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The sky is blue (I'm told) because nitrogen in the atmosphere scatters short blue wave lengths of light from the sunlight, which is also why the sun appears somewhat yellowish rather than white, as it does in space. I know that the moon is not bright enough to make the sky blue but shouldn't the 'blue' part of it's spectrum still be filtered out of its image regardless making it more yellow or red in colour?

Qmechanic
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Ummdustry
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    The moon is yellow! Have you seen it on the horizon? – knzhou Apr 21 '18 at 15:48
  • https://physics.stackexchange.com/q/244922/ https://physics.stackexchange.com/q/193585/ – ProfRob Apr 21 '18 at 15:55
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    Your dark adapted vision is mostly black-white, you have poor colour senstivity in low light – Martin Beckett Apr 21 '18 at 15:56
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    Part of the problem with this question is that different people will perceive the Moon's color differently. To me it does look yellow more often than not, to others it may not. Color perception is more complex than simple physics. – StephenG - Help Ukraine Apr 21 '18 at 16:12
  • Possible duplicates: https://physics.stackexchange.com/q/15184/ , https://physics.stackexchange.com/q/193585/ , https://physics.stackexchange.com/q/244922/ and links therein. – Qmechanic Apr 21 '18 at 16:18
  • The Sun looks white to me, unless it's close to the horizon, and then it goes through yellow / orange / red shades. Of course, I tend to avoid looking at the Sun, unless it's very close to the horizon. ;) FWIW, even Venus looks reddish if it's close enough to the horizon; I once mistook it for Mars for a few moments before I decided that it was far too bright. – PM 2Ring Apr 22 '18 at 05:55

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Moonlight is actually slightly redder than sunlight, because the reflectance spectrum of the full Moon is higher in the red part of the spectrum than the blue.

The effect of the atmosphere (which of course is exactly the same for sunlight and moonlight) is not very large when looking at both objects when they are well above the horizon. If both were overhead then something like 50% of the bluest light you can possibly perceive (maybe 350 nm) is scattered/absorbed compared to about 20% at 550nm and perhaps 15% at 700nm. This sounds like a lot, but the eye/brain does not perceive colour in a way that is linearly proportional to the flux received at any wavelength and this level of modification is perhaps equivalent to changing the perceived colour from that of a G2 star to a G5 star (not a big change -- see here). It is not until the Sun gets very low towards the horizon that there is a very noticeable change in colour, and as I say, this affects moonlight in exactly the same way.

I think if you are claiming that the Sun and Moon appear to be different colours then I won't argue. But it is a question of perception for two objects whose brightness is different by many orders of magnitude and therefore the answer probably lies in the physiology of the eye/brain and not Physics.

