Why is the surface of the Moon white? Rather, what makes the surface white?
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asked Jul 11, 2015 at 21:37
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5 Answers
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The moon appears white from the Earth for two reasons.
The first is that the reflected spectrum of sunlight is very broad and contains no very significant features. On this basis, the spectrum of the moon could be considered pinkish, as the reflectance of sunlight (which appears almost white to the human eye) is twice as effective at red wavelengths than blue. See the reflectance below from Sakaki et al. (2013) (see p.84 of the linked document).
However, the moon does not appear pinkish, and that must be because of the way the eye works (not physics). At the relatively low intensities of moon light, it is possible that the cones in the eye start to switch off, leaving the rods responsible for colour-insensitive mesopic or even scotopic vision (NB: I think the light levels are too high for true scotopic vision). When applied to the "silvery moon" (this is also known as the Purkinje effect) - the pinkish hue of the moon is shifted back blueward by our perception - resulting in a silvery-white appearance.
See for example Ciocca & Wang (2013) for further discussion - apologies, I cannot find a version that isn't behind a paywall - however, here is a comparison of the sunlight and moonlight spectra from that paper. As you can see, the moonlight is redder than sunlight. These authors suggest that the illumination from the moon is low enough that mesopic vision results in a weakened perception of the red tinge that the moon should have in comparison to sunlight.
However, I think that the full moon is sufficiently bright that photopic vision ought to be feasible. Therefore it appears that a plausible explanation is nothing more than that the two spectra below are insufficiently different for our eyes to perceive them as differently coloured. As sunlight appears (almost) white, then the reflected light from the moon is also (almost) white. I'll leave optometrists to argue about whether the Purkinje effect could play a role in diminishing the perceived difference.
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1$\begingroup$ There's something wrong with that article and with that spectrum. The Purkinje effect explanation is nonsense. That explains why a white car looks bluish under moonlight, but not moonlight itself. Mars is obviously red. So is Antares ("not Mars"), as is Betelgeuse. All three are orders of magnitude less luminous than is the Moon. $\endgroup$ Commented Jul 12, 2015 at 17:33
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$\begingroup$ @DavidHammen Which spectrum do you refer to. I show the reflectance. To get the lunar spectrum you multiply the solar spectrum by this and then modify slightly for transmission through the Earth's atmosphere. The stars/planets you mention are very much redder than moonlight and appear only slightly red to the naked eye. They are fainter than the moon, but perhaps not orders of magnitude fainter when expressed in terms of surface brightness over the cells in the eye. (The moon is apparent mag about -5 per sq arcmin). $\endgroup$– ProfRobCommented Jul 12, 2015 at 17:54
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$\begingroup$ @DavidHammen However, I do agree that even a preliminary bit of research does suggest that the brightness of the full moon surface ought to be enough to engage photopic vision. $\endgroup$– ProfRobCommented Jul 12, 2015 at 18:03
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$\begingroup$ That second set of spectra looks much better than your first spectrum. Also note that the scale is linear. Our eyes are logarithmic devices. Those two spectra would be much closer to one another had intensity been placed on a logarithmic scale. $\endgroup$ Commented Jul 12, 2015 at 19:09
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3$\begingroup$ In this answer you say "could be considered pinkish", but conclude that the spectrum is "close enough" to be called white. Then in comments to other peoples' answers, as a critiscm, you say that the moon objectively is "not grey" since its spectrum is not precisely flat. This seems an uneven application of the meanings of colour-words. Once bright enough that colour vision kicks in, the moon is by definition exactly as grey as (ignoring the effect of Earth's atmosphere) its reflected light appears white, isn't it? That is to say, pretty grey. $\endgroup$ Commented Jul 12, 2015 at 23:38
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The moon is actually grey.
You can see this if you look at images taken in space, or, preferably, on the moon itself. For example, this one, of Buzz Aldrin: (Courtesy of NASA)
But, seeing as how at night you compare it to a black sky, it appears white.
