Why can't moon light (reflected sun light) turn the sky blue?

  • 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).


    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.

    @fractalspawn - Or, conversely, in a very light-polluted area, so that the coloring is attributed to the ambient light level.

    @fractalspawn I live in a big city, it's almost impossible to see low magnitude stars.

    @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.

  • userLTK

    userLTK Correct answer

    6 years ago

    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.


    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.

    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.

    @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.

    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.

    Can confirm the sky actually *is* blue at night, moon or not, if my many attempts at long exposure night photography (outside of the reddish/orange of city lights) are any indication.

    @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.

License under CC-BY-SA with attribution

Content dated before 7/24/2021 11:53 AM