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What makes the Moon shine white in the sky, although it seems to be as dark as asphalt?

Earth's average albedo is 3 and 4 times that of the Moon and Ceres respectively (0.37 vs 0.12 and 0.09). But except for the clouds, Earth doesn't shine bright white in images of Earth taken from cis-lunar space.

Here's an image of Earth seen from the Moon. Although it has 3 times as high albedo, it does not seem to be as bright as the white Moon is as seen on Earth's blue sky.

enter image description here

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  • $\begingroup$ Can you link such an image where the Earth does NOT shine as seen from the Moon? Just as a clue, why does the Moon is not always as bright as the full Moon? $\endgroup$ – Py-ser Mar 17 '15 at 9:28
  • $\begingroup$ @Py-ser I've added an example image. You know how the Moon looks from Earth. $\endgroup$ – LocalFluff Mar 17 '15 at 12:27
  • $\begingroup$ W respect to Earth appearing dark, liquid water has a very low albedo: en.wikipedia.org/wiki/Albedo Still, I think you'd notice the oceans against a black sky, if it weren't for those glaring white clouds. $\endgroup$ – Wayfaring Stranger Mar 17 '15 at 13:46
  • $\begingroup$ One cause is that Lunar dust has a tendency to reflect light back in the direction of the light source. That means that when the Moon is roughly full, we see it as brighter than we would if it scattered light isotropically, since in that case we have the Sun in our back. This doesn't explain <half Moons, though. $\endgroup$ – pela Mar 17 '15 at 14:06
  • $\begingroup$ @pela On images from Mars orbit, sand dunes made up of fine tiny particles look dark. As I understand it, because they are tiny, not because they are intrinsically dark individual by individual. And the dark Moon looking white at a distance cannot be an atmospheric phenomenon since images from the Dawn spacecraft approaching Ceres shows the same effect. $\endgroup$ – LocalFluff Mar 20 '15 at 18:15
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Here's my take on it. I'm not sure what the mystery is - there appear to be two contributing factors.

Both the Moon and the Earth simply reflect/partially absorb the light that is incident upon them from the Sun. The overall of albedo is of little consequence, since the reflection of any light will result in an object that appears bright against the night sky. The moon reflects quite uniformly across the whole of the visible range - it actually reflects red a bit better than blue. On the other hand the Earth's albedo is strongly wavelength dependent depending on what is doing the reflecting on the visible hemisphere. See the image below from this site. This injects some perceived colour into reflected earthlight.

Reflectance from Earth features

The second point is the way the eye works at low illumination levels. Although you might think so on a dark night, the moon is not very bright. It is faint enough that the eye starts to switch over to mesopic or even scotopic vision where the light levels are too low to stimulate the colour-sensitive cone cells and just stimulate the colour-insensitive rod cells.

In the case of the moon, this has the effect of removing the red tinge it would have otherwise and giving the moon a "silvery"/white appearance. This is known as the Purkinje shift.

How the Earth appears to the human eye from the moon I could not say. It is possible that it is bright enough to stimulate the cone cells and thus appear as coloured as the photographs suggest. Either way, you can't tell anything from a photograph other than that the light from the Earth definitely is coloured, whether we perceive it to be or not. Photographs of the moon however do not show any strong colouration. In terms of brightness you also can't tell anything from a photograph showing just the Earth. The Earth is much brighter than the moon in terms of reflected light per unit area and integrated over its disc as seen from the moon.

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  • $\begingroup$ So we are color blind at low illumination. That must be an important part of the explanation. The Moon is indeed quite faint in daylight. But stars range from blue via yellow to red to the naked eye, so some distant objects do look colored. And Mars is red while Venus is white. Both are CO2 rich atmospheric reflectors of sunlight. Jupiter has gray scale features in a simple telescope. But to a probe out there it seems to be a very colorful place, especially with Io. Maybe the subjective human perception of color doesn't go all the way together with today's physics. $\endgroup$ – LocalFluff Jun 10 '15 at 11:47
  • $\begingroup$ @LocalFluff See physics.stackexchange.com/questions/169969/why-are-stars-white/… Only the brightest stars/planets (or stars through a telescope) have perceptible colours. The full moon is bright enough that if it were very coloured we would perceive it. $\endgroup$ – Rob Jeffries Jun 10 '15 at 12:50
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The photo in your question is -- well, not exactly fake, but a composite. The biggest clue is that Earth is too close to the horizon; it would have had to be taken from within a few degrees of the boundary between the near and far sides of the Moon, and none of the Apollo missions landed there.

Furthermore, take a close look at the cloud patterns. The view of Earth appears to be identical to that in famous Earthrise photo taken from lunar orbit by Apollo 8 (click to see a larger version):

Earthrise

As you can see in that photo, the surface of the Moon is considerably darker than the Earth.

In the photo in the question, ignoring the inserted image of the Earth, the sunlit surface of the Moon is the brightest thing in the photo. The light balance must have been adjusted to make everything in the image easy to see.

As for why the Moon looks white in the night sky, it has an albedo of about 0.37, whereas the night sky has an albedo of about 0.00. Human eyes are very good at adapting to varying lighting conditions. When you look at a dark object against an even darker background, it's going to look white or light gray, even if it's intrinsically dark gray.

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Distance is what it's all about. The distance of view. And the behavior of sunlight. Look at Jupiter in the night sky. It's a brilliant, but apparently white, dot. Look through a good telescope. It isn't white in any sense, but rather, brown and red. It is exactly what you see in those pictures. Venus? Same deal. It's a golden brown in reality, but through the naked eye on Earth, it's white. I bet Earth does appear white to, say, an observer on Pluto.

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  • $\begingroup$ that's only mentioning the effect, but not explaining it. $\endgroup$ – Astrony May 25 '15 at 18:15

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