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Why does the Moon sometimes seem red?

  1. Aug 13, 2017 #1
    Hello all, as i write in the title my question is about the colour of the moon. Yesterday evening, i saw the moon rising to the sky in a red colour. Why is this happening? Is it an atmosphere/ gas problem?
  2. jcsd
  3. Aug 13, 2017 #2

    Charles Link

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    I do think we may have been close to a lunar eclipse the other night, with the solar eclipse coming up in another week an a half. I saw the very reddish moon also. Normally as the moon rises, instead of being white, it appears somewhat orange as the light passes through about 100 miles of atmosphere before it reaches us when the moon is on the horizon and the blue light gets scattered out. (Overhead, the atmospheric layer is about 10 miles, and that encircles the earth which is a sphere of radius 4000 miles, so that as the moon rises, it may have 100 or more miles of atmosphere to pass through to reach us). The blue light and shorter wavelengths gets scattered much more than the longer wavelengths such as orange and red, so we see the orange and red that reaches us directly. In this latest instance, I think that some of the light reaching the moon may have traveled through the earth's atmosphere before reflecting off the moon, so that the blue light and other shorter wavelengths got filtered out much more than usual, where normally the earth's atmosphere isn't in-between the path of the light from sun to the moon. ## \\ ## When the moon is overhead, the light only has 10 miles of atmosphere to pass through, so that most of the blue light, (and violet and green and all of the other colors), from the moon makes it directly to us along with the orange and red and the moon appears white in color. ## \\ ## Incidentally, this atmospheric scattering that scatters the shorter wavelengths much more than the longer wavelengths is known as Rayleigh scattering, and this also explains why the daytime sky appears blue. Most of the scattered light is light of shorter wavelengths. It also explains why the sun appears red or orange during a sunrise or sunset.
    Last edited: Aug 13, 2017
  4. Aug 13, 2017 #3
    thanks a lot for your anwer. So normally, the colour we should be looking at should be red but it gets filtred by the atmosphere and we see it white? I mean normally the moon is grey. We are able to see it because it is lightened by sun. The reflection of the sun by the moon which is not filtred makes the early moon seems red? Does hat low explains the waves as well?
  5. Aug 13, 2017 #4

    Charles Link

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    The white moon that we see overhead at night is a mixture of all colors that have minimal atmospheric filtering. The gray spots that we see when the moon is overhead at night are craters and other surface features that absorb the light. Meanwhile, in the daytime when the moon is overhead, the scattered (by the earth's atmosphere) blue light from the sun adds to the white and gray from the moon and it appears bluish white and gray. ## \\ ## When the moon is on the horizon, (it is 240,000 miles away as always), the light has approximately 100-200 miles of atmosphere to go through. The blue light and shorter wavelengths are strongly scattered and unable to get through this 100 miles without getting scattered. Draw a circular earth, 4000 mile radius, with a 10 mile ring of atmosphere around it. Notice when the moon is located (as the earth is rotating), so that the moon is on the horizon of the viewer standing on the surface of the earth, (rather than straight above the observer), the light will have a much longer path through the atmosphere to reach the viewer. As the earth rotates over the course of the day or night, the moon rises and sets, just like the sun. Approximately 6 hours after moonrise, the moon is directly overhead.
  6. Aug 13, 2017 #5


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    No, it's the other way around. It would look white if there were no atmosphere. (or rather white-ish, since it's sun's white light reflected off of dirty grey surface)

    The following is mostly just rehashing Charles' answers, but maybe it'll click if put in different words:
    What you saw is what happens when light coming from some source is being scattered by gasses or particulates in the atmosphere. This scattering is most efficient with shorter wavelengths, meaning that colours from the blue end of the spectrum are removed from the light, and what gets through is the longer wavelengths, on the red end of the spectrum.

    The more gasses and particulates are there in the way, the more scattering occurs. This means that you're most likely to see yellowish or even reddish moon when it's just above the horizon, where there's the most atmosphere for the light to get through. You're also more likely to observe red moons in polluted air.

    Think of it in terms of RGB colours. Red+Green+Blue netts white. Scattering removes first blue (turning the source yellowish), then green (looks reddish), then red (it's completely obscured by the thick haze in the atmosphere).

    This is the same mechanism that makes rising/setting sun look red, yellowish most of the time, and nearly white when it's approaching the zenith (because that's where there's the least atmosphere to scatter its light).
  7. Aug 13, 2017 #6


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    Two days ago I saw the moon at about 30-40 degrees above the horizon.
    It was a strong orange, more so than I have noticed recently.
    Where I live (Eugene, Oregon) and we have recently had a lot of smoke in the area due to forest in Canada (and other places), I assumed the color of the moon was due to the smoke in the air doing the filtering.
    Local conditions might also have an effect.
  8. Aug 13, 2017 #7

    Charles Link

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    Yes, perhaps that is why I observed it to be more red on the horizon than usual, rather than an effect such as a near lunar eclipse. Since the atmospheric layer isn't more than about 20 miles, a near lunar eclipse should not produce the effect that was observed. I observed it to be reddish looking east from Chicago, but perhaps the smoke from the west traveled far eastward.
  9. Aug 13, 2017 #8
    I think i got it now!!! You guys are amazing thanks :)
  10. Sep 6, 2017 #9
    Presently, In Tacoma Washington area, we have a VERY Reddish-orange moon due to the huge amount of smoke from forest fires all along the coast. We had a near full moon, and it was a dim circle in the sky with the maria barely visible. So, aside from the various geometric positions that can give an orange or red moon, the amount of smoke in the air can have an immediate and far reaching impact.
  11. Sep 6, 2017 #10
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