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Why does the Moon, being close to the horizon, turn red only slightly, or not at all?

Aren't the causes that lead to the coloring of the Sun (atmospheric refraction) are also true for the Moon?

In addition, the spectrum of moonlight is more red-shifted than that of the sun, so shouldn't that contribute to an even more intense reddening of the lunar disk than the sun?

From Universe Today Rising Full Moon time lapse

A series of photos combined to show the rise of the July 22, 2013 ‘super’ full moon over the Rocky Mountains, shot near Vail, Colorado, at 10,000ft above sea level in the White River National Forest. Moon images are approximately 200 seconds apart. Credit and copyright: Cory Schmitz

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    $\begingroup$ Your example picture isn't that good; rising behind mountains means it is not actually close to the horizon when it comes into view. Degree of reddening of both moon and sun depends more on dust scattering rather than refraction, and so on atmospheric conditions which vary (I've see sunrise looking very yellow rather than red)-- when there is heavy extinction (very red sun) moon will be more difficult to see. $\endgroup$
    – antlersoft
    Aug 22, 2023 at 18:50
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    $\begingroup$ Further, the moon is not a perfect reflector of the sun across the visible spectrum. In particular, it appears that red and blue are reflected less than the intermediate wavelengths. $\endgroup$
    – Jon Custer
    Aug 22, 2023 at 18:53
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    $\begingroup$ Does this answer your question? Why does the moon sometimes appear giant and a orange red color near the horizon? $\endgroup$ Aug 23, 2023 at 1:51
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    $\begingroup$ related Why is this moon red? $\endgroup$
    – uhoh
    Aug 23, 2023 at 3:53
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    $\begingroup$ Please do not post content that you didn't create without mentioning in some way that you didn't create it. You also need to mention who the creator was, if possible, and give a link to the source. Please see astronomy.stackexchange.com/help/referencing $\endgroup$
    – PM 2Ring
    Aug 23, 2023 at 21:13

4 Answers 4

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The moon does significantly redden when it's close to the horizon, especially if you can see it over the ocean or from very flat ground, where you have a clear view all the way to the horizon. A clear view is very important; even 10 degrees up, such as looking over a distant mountain range, will produce a much less red moon.

https://en.wikipedia.org/wiki/Full_moon#/media/File:Harvest_moon.jpg

But, arguably, it's still less red than a sunset. Why is the moon's reddening less obvious than the sun's?

The moon is already a little reddish

I think it's important to think about what 'reddening' means. You said "the spectrum of moonlight is more redshifted than that of the sun, which should contribute to an even more intense reddening" -- but that's actually the opposite of the case. (I would not use the term "red shift" in this context; that term has a specific astronomical meaning related to relativistic speeds.)

The sun at the horizon turns red because the blue light is being scattered by the atmosphere (contributing to blue skies for the day side of the world), and the red passes straight through. Consider if the sun produced only red light -- then you would see no additional reddening near the horizon because there's no blue light to remove from it.

So if the moon's spectrum is already redder than the sun's, then we would expect its reddening to be less intense than the sun's, not more -- there's not as much blue light to remove, so the change is smaller. The moon may be as red or redder than the sun from an objective "I am measuring light frequencies" perspective, but if you take two moon photos, one at the horizon and one high in the sky, and lay them side by side, the difference will be less impressive.

How red is the sun, really?

In your question you state that the sun "turn[s] bright red when it's close to the horizon", but I don't think that's true. Not every sunset features a really red sun; a middling orange is far more common. The deep crimson is usually something you usually only see when there's a lot of particulate in the atmosphere, such as from a volcanic eruption or large fire. So I don't think the sun's color shift is as dramatic as you think, and thus the moon's less dramatic color shift matches it more closely than you're expecting. The moon does indeed turn a deep blood red when the atmospheric conditions are right for it.

But also, photos are tricky things, because photographers almost always adjust the color of a picture, and the display unit you're viewing it on will alter the perceived colors. Moon photos don't typically try to enhance the moon's redness -- for example, the photo you posted is clearly meant to retain some realistic green color in the trees, which can't be compared to a sunset photo that's been tweaked for maximum intensity.

Your eyes aren't perfect either.

