I’m having an argument with someone. I said I believe the moon turns red because the light from the sun is shining through more of the natural gasses in the atmosphere, thus making it appear red. He says it’s the pollution in the air, and it wouldn’t turn red if we didn’t have any in the atmosphere. I know I’m right, but I just can’t prove it. Any astronomers or chemists out there that can help me?
I think you and your friend are both right, but you’re thinking about two different situations.
During a lunar eclipse, when the Earth’s shadow falls on the Moon, some light from the Sun that is refracted (bent) by the Earth’s atmosphere strikes the Moon. Because the light has traveled through so far through the Earth’s atmosphere, most of the bluer light is scattered away (that’s why the sky’s blue), leaving only reddish light to illuminate the Moon, and it turns a rather lurid blood red. (Fred Espenak’s web site has lots of good information about lunar eclipses, including a table of recent and upcoming eclipses.)
Lunar eclipses are fairly rare, though–just a couple per year. Most of the time when the Moon looks red, it’s due to pollutants and dust in the atmosphere. (Note that the dust can come from natural sources: dust storms, volcanic eruptions, etc., so it’s not all manmade pollution.) If the Moon is reddish but it is not full, you can be sure that it’s not a lunar eclipse.
The moon looks redder near the horizon for the same reason the sky is blue. The sky appears blue because more blue light is scatter by atmospheric gasses. Thus, when the moon is low to the horizon, more red light passes through because the blue light has been scattered away by the atmosphere. Dust and atmospheric water vapor enhance this effect, so the moon looks redder some days than others. Pollution might enhance this effect, but it is not the sole cause of it.
Are you talking about the sun, or moon, or both?
The red colour of the sun or moon on or near the horizon is caused by scattering by dust or particulate matter in the Earth’s atmosphere- this dust can be human pollution or natural dust from deserts or volcanoes-
in fact much more frequently it is from natural sources.
Most gases such as oxygen, nitrogen and carbon dioxide are transparent, and don’t scatter the light.
Large mist droplets dont scatter the light, but dim the sun, and can cause halos (parhelia)
Wonderful link.
Ah, but of course, if you are talking about eclipses, the refraction effect of atmospheric gases is more important.
Sorry, eburacum45, but that’s not right.
The Sun looks red at sunset (at the sky is blue) due to Rayleigh scattering, in which shorter-wavelengths of light are scattered by the molecules of gas in the Earth’s atmosphere. No dust is required, though it can contribute. Volcanic eruptions that throw lots of ash into the atmosphere tend to result in dramatic, fiery sunsets, for example.
Due to the Moon’s grey spectrum (roughly equally bright at all wavelengths) the reddening of the Moon when it’s rising or setting isn’t as dramatic as the redding of the Sun at sunrise and sunset. So usually you need some unusual atmospheric conditions to make the moon look really red.
OK…Podkayne’s first comment was the first one that jumped to my mind. You’re both right. But if it makes you feel better, you’re more right.
Your argument parter’s first premise is correct: pollution CAN have such an effect, but it would still happen even if there were absolutely no pollutants in the air.
One slight (nit-picky) correction on your part is that the moon turns red because the light REFLECTED FROM the sun goes through more atmosphere.
Q.E.D.'s comment “The moon looks redder near the horizon for the same reason the sky is blue” is accurate, but not because all the blue light has already been scattered away (there is ALWAYS blue light). It’s because the amount of atmosphere that the light is passing through at any given time scatters a certain wavelength of light the best. The sky is blue because during the day, the “thickness” of atmosphere that the sunlight passes through scatters that shade (wavelength) of blue the best.
The frequency of blue light is much “faster” than that of red light. Therefore the individual wavelengths are much shorter. This is why the comparatively “thin” layer of atmosphere scatters “sky blue” the best. As that “thin” layer gets “thicker” (as the sun sets), corresondingly longer and longer wavelengths get scattered, eventually getting to “Red”.
If you’ve ever noticed at sunset, when the sun is still above the horizon, but it’s almost dark, there is a spectrum across the sky. That is, it’s red/orange where the sun is, and gradually cycles through the rainbow until its dark.
Hope that helps (WHEW)!
Oops- dust free scattering does occur-
from the same link I posted just now-
Einstein was a busy little bee, wasn’t he?
Thinking about the colour of the moon in a eclipse, I can see now that refraction has very little to do with the red colour… there might be a refraction event at the edge of the earth’s shadow, but during totality the light reaching the moon will be scattered through the earth’s atmosphere. mostly by pure air particles in the upper atmosphere by Rayleigh scattering, and really represents the light that is visible after sunset.
(this might have a small pollution component as well- the areas of dusty atmosphere like the Sahara or Western Europe might be visibly more red if observed from the moon)
I have seen about ten different lunar eclipses over the years- one in about 1990 was about a L=0 on the Danjon scale
http://sunearth.gsfc.nasa.gov/eclipse/OH/Danjon.html
and the moon went purplish blue.
Finally if you go to about a million km out the earth’s atmosphere will focus all the sunset red light onto a small area-now that is refraction.