Why does the moon appear white/grayish and the sun yellow? [duplicate]

Question

The Sun, as we all know, is white. Because our atmosphere scatters light the sun appears yellow to us hear on earth. So, why isn't the moon also yellow. Doesn't our atmosphere scatter its light. How about those little stars in the night sky, they don't look yellow.

I have heard the explanation that "the moon is dim and mostly stimulates the rods in our eye and not cones" but that's impossible! The moon does look orange/yellow when its low in the sky due to our atmosphere. So, why isn't it bright yellow when its up above our head like the sun? Same thing with the stars. Whats going on with the light?

Answer 1

First of all, a celestial body could appear blue for a simple reason: it may be blue, indeed. This is the case of Neptune. Its being blue simply means that the probability that a photon is reflected off Neptune is higher if the frequency (color) of the photon is basically blue than when it is green, yellow, or red.

However, the Moon isn't actually blue. The percentage of the photons of different colors is basically the same in the sunlight and the moonlight. The fact that we think that the moonlight is bluish is an optical illusion known as the Purkyně effect after a Czech 19th century biologist.

The reason of the illusion is rather simple and you sketched it. The human eyes depends on cones and rods to see. Cones are able to distinguish colors – basically, they give us the detailed colorful information about the amount of red, green, blue (like the RGB information about colors on computer displays).

Cones are most sensitive to the green-yellow light in the middle of the visible spectrum. The most neutral color of light is "white" but the light from the Sun is a bit separated. The blue, high-frequency component, changes the direction a lot (scatters) which is why the sky is blue, while the remaining light arrives from the direction of the Sun. The "opposite" color to blue is basically yellow which is why the Sun appears a bit more yellow, not quite white.

Cones with this RGB color sensitivity could be enough to see everything in color but they have one disadvantage: their sensitivity is worse. That's why the eyes also have rods. They can't distinguish colors but they can see even dimmer light. Their sensitivity just happens to be maximized for colors that are blue-green or something like that.

So when thc ones already "see nothing" because the light is too dim (weak), the eye relies on the rods. The eye effectively becomes color-blind but the brain must still interpret the colors in some way. It interprets them as the color that is normally maximally correlated with the rods' seeing something, i.e. with the frequency at which the rods are most sensitive. And this mechanism gives our perception the bluish color. It's because the eye starts to rely on different cells which are maximally sensitive to slightly different wavelengths (colors) than the average cones.

Answer 2

I think any rods-and-cones explanation, such as the Purkinje effect, for the perceived colour of the Moon itself is nonsense, simply because the light reflected from the (full, overhead) Moon is easily bright enough to be perceived by the cones in our retina. As you say, if the contrary were true, it would also prevent us from seeing the Moon as orange when it is close to the horizon, but that we definitely do. On the other hand such explanantion could apply to the appearence of the (rest of the) sky on a moonlit night, which is very dim; however from personal experience I cannot recall the sky appearing anthing but pitch black once the Sun is well gone.

We are talking about a subjective phenomenon here; I am pretty sure that careful spectral analysis would show all kinds of variations for moonlight, according to conditions of observation, and hardly ever the same spectral composition as that seen from outside our atmosphere. The question however was why with the naked eye, we do not perceive the Moon as coloured (unless it is very low). For that I mention two factors.

1. Thought Rayleigh scattering produces a markedly blue colour for the sky (where there is no other source of light reaching our eye) it only slightly changes the spectrum of direct sunlight. Of the direct light of the overhead Sun only about 25% gets deflected by scattering, so the colour change cannot be more than slight, even if all the scattered light were blue (which it isn't). Given the extreme brightness of the Sun, I'm not even sure what it means precisely to say the we see the Sun as yellow rather than white (with what definition of white light; the average colour of the spectrum as seen by astronauts in orbit?). Doubtless direct sunlight is more yellowish than the total spectrum obtained if one adds in the blue skylight, but that is not really saying much more than that yellow is complementary to blue. In short, any effect for the Moon would be small, and we don't have much of a White Standard to compare with (for those of us who have never seen the Moon from orbit).

2. While we are good at detecting slight variations of colours reflected from different objects lit by the same source, we are pretty bad at detecting the colour variations of light sources themselves when used separately. This is because our brains automatically compensate for the general colour temperature of the light source, to better estimate the true colour of objects. We don't perceive our indoors as turning yellow when evening falls and incandescent lighting replaces daylight; yet as any photographer knows the actual colour of the scene does shift heavily towards the yellow. Since at night the Moon is by far the brightest object in the sky, and (barring artificial light) the main source of light around us, we are bound to perceive it as white, regardless of what its spectrum is exactly (except in extreme cases where the Moon is very low, and the Sun is not quite gone). Actually I just learned from WP that the spectrum of the Moon is somewhat brownish when compared to that of the Sun, but I am sure I would never have noticed that by direct observation.