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6$\begingroup$ How does this answer the question? $\endgroup$ Commented Jul 11, 2015 at 21:57
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19$\begingroup$ "Why is the surface of the moon white?" "It isn't, it is (something), but it appears like that because (something)" $\endgroup$– OmryCommented Jul 11, 2015 at 21:59
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1$\begingroup$ I'm doubtful of the explanation you give. $\endgroup$ Commented Jul 11, 2015 at 22:00
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1$\begingroup$ @HarryCBurn No, it is not a "different colour". Grey is achromatic and colourless. A grey object reflects light of all wavelengths equally with less than 100% efficiency. Therefore the moon is not grey, because it does not have this property. en.wikipedia.org/wiki/Grey $\endgroup$– ProfRobCommented Jul 12, 2015 at 16:59
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4$\begingroup$ @RobJeffries, Our perception of "white", and "gray" and "black" is highly subjective. Google for "color constancy" or, for "checkerboard illusion". The ground in the photograph looks "gray" because it reflects less light than the astronaut's suit. When you see the same surface up there in the night sky it looks "white" because there's nothing brighter up there against which your eye can compare it. $\endgroup$ Commented Jul 12, 2015 at 19:14
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For emphasis: The Moon is grey, but it looks white because of scattering along with sunlight.
The dark parts - which are less common - are maria, plains of volcanic rock (basalt). They are relatively old, as there has been no recent volcanic activity on the Moon. Most are on the near side of the Moon. By contrast, the white parts are often referred to as terrae, or highlands. They are made of anorthosite, which happens to be, in general, light grey, and breccia, which has a similar color (see also here).
There are also other rocks in the highlands:
That gray color you see comes from the surface of the Moon which is mostly oxygen, silicon, magnesium, iron, calcium and aluminum. The lighter color rocks are usually plagioclase feldspar, while the darker rocks are pyroxene.
. . .
During the day, the Moon has to compete with sunlight, which is also being scattered by the atmosphere, so it looks white.
The Moon is covered in regolith, which is a fine powder. This is often referred to as "Moon dust".
answered Jul 11, 2015 at 21:49
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4$\begingroup$ The article you quote from is mostly irrelevant. The transmission properties of the Earth's atmosphere on the apparent colour of the moon are very small compared to the reflectance spectrum which is heavily weighted towards the red. If the eye was more sensitive the moon would appear pink/red even when high in the sky. Similarly, elements do not have some intrinsic colour, and they are not emitting "grey" light. We see reflected sunlight. In fact, what colour is "grey" - isn't it just white at lower intensity? $\endgroup$– ProfRobCommented Jul 12, 2015 at 11:21
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$\begingroup$ @Rob: There is no such thing as grey light, but there is such a thing as a grey object. It is perfectly possible for reflectance, which is a fraction, to be lower than 100%. $\endgroup$– user300Commented Jul 12, 2015 at 15:09
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$\begingroup$ @Rahul Grey is achromatic - colourless. IN RGB units it has equal R, G and B. Yes, a grey object might reflect all wavelengths equally at less than 100%. The moon is not grey, because it does not reflect all wavelengths equally. $\endgroup$– ProfRobCommented Jul 12, 2015 at 16:57
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$\begingroup$ I know this is an old answer/question, but aren't those pictures correct? Isn't it more yellowish with blue ? Where do these colors come from? I believe what we see from earth could be affected by the atmosphere. $\endgroup$ Commented Sep 21, 2015 at 13:09
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Gray is not a color - it is a shade. A shade of white. We perceive something as gray when
- it has no obvious "color" (that is, the amount of red, green, blue stimulus that the cones in our retina receive are roughly the same), and
- Something else in our field of view is brighter
Our eyes adjust our perception of gray or white relative to "something else". You eyes receive exactly the same information from a dimly illuminated object with a reflectivity of 1.0 as from a more brightly illuminated object with reflectivity of 0.1 (gray). When you see the moon "by itself", brighter than anything else in the night sky (or brighter than the blue sky in the day) our brain says "that's white". But if you put a big white sheet on the moon it would be a lot whiter - as you see in the photo of the astronaut whose white suit makes the moon look gray.
