Okay we all know about Raleigh Scattering, which makes the sky blue. And by the same token, sunsets appear red because sunlight traveling through more atmosphere will "lose more blueness" as it's scattered away.

But what about the Moon? The Moon is just reflected sunlight, so when the Moon is setting on the horizon, it should appear reddish right? But I've never seen that happen.

Now I know lunar eclipses are red, so I'm not discounting the principle of Raleigh Scattering or anything. But there seems to be something else at play causing the normal rising/setting Moon to not turn red.

  • $\begingroup$ Did you try to take a series of shots with a camera for comparison? $\endgroup$ – CuriousOne Oct 5 '14 at 4:44
  • $\begingroup$ Afraid not, all i have are my own two eyes and a fairly unsophisticated camera phone. $\endgroup$ – DrZ214 Oct 5 '14 at 5:24
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    $\begingroup$ Yeah, unless you can force your phone's camera to take raw images, that's not very helpful. $\endgroup$ – CuriousOne Oct 5 '14 at 5:26

This is just an opinion, but the moon on the horizon is simply less visible than the sun is. I suspect that color changes it makes are more subtle and less easily noticed. However full moons are often noticeably orange. Here is a page with a wonderful time lapse view.

enter image description here


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    $\begingroup$ That's a beautiful picture, but the huge jump from yellow to red on the last sequence makes me wonder if there is any smog or pollution on the immediate horizon. The sun also can turn very red when just above a smokestack for this reason. $\endgroup$ – DrZ214 Oct 5 '14 at 5:26
  • $\begingroup$ Yes, it may be. Aerosols are a factor in the sun's color as well. The sun is always "reddened" at the horizon, but is not always "red". $\endgroup$ – BowlOfRed Oct 5 '14 at 7:55
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    $\begingroup$ @user1705043 - That extreme reddening right at the horizon is because of the huge amount of air that moonlight has to pass through when the moon appears to be at the horizon. An obvious contributor is the low angle. A less obvious contributor is the atmosphere's density gradient. This results in atmospheric refraction. That very red moon is actually below the horizon. You can see the effects of refraction in the first two full images of the moon. Those images aren't quite round. $\endgroup$ – David Hammen Oct 5 '14 at 19:44

But I've never seen that happen.

You haven't looked then.

The rising or setting Moon is rather reddish, just as is the rising or setting Sun. However, there is a difference between the Moon and the Sun. You can look directly at the Moon, even a full Moon, regardless of where it is in the sky. On the other hand, you can only glance at the Sun when it is rising or setting. You cannot even glance at the Sun if it is at any appreciable distance above the horizon.

Both the Sun and the Moon appear reddish when they are very close to the horizon. This reddening diminishes rapidly with increasing angle above the horizon. You don't see this with the Sun because you cannot look at the Sun except when it is extremely close to the horizon. People perceive the Sun as yellow or red because the only time you can look at the Sun is when it is at sunrise or sunset. During most of the day, sunlight is white (as perceived by the human eye).

On the other hand, you can look directly at the Moon, regardless of its elevation. The rising Moon appears to be quite red on the horizon. This quickly changes to yellow and then to white as the elevation angle increases. You would see the same phenomenon if you could look directly at the Sun. Your camera can look directly at the Sun. The color of the rising Sun changes from red to yellow to white quite rapidly.

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    $\begingroup$ "You haven't looked then." is a pretty bold statement to make to a stranger. In fact i have looked at the rising Moon, and i've never seen it turn red or reddish. The biggest color change i ever witnessed is a pale yellow nowhere near red. $\endgroup$ – DrZ214 Oct 5 '14 at 5:27
  • $\begingroup$ Unless you live in a mountainous area where you can't see the true horizon, you haven't looked. What can I say? I've seen plenty of yellowish and reddish moonrises and moonsets. You need to look. $\endgroup$ – David Hammen Oct 5 '14 at 5:58
  • $\begingroup$ Another thing thing to consider is that there is more variability in the amount of reddening of the moon, compared to that of the sun. This because sunset is always at the end of the day (to state the obvious) which is when typically dust and other such stuff is at a maximum, so there is a lot of scattering. Moon rise/set, on the other hand, can be at any time of day or night, and so there will be more variation in the amount of reddening. However there is definitely reddening. $\endgroup$ – Dr Chuck Oct 5 '14 at 8:28

Is this not a question about human physiology rather than physics?

We are discussing the variations in observed response of the human eye to light with different qualities: overall intensity, and distributions over the visible spectrum.

This question seems to ascribe all of the observed differences (or lack thereof) to the quality of the light; "the Moon [is] not redder" is the basis of the question.

The appearance (to a human observer!) of a color wheel under sunlight and moonlight changes greatly. Yet we do not theorize about the changes in the pigments under various light levels. We work out the sensitivity of the rods and comes in the retina, and conclude that moonlight is not intense enough to trigger color vision in the human eye.

Perhaps the question should be deferred, pending measurement of the spectrum of moonlight for the moon at different elevations.

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    $\begingroup$ This is a question about physics. The reddish/yellowish sunrise, sunset, moonrise, and moonset are real phenomena and are caused by Rayleigh scattering. $\endgroup$ – David Hammen Oct 5 '14 at 16:49
  • $\begingroup$ The last line of the question is quite explicit: The OP wants to know why the rising/setting moon does not turn red. $\endgroup$ – DJohnM Oct 5 '14 at 19:25
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    $\begingroup$ But it does turn red. The OP is mistaken. $\endgroup$ – David Hammen Oct 5 '14 at 19:28

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