hand, the scattered blue sunlight makes the sky appear blue. And since this blue light is missing from the actual sunlight, its color shifts to longer wavelengths – thus into the yellow. This is the standard explanation mentioned above. It is correct, but not quite complete. Because the share of scattered light is not very large as long as the sun is high in the sky.

If the sun is high in the sky (like here on a winter afternoon), the light scattering in the earth's atmosphere is not enough to noticeably shift its color.  It glows white - but is too bright to be looked at directly.
If the sun is high in the sky (like here on a winter afternoon), the light scattering in the earth’s atmosphere is not enough to noticeably shift its color. It glows white – but is too bright to be looked at directly.

(Another wrong explanation claims that the yellow sun is only an optical illusion: Because the sky is blue, and blue is the complementary color of yellow, we believe that the sun appears yellow. But first, a white circle against a blue background does not automatically appear yellow, as you can try out for yourself (at least I still see it as white). Second, the white clouds against the blue sky background would then also have to appear yellowish. But they don’t.)

We approach the solution by asking ourselves when we see the sun at all – in the sense of “looking at it directly”. As long as it is high in the sky at noon, it is absolutely not advisable to look directly into the sun disk. It is much too bright. That means: For the most part of the day we can’t know what color the sun is.

Only when it is low enough in the late afternoon (or early morning) that its light is sufficiently attenuated by the effect of the Earth’s atmosphere can we risk a brief glance. Then the scattering has already disposed of a more stately part of the blue light, and the sun actually appears yellow. If it is very low above the horizon, its color even changes to red-orange.

If the sun is low above the horizon, the effect of light scattering becomes noticeable: the sun first takes on a faint, then increasingly strong yellow tone.  At the same time, its light is weakened so that you can take a quick look.
If the sun is low above the horizon, the effect of light scattering becomes noticeable: the sun first takes on a faint, then increasingly strong yellow tone. At the same time, its light is weakened so that you can take a quick look.

Sunlight Science

Optical engineer Stephen R. Wilk stressed this theory a few years ago. To do this, he modeled the change in color perception of an originally pure white light source with increasing scatter. As he reports in the magazine Optics & Photonics News (March 2009), the result corresponds exactly to the expectation described above. As soon as his model sun had sunk so far that one could risk a quick glance, it appeared yellow. And it stayed with this color until shortly before its artificial demise. Then its color swung rapidly from yellow to red due to the rapidly increasing thickness of the atmosphere.

Conclusion: The paradox of the yellow sun is on the one hand an effect of light scattering in the earth’s atmosphere. This robs the sunlight of part of its blue color and shifts its color focus from white to yellow. But it also has to do with the fact that we can only look at the sun when it is low enough on the horizon – where the color can also be seen. As long as the sun is high in the sky, it shines white – and usually much too bright to look at directly.


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