Here is another example of just how our eyes fool us:
It is reasonable to say that the little rectangle on the left looks white, while it looks gray on the right; and whether you consider it gray or white in the middle depends on your screen brightness, probably. But they are in fact all the same shade of gray.
There is a good description of the composition of the moon and its color at this link
But there is one other very cool thing. When you look at the intensity of the moon as it goes through its phases, you expect there to be more moonlight when the moon is full - obviously, since the "disk" of light is bigger. BUT! The intensity of moonlight does not scale with the phase (apparent area) of the moon. If it did, you would expect the intensity of moonlight to follow some smooth curve - but in particular you would not expect the intensity to change much right around the full moon (because the illuminated area doesn't change a lot). But that is not what is observed with information from this original source. Borrowing a curve from this link:
The moonlight gets considerably brighter (the moon "whiter") right around the full moon. There are two effects in play:
- Since the moon surface is rough, when the sunlight is shining across the surface at an angle, you will see some illuminated surfaces and some areas that are in the shade. This means that the average intensity will seem lower - an effect that is stronger as the light is more glancing: you see this very clearly in this image source
- The surface of the moon (moon dust) has a lot of tiny quartz spheres on it: these are the result of meteor impacts that heated the soil to melting point, after which the molten droplets fall back to the surface as little "glass spheres". And these have the interesting property of retroreflectivity: when light come from directly behind you when you look at a sphere, you will see a very bright reflection - because the sunlight is focused onto a point right behind the sphere, and you are looking at this bright spot "through a lens".
This second point explains the really sharp peak right around full moon - and this is why the moon looks particularly bright and white at that time (barring atmospheric effects).
This is described in more depth (and I just discovered it has a name: Heiligenschein which is German for "saintly glow") at this link
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It is not white, see this link for photos: http://www.mikeoates.org/mas/projects/mooncolour/intro.htm
See also this image I took, it does not appear white, one of the seas looks blue.
See this photo with increased saturation.
It appears that on average the Moon is grey with light coloration aligned with the terrain ranging from yellow to blue. The saturated photo shows this and a discerning eye can perceive the light color tint in the non-saturated photo.
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1$\begingroup$ As others point out above you can see bright stars as coloured against the same background. And actually the moon does appear coloured when it is low in the night sky and affected by atmospheric scattering. The answer can only be that the lunar spectrum is close enough to (what we perceive as) white that we are unable to see it as coloured. $\endgroup$– ProfRobCommented Jul 12, 2015 at 20:53
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$\begingroup$ The apparent magnitude of the brightest star is dim (Sirius at -1.4) compared to the Moon at -12. So the moon is extremely bright compared to those stars. I'm looking for the apparent magnitude of the moon on the horizon, my assumption is that it is not near -12. $\endgroup$– scmCommented Jul 12, 2015 at 21:06
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$\begingroup$ I also should point out that though this is a physics forum where we are used to objective instrumentation, the human visual system is not a spectrum analyzer. If you are in a dim room at night watching TV, your perception of what is white and what colors are appear normal. Walk outside and look at the window of the room. The room will appear to be bathed in blue light. This is because the white point of the TV is decidedly blue but you adapt to it as white. When you are outside looking in, you are no longer adapted to that white point. $\endgroup$– scmCommented Jul 12, 2015 at 21:29
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$\begingroup$ It is not the integrated magnitude that matters is it? Are you claiming that the brighter the object, the less likely we discern its colour? Do you have a reference for this? The other way around is certainly true. $\endgroup$– ProfRobCommented Jul 12, 2015 at 21:59
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$\begingroup$ Ok. I will witdraw my answer as to why it is perceived as white because I do not have a reference for white adaptation and the perception of bright objects. And I will make it clear what the photographs show in my answer. $\endgroup$– scmCommented Jul 12, 2015 at 22:29