ProfRob
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  • The brightness perception in the eye is in some sense logarithmic. The color perception is not. Under normal circumstances, absorbing 50% of the bluest light would make a relatively large difference in the color. The answer to the OPs question lies in the way the brain processes the signals from the retina. – Peter Shor Apr 21 '18 at 23:51
  • @PeterShor Genuine question - does not the colour perception rely on comparing the signal from different specialised cells that have a logarithmic sensitivity? For example, the flux ratio of 550nm/400nm for sunlight compared with blue sky is about 2.5 vs 0.4. A very dramatic difference. – ProfRob Apr 27 '18 at 18:28
  • Color vision is much more complicated than that. Evolutionarily, it makes sense for brightness to be logarithmic, but it doesn't make much sense for color vision to be logarithmic, because then we couldn't see color very well. And evolution has figured out how to make our eyes do that. – Peter Shor Apr 27 '18 at 18:59
  • Look what happens when you decrease the blue content by 25% on this website. It changes the color from white to a definite yellow. – Peter Shor Apr 27 '18 at 19:02
  • @PeterShor You might be right, but that website doesn't demonstrate it. A linear mapping in RGB colour value is unlikely to correspond to a linear mapping of flux density. To put it another way - I doubt that changing the b value from 255 to 254 corresponds to a 1/255 reduction in the flux at 400nm. – ProfRob Apr 27 '18 at 19:32
  • The RGB color value map is indeed linear. See Wikipedia. Given that what you're seeing on that website is filtered through a computer screen, it's probably not exactly the RGB color model, but I would hope it's relatively close. – Peter Shor Apr 27 '18 at 19:52
  • @PeterShor I don't see where the mapping from flux to RGB is defined to be linear in that article. Here are RGB values that have been calculated for stars. http://www.vendian.org/mncharity/dir3/starcolor/ Look at the RGB Vs spectral type table. See how little it changes despite gross changes in the stellar spectra. e.g. G=B for an F8 star, and only changes to G=0.7B for an O5 star. A glance at a spectral atlas reveals that the 550/400 nm flux ratios for such stars are actually 1.3 vs 0.35. If the RGB values were linear than G should be 0.27B for the O5 star. – ProfRob Apr 27 '18 at 23:40
  • Let us continue this discussion in chat. – ProfRob Apr 27 '18 at 23:40
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The colors of most things don't appear to change when you move from sunlight to fluorescent light to incandescent light to firelight, even though the actual ratios of wavelengths that are hitting your retina are quite different. This is because your brain automatically adjusts colors based on the ambient light. Since the moon is providing the ambient light, your mind automatically adjusts its color to be close to white.

So if you try to answer this question using physics alone (like some of the other answers do), you're likely to get the answer wrong; the perceived color of the moon depends on a combination of physics and neuroscience. See this question in biology.stackexchange.

In fact, you would expect the moon to be the same color as the sun, because both of these colors are produced by having the blue light refracted by the atmosphere. So why aren't they? I would guess that the eye is not good at perceiving the moonlight scattered by the sky and off objects; it's so dim it just stimulates our rods and not our cones. Thus, when the color of the ambient light is averaged over by our brain, it is very close to the actual light coming from the moon, so the moon looks white. This explanation would predict that a bright full moon would look yellower than a half-moon. I believe it does.

Peter Shor
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To me the moon appears yellow enough but here are its true colours: http://www.olino.org/us/articles/2015/10/05/spectrum-of-moon-light

my2cts
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We need to start with the fact that white light is a combination of all colors produced equally by a glowing object. A glowing object that appears blue is blue because it's producing more blue light than it is producing red, orange, yellow, green light. The color of a glowing object depends on the temperature of the object. Now we can proceed to your question. Two Reasons why the Sun appears yellow:

  1. The Sun's surface temperature (5,500 degrees C) produces a range of visible light (red to blue) in which yellow is the most plentiful, but not much more than other colors it produces. If the Sun were cooler, say 2,500 degrees C, it would look red, like the stars Antares and Betelgeuse. Or if the Sun were hotter, say 15,000 degrees C, it would look blue, like the star Rigel.

The Earth's atmosphere acts as a kind of light filter. Some colors are filtered more than others. The Sun is a yellow star, but the Earth's atmosphere makes the Sun look more yellow than it appears than if you were to observe it from space where it would appear more white than yellow. But you don't have to leave Earth to see that the Sun is really less yellow than it appears. If you are in the Rocky Mountains at 11,000 ft elevation, the Sun looks less yellow and more white than it does at sea level. There are fewer air molecules at this elevation to filter the Sun's other colors. Imagine what the Sun would look like from an airplane at 40,000 ft altitude--quite white! Also, when you are able to look at the Sun where you live, it's morning or late afternoon. It's easier to look at the Sun for a few seconds than it is a noon. The Sun appears more yellow at those times than it would if you were to observe it at noon (12 PM) when Sun is highest in the sky for the day; it's at its brightest and whitest--hard to look at. Because of the Sun's high position at noon, the sunlight has less air to travel through. Less air means less filtering of other colors. Remember: Light appears white because all colors are equally reaching your eyes. So, at noon the Sun appears to be more white, less yellow--closer to the way it really is! (Don't try to make this observation without hi-tech eye protection).

Godefroy
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