MadSci Network: Physics

Re: Why is the sun yellow but the moon is white?

Date: Wed May 28 09:56:31 2003
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 1053886965.Ph

You write:

The Sun is yellow in colour ...

Well, I just went outside and looked. When I peeked directly at the Sun, I couldn't really tell what color it was: it was too bright to judge. So, I took out a small telescope I have and projected an image of the Sun onto a piece of white paper. When I held the paper close to the telescope's eyepiece, the Sun's image was very bright and appeared white. As I moved the paper farther away from the eyepiece, the image became larger and dimmer; I noticed that its color shifted slightly from white towards a very pastel shade of yellow.

One of the problems in discussing the color of the Sun is that human eyes perceive the color of a light source to depend on its intensity. When you ask about "the color" of the Sun, do you mean "as one sees when looking directly at the Sun when it is high in the sky?", or "as one sees when projecting a small image onto paper?", or "as one sees when projecting a large image onto paper?" Yes, it's the same mix of wavelengths in each case, but our eyes don't send the same signal for "color" to our brains.

Another complication is that we live on Earth, under a blanket of several miles of air. As sunlight passes through the atmosphere, the blue component of its light is absorbed and scattered more than the red component. The effect is obvious at sunrise or sunset -- the Sun turns a distinct shade of orange or even red. But even when the Sun is high in the sky at noon, the atmosphere still shifts its color slightly to the red. If you were to look at the Sun on a clear day from the ground, and then jump up above the atmosphere into space and look again, you would probably notice a small change in its color (and its brightness, too).

So what color is the Sun? I would say that the best answer is "whitish, with a hint of faint yellow." Or, perhaps, "definitely not blue, or green, or red, but, if anything other than white, yellowish." It might be easier to label the Sun with a color if we were to compare it to some other star, because human eyes are good at detecting the difference in two shades of color. Just visit any hardware or home decorating store and look at all the different versions of "white" that you can purchase. If you look at just one little sample of paint, you'd say, "Well, that's basically white;" but if you see it on a page with ten other samples around it, you can add more to the description: "Oh, it's somewhat darker than 'bone', and has less blue in it than 'desert', with maybe a hint of green."

The Sun's photosphere (the outermost layer visible to human eyes) has a temperature of about 5600 Kelvin, which places it in the middle of stellar temperatures. Take a look at this photograph of the constellation Orion: ap030207.html

Rigel, the bright star at the bottom of Orion, has a photospheric temperature around 11,000 Kelvin. If we moved the Sun very far away from the Earth, so that it was roughly the same apparent brightness as Rigel, we'd say, "Rigel is a bluish-white color, while the Sun is orangish-white." On the other hand, Betelgeuse, the bright star at the top of Orion, is only about 3400 Kelvin. If we moved the Sun so that it had roughly the same apparent brightness as Betelgeuse, and was next to it in the sky, we'd say "Betelgeuse is orangish, but the Sun is yellowish." Note that our perception of the Sun's color would depend a little bit on exactly which star was being used as a comparison.

Let me get back to one of your questions: you write:

the moon is white even during the day even though it is the same
light from the sun that has been reflected by the moon's surface.

This brings up another complication. Human eyes have two sorts of light-sensitive cells, which are called "rods" and "cones". The "cone" cells can distinguish colors, but they only work when struck by lots of light. The "rod" cells, on the other hand, respond to low levels of light, but provide no color information. If you don't believe me, go into a room without windows and look around you: what color is the floor, and the ceiling, and the furniture? Then turn off the lights, and look again. You won't see the same colors so clearly under dim lighting.

Even the full moon is a pretty dim object. It reflects only about seven percent of the (whitish) light which strikes it (about the same as an asphalt road), and then that light gets spread out as it travels from the moon to the Earth. By the time it reaches us, it is so feeble that it doesn't excite the cone cells in our eyes very much. Since it is the black-and-white rod cells which provide most of the signal to our brains, we perceive the moon in shades of grey.

Your final point,

clouds are white yet they are reflecting light from the sun.
is a good one. Puffy cumulous clouds on a sunny day are certainly bright enough to excite the cone cells in our eyes, so we should be able to detect any hints of color in them. They do reflect equally well all the wavelengths of light striking them, most of which comes from the Sun. So if the Sun is really yellow (and not white), then clouds should be yellow. But they aren't. And that goes to show that the Sun itself is much more whitish than it is yellowish.

I'm sorry to have taken so long to address your questions, but the bottom line is that "color" is very subjective. Different people will claim to see different colors when they look at the same item, and the same person will perceive different colors when he looks at exactly the same item under different conditions. You just asked a really tough question!

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