Another contributing factor is that the moon is much less bright than the sun, and seen against a generally dark sky. Under those conditions your eyes do a worse job of color discrimination, especially as the more sensitive rod cells in your eye take over for the color-vision cone cells. However, the blue-sensitive cone cells retain their responsiveness better than your red and green cones do under low-light conditions, which means colors under lower light levels tend to look bluer. This is called the Purkinje effect. That means even in person, it's likely that your eyes will perceive the moon as being less red than it truly is.

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    $\begingroup$ Also if the sun produced only red light, Superman would get weak, so don't do that. $\endgroup$ Aug 22, 2023 at 20:14
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    $\begingroup$ Great answer! Here's the spectral albedo of the Moon: astronomy.stackexchange.com/a/28862/7982 $\endgroup$
    – uhoh
    Aug 23, 2023 at 3:54
  • $\begingroup$ The question is why the maximum redness in moon is less than sun. It is answered only in the last section which is handwavy. $\endgroup$
    – tejasvi88
    Aug 23, 2023 at 5:58
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    $\begingroup$ "So if the moon's spectrum is already redder than the sun's, then we would expect its reddening to be less intense than the sun's, not more" — this is completely wrong. First, there's quite a lot of blue scattered by the lunar surface, the spectral albedo in the blue part is about half as high as in the red part. Second, even if there were zero blue in albedo, we'd still get a deeper red after extinction through the atmosphere, because a product of two decreasing function is a faster decreasing function. $\endgroup$
    – Ruslan
    Aug 23, 2023 at 8:14
  • $\begingroup$ @DarthPseudonym Luckily it's the yellow that powers Kryptonians, so the filtering of blue at sunset doesn't affect him. $\endgroup$
    – Barmar
    Aug 23, 2023 at 13:35
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Your mountain is about a degree or more above the mathematical horizon. The most intense reddening (and vertical flattening) happens in the last few degrees of elevation, and your image just doesn't record the Moon in these moments.

From my personal experience, on days when the Sun looked orange at sunset, the lunar crescent was almost red (surprisingly red!) at moonset. So it was definitely much redder than the sun, contrary to what the other answer says.

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  • $\begingroup$ this photo just illustrates the usual colors of the lunar disk. "the lunar crescent was almost red (surprisingly red!) at moonset." An interesting observation. I have never seen anything like it, not even during a partial lunar eclipse. $\endgroup$ Aug 23, 2023 at 20:19
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    $\begingroup$ Once I got to experience a nighttime moonrise from a plane in a humid weather. I saw a big, deep orange semicircle. The color was so deep, so fascinating and almost unnatural, that I couldn't recognize what I was seeing for a long time. But as soon as it left the horizon, the color turned the usual grey-yellow on OP's photo. It's just a sight you rarely ever see from the ground. $\endgroup$
    – Neinstein
    Aug 23, 2023 at 21:26
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    $\begingroup$ @Neinstein well, your case likely increased redness even further from what you'd see at ground level by two mechanisms: 1) higher altitude which lets you see moonrise earlier and through a thicker layer of air, leading to higher extinction; 2) high humidity can result in wavelength-selective Mie scattering that would e.g. make faraway cumulus clouds yellow at daytime (which doesn't usually happen at lower humidity). $\endgroup$
    – Ruslan
    Aug 24, 2023 at 21:06
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Photons entering the Earth's atmosphere do not "know" from what object they have been emitted. The scattering coefficient as a function of wavelength is the same for light reflected from the Moon as for light arriving directly from the Sun.

Thus the answer is in terms of your perception. There is no physical reason why the overall reddening of moonlight should be different to the reddening of sunlight.

However, although moonlight isn't (significantly) "redshifted" in the Doppler sense, it is intrinsically slightly redder than direct sunlight. The small relative deficit of blue photons from moonlight compared with sunlight could mean that you perceive the reddening effect of the atmosphere to be less, since blue photons are more readily scattered than red photons (this is what we mean by reddening), but I suspect the difference in the pre-atmosphere spectrum of Sun and Moon is too small for this to be perceptible by the human eye.

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One possible reason could be the additional scattering of sunlight after hitting the moon surface. Higher frequency light is more likely to scatter from the moon surface and reach the earth. [citation needed] This will blue shift the reflected sunlight reaching the earth which does not occur when the sunlight reaches directly from sun.

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  • $\begingroup$ If this were true, the Moon would appear bluer than the Sun when in zenith, but if anything it is yellower. $\endgroup$ Aug 25, 2023 at 14:36

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