About the sun and moon

I sent this question to cecil ages ago, but i guess he doesnt consider it to be worthy of an awnser shakes fist at cecil, but its buged me since i was a kid, so i guess i’ll let the teeming millions awnser it.
Okay, first of: assuming the colour means light, and more colours means whiter, and all colours mean white.
Looking directly at the sun, it appears Yellow, probally due to the atomsphere, in space i assume it would appear white (all colour)
But looking at the moon, it appears grey / white. but if the moon shining is only light reflected from the sun, why does it appear whiter (more colours / light) than directly at the sun?
Shouldnt the moon absorb some light instead of giving back more light than the sun appears to have?
-Mikey:cool:

  1. Don’t look at the sun - it will damage your eyes.
  2. In a vacuum, I believe the sun does indeed look white, like any of the other stars.
  3. I believe the moon’s colour is down to the colour of the moon rock. It’s just giving off reflected white sunlight.
  4. The moon doesn’t give back anywhere near the amount of light that the sun does - it’s nearer, so appears quite bright, but it doesn’t light up the earth like the sun does during the day, or provide any discernible heat (infra red light).

Well that still doesnt awnser it, i knew the sun doesnt give back as much light…
just how come indirect sunlight is white (all 3 prime colours), and direct is yellow (red and blue absorbed)
what colour would the moon be if it was a perfect mirror?

The sun would look yellowish even in space because a star’s color is an intrinsic function of its temperature.

If the moon were a perfect mirror, it would look black with a few small white spots, because that’s what space looks like.

Interesting fact - the moon is (in reality) a dull grey colour, with an albedo (reflectivity) of about 7%. IOW it reflects about 7% of the light that hits it (Cite). The reason that it looks so white and bright is that there is not a whole lot to compare it with in the night sky…

Gp

When you mix all colors of light you do get white, but the sun is not white, it is yellowish. There is some white light in sunlight, but there is more yellowish light than the other colors.

The moon only reflects certain colors, white mostly; the rest of the spectrum is absorbed by the moon. So the light being reflected from the moon will be mostly gray.

When you mix all colors of light you do get white, but the sun is not white, it is yellowish. There is some white light in sunlight, but there is more yellowish light than the other colors.

The moon only reflects certain colors, white mostly; the rest of the spectrum is absorbed by the moon. So the light being reflected from the moon will be mostly gray.

Is there anything to do with the scattering of the blue light by the atmosphere? Wouldn’t that enhance the apparent yellow of the sun?

Technically, yes – and, of course, this is pretty much what happens at sunset. But at noon it only requires a small fraction of the blue light from the Sun to be scattered to get the blueness of the sky; the Sun is, after all, very much brighter than the sky. Oddly enough, Minnaert suggests that we tend to think that sunlight is white because we’re used to looking at a yellow Sun in a blue sky and our brain is integrating the two colours together.

As you’d expect, his The Nature of Light and Colour in the Open Air (Dover 1954, p298-300) has a pretty through discussion of what colours we perceive the Sun and Moon to be under different conditions. There’s quite a subtle range for both.

Incidentally, the usual comparison for the Moon’s albedo being 0.07 is to note that this makes it blacker than coal dust.

Technically, yes – and, of course, this is pretty much what happens at sunset. But at noon it only requires a small fraction of the blue light from the Sun to be scattered to get the blueness of the sky; the Sun is, after all, very much brighter than the sky. Oddly enough, Minnaert suggests that we tend to think that sunlight is white because we’re used to looking at a yellow Sun in a blue sky and our brain is integrating the two colours together.

As you’d expect, his The Nature of Light and Colour in the Open Air (Dover 1954, p298-300) has a pretty through discussion of what colours we perceive the Sun and Moon to be under different conditions. There’s quite a subtle range for both.

Incidentally, the usual comparison for the Moon’s albedo being 0.07 is to note that this makes it blacker than coal dust.

Technically, yes – and, of course, this is pretty much what happens at sunset. But at noon it only requires a small fraction of the blue light from the Sun to be scattered to get the blueness of the sky; the Sun is, after all, very much brighter than the sky. Oddly enough, Minnaert suggests that we tend to think that sunlight is white because we’re used to looking at a yellow Sun in a blue sky and our brain is integrating the two colours together.

As you’d expect, his The Nature of Light and Colour in the Open Air (Dover 1954, p298-300) has a pretty through discussion of what colours we perceive the Sun and Moon to be under different conditions. There’s quite a subtle range for both.

Incidentally, the usual comparison for the Moon’s albedo being 0.07 is to note that this makes it blacker than coal dust.

