Does turning the colour of the sky blue need more luminous light? Does it depend on luminosity or some other factors are also responsible for this phenomenon? Why can't the moon light turn the sky blue even a little bit (at least the area near the disc).


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    $\begingroup$ You must live in an incredibly clear and/or elevated area. Anywhere that I've been, even in very Class 2 dark sky sites in Western US, the moonlit sky is a dark dark "midnight blue" during a full moon. Hell, even a quarter moon will color it something like "navy blue". You'll have to tell me where you live so I can go there and do some astrophotography. $\endgroup$ – coblr Mar 11 '16 at 0:31
  • $\begingroup$ @fractalspawn - Or, conversely, in a very light-polluted area, so that the coloring is attributed to the ambient light level. $\endgroup$ – Bobson Mar 11 '16 at 16:04
  • $\begingroup$ @fractalspawn I live in a big city, it's almost impossible to see low magnitude stars. $\endgroup$ – Sirius Mar 20 '16 at 7:21
  • $\begingroup$ @Sirius, yes, I was actually noticing this over the weekend. I live right in the center of Los Angeles and the sky was a noticeably dark blue near zenith even with a 3/4 moon, but it quickly got purple-brown and then orangish as you tracked down towards the horizon. I thought you had really clear sky since it's this haze that turns the different colors. $\endgroup$ – coblr Mar 21 '16 at 19:49

The simple answer is that it does, but it's not bright enough to be visible to the naked eye. Earth's atmosphere scatters the moon light just like sunlight.

The full moon (like the sun) fills about 1/2 of 1 degree of the sky, the entire sky being 180 degrees, give or take, so the full moon fills less than 1 part in 100,000 of the night sky, so there simply isn't enough blue light to be visible over the brighter stars even with the brightest full moon. Our eyes are very good at seeing variations in brightness, but not that good. . . . and, for what it's worth, the night sky has always appeared to have a dark bluish tint to me, but that might just be my brain playing tricks on me because logically I know it's there. I'm not sure whether it's actually visible.

With a good sized telescope, moonlight scattering acts as a form of light pollution. Telescope users know that you get better visuals when there's no moon.


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    $\begingroup$ It's not just you - the blue tint is there. It's quite easy to see when you have a reference - a guy in a black "ninja" suit stands up against the background (both the horizon and something like a distant hill, for example) like an idiot. Dark blue is the colour you want to minimize visibility at night, especially during full moon. $\endgroup$ – Luaan Mar 10 '16 at 13:27
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    $\begingroup$ While I agree with your conclusion, I think it is pointless to talk about the apparent size of the moon : the moon and the sun have the same apparent size, so the sun should make the sky as blue as the moon ! What really matters is the intensity of the source : diffused light is somewhat proportional to the intensity of received light, so because the sun is brighter, its diffusion is brighter, and we can see it. $\endgroup$ – Quentin Mar 10 '16 at 14:28
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    $\begingroup$ @Quentin Exactly so. The more relevant bit of info is that the sun has an apparent magnitude of -26.7 or so, and the moon is -12.6 or so. The difference there, 14 magnitudes, means that the sun is over 400,000 times brighter than the moon, and ergo the "blue sky" from the moon is 1/400000 as bright as daytime skies. If the sky is completely clear (no haze etc), it's just not noticeable. $\endgroup$ – dcsohl Mar 10 '16 at 16:30
  • $\begingroup$ The relative brightness of the sun is more relevant, but I don't think the size of the moon is completely irrelevant. As the blue light from the moon is scattered over so much area, but point taken. The relative brightness is a better point. The orange/red color of the moon at Moonrise and Moonset is also proof of blue-light scattering. $\endgroup$ – userLTK Mar 10 '16 at 17:38
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    $\begingroup$ @Michael The night time sky is bluer than sunlight. This is well known to any astronomers that do imaging/spectroscopy of faint objects. The presence of the moon affects the sky background at blue wavelengths, far more than red. $\endgroup$ – ProfRob Mar 11 '16 at 10:06

It does actually, but the human eye can't see it. But long exposure photography can see it easily.

enter image description here

Or this photo, taken about three hours after sunset and lit by a nearly-full moon.

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    $\begingroup$ That photograph looks, to me, as if it was taken shortly after sunset (or shortly before dawn), at which time the sky is strongly blue even to the naked eye. Long exposure photographs taken in the middle of the night look much blacker than this. $\endgroup$ – David Richerby Mar 10 '16 at 20:27
  • $\begingroup$ @DavidRicherby, is this photo, taken about three hours after sunset and lit by a nearly-full moon, any better? $\endgroup$ – Mark Mar 10 '16 at 22:09
  • $\begingroup$ @Mark Yes, that works. (As would a statement to the effect that the photo in the answer wasn't actually taken near dawn or dusk.) $\endgroup$ – David Richerby Mar 11 '16 at 1:08

userLTK nailed it, but I'll add the answer to the last part of your question. Close to the Moon we often do see scattered light. This is a phenomenon called Mie scattering where it cannot be assumed that the scatterers are much smaller than the wavelength of light (water droplets etc). Mie scattering is roughly wavelength independent and much stronger in the forward scattering direction. Therefore any "glare" around the Moon is usually the same colour as the Moon itself.

The sometimes-seen halo (at about 22 degrees) is a different phenomenon caused by refraction through ice crystals in the upper atmosphere.

  • $\begingroup$ That begs the question as to why the affect you note is specific to the moon, and doesn't show for the sun. $\endgroup$ – JDługosz Mar 11 '16 at 3:10
  • $\begingroup$ @JDługosz because usually you don't stick your eyes that close to the Sun? $\endgroup$ – John Dvorak Mar 11 '16 at 6:48
  • $\begingroup$ The halo can be significantly larger than the moon so not "that close" $\endgroup$ – JDługosz Mar 11 '16 at 7:10
  • $\begingroup$ @JDlugosz Are you trying to argue that there is no such thing as Mie scattering from aerosols in the atmosphere? $\endgroup$ – ProfRob Mar 11 '16 at 7:31
  • $\begingroup$ No. I'm saying that stating that "the moon does something the sun doesn't do" doesn't answer the question of "why does the sun do something the moon doesn't do?". $\endgroup$ – JDługosz Mar 11 '16 at 7:51

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