Apologies for the multiple post - that’s what happens when you compose a reply and then the board slows to a dead-stop when you try to post it.

1- White is not necessarilly the mixing of all colors or even of many colors. White, as a moment’s thought will show, can be obtained by mixing three or even two colors.

2- Proportion is of the essence. What makes white, white is the correct balance.

(I can’t believe people who use computer monitors and TVs need to have this pointed out to them)

3- The Sun’s light is not white and it does not even come close to being white nor is there any good reason why it should be white. Different stars have different colors depending on that time of the month.

Um, which two? Or even three? I’ve given this several moments’ thought and I can’t think of any two (or even three) colors which, when combined, would yield true white.

Er, they do? What stars do this? Got a cite?

>> Um, which two? Or even three? I’ve given this several moments’ thought and I can’t think of any two (or even three) colors which, when combined, would yield true white.

Maybe you’re thinking too hard. Just look at your monitor.

>> Different stars have different colors depending on that time of the month.

>> Er, they do? What stars do this? Got a cite?

I guess that was too obscure. Notice I said “that” time of the month (when their mood changes). :wink:

In other words, what I meant to say is that different stars have a very different combination of colors and there is no reason whatsoever to assume the balance should be white. As we all know (don’t we?) stars can range from red to blue depending on their temperature (and time of the month if you can understand I’m joking).

Depends on what you define as “color.” If you mean a particular wavelength, you need to mix at least 3 colors to make it appear white to human eyes. If you are going to claim “cyan plus red is white,” you might as well define “white” to be one color. Cyan is not one particular wavelength, but a mix of blue and green.

Now you’re using a different definition of “white”? Sunlight does appear white to human eyes. Of course it’s not “white” in the sense of having an absolutely flat spectrum. (i.e. “white” as in “white noise.”) It’s almost a blackbody, and has numerous absorption lines as well.

Our eyes are very relative sense organs. They can adjust to a wide range where we perceive different inputs of their three cone types as evenly distributed or white.

It’s called chromatic adaptation: http://www.cis.rit.edu/mcsl/faq/faq1.shtml#q8
an extreme example http://www.colorcube.com/illusions/chrmadptb.htm

We assigned a technical white point (D[sub]65[/sub]) to the color of the light a body at a temperature of 6500 K would emit. It comes closest to the mixture of direct yellowish 5780 K sunlight and scattered bluish 9000-12000 K skylight on what we would call an average bright overcast day. Your eyes accept this light as white, and compared to it the sun actually has the peachy pink this site explains why it chose it for its background color.

At night, however, the light of the moon is the only reference your eyes get. It blue-scatters too, but doesn’t illuminate your surroundings above the threshold to make your eyes accept them as reference point instead.

Or, for another POV, I think the Sun is very close to white when it’s high in the sky. The difference is that you can actually look at the Moon when it’s up, but you can’t really look at the Sun until it’s to a point where the atmosphere makes it less intense by scattering a large portion of its light (therefore will look yellow-orange-red).

In the book Bad Astronomy written by this board’s poster BadAstronomer, he notes that clouds scatter the light that hits them pretty evenly, so they’re a good indicator of what color the Sun really is. That’s pretty white if you ask me. On the other hand, he discounts my explanation for why the Sun seems yellow (but I’m not convinced).

Okay, lots of misconceptions to clear up here:

  1. The intensity of light doesn’t have anything to do with its color. Something brighter does not mean it is whiter, it just means it’s brighter. A red object can be brighter than white one. So if the Sun is yellow, and the Moon white, the Sun can still be brighter than the Moon. This answers the OP.

  2. The Sun isn’t yellow, it’s white. The colors we perceive are a combination of the mix of colors emitted by an object which is then interpreted by our eyes. Many people claim the Sun looks yellow, and I don’t think it’s well understood why. It may be that you are comparing it to the blue sky (and, as someone earlier noted, the amount of blue light lost from the Sun shouldn’t make it any yellower as seen to the eye, as far as I know).

  3. The Moon is indeed very dark, as it reflects only a small fraction of the light that it receives. But against a black sky, it looks very bright. The Apollo astronauts said the lunar surface was very grey, even gunpowder-like.

  4. Three colors can indeed make white. Red+green+blue light at equal intensities should make white, or a reasonable facsimile thereof (think of the RGB colors 255+255+255, which make white).

  5. I am baffled about the “different times of the month” comment. A star’s color is its color. They only change if there is some distortion in our atmosphere (twinkling) or the star itself changes, like when a star like the Sun becomes a red giant. That only happens near the end of its life.