Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 15/12/2011 08:08:43
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I was asked why a lunar eclipse looks red. What's the reason?
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The sky is blue due to molecules clouds etc, scattering the 'white' sun light in the atmosphere. When the sun goes down it can look red due too the longer path that light has to travel in the atmosphere before reaching your eye. And a moon eclipse is created by the shadow of our earth falling on the moon. To create that shadow the light from the sun has to pass through our atmosphere, and at a total eclipse that same light, reaching the moon, then reflected back to your eyes, must use the longest path possible through Earths atmosphere.
"A simple way to describe what happens would be to say that the air molecules, which are about 0.0004 millimeters in diameter, are very close in size to the waves of blue light."
And "The red colour in the sky at sunset (and sunrise) is due to an effect called Rayleigh scattering. There is a similar form of scattering called Mie scattering which is responsible for the white colour of clouds.
Particles in our atmosphere that are approximately the same size as the wavelength of visible light cause the white light from the sun to scatter and split into individual components. Oxygen and Nitrogen (the main components of our atmosphere) scatter violet and blue light due to their small size. This is why the sky appears to be blue in the day time, especially at midday when the Sun is closest to us.
During sunrise and sunset the distance that the light has to travel from the Sun to an observer is at its greatest. This means the a large amount of blue and violet light has been scattered so the light that is received by an observer is mostly of a longer wavelength and therefore appears to be read."
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Scattering of Light (http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/opt/mch/sct.rxml)
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During a total lunar eclipse the moon is only (dimly) illuminated by light which has passed through Earth’s atmosphere. Passing through Earth’s atmosphere has filtered out some blue from the sun's white light,
(see “why is the sky blue”), leaving orangey-red light which reaches the eclipsed moon,
(so same mechanism which is responsible for the colours of sunrise/set).
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Yeah, another thing that I think is correct, although I can't swear to it, is that the red light we see actually has 'bent' around Earth following the atmosphere. It is the simplest explanation for why the moon can be seen as having a reflected color. Either the atmosphere acts like some lens or prism, or that plus some weak combination of earths gravity.
I looked to see if it was correct but I didn't find anything that had it calculated.
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... the red light we see actually has 'bent' around Earth following the atmosphere. ... the atmosphere acts like some lens or prism
If the orange-red colour is due to refraction of the sunlight by the Earth’s atmosphere, (like a prism), then the moon should turn many colours during the eclipse as it passes through the spectrum created, not just orangey-red.
(https://upload.wikimedia.org/wikipedia/en/thumb/4/47/Lens6a.svg/415px-Lens6a.svg.png)
https://en.wikipedia.org/wiki/Chromatic_aberration (https://en.wikipedia.org/wiki/Chromatic_aberration)
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Well, not that then, didn't think the words through. But still, to fill the shadow red you need something to to be reflected from that 'shadow' as I see it, and that should mean that the light somehow 'bends', and assuming that the atmosphere does it seems reasonable to me.
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I guess I haven't paid a lot of attention to Lunar Eclipses.
When the sun or moon is near the horizon, as viewed from Earth, they will often turn very orange colored. Perhaps partly due to smog and local effects. But, if light is cutting a tangent through Earth's atmosphere, it would essentially travel twice the distance through the atmosphere than what we are seeing when viewed from Earth. At least the light traveling through the lower atmosphere. That traveling through the upper atmosphere would have a shorter, lower density path to travel through.
Keep in mind that the Earth is bigger than the moon.
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Of course, the sun is larger than both the Earth and the moon, but the moon is small enough that if you stood at any place on the moon during a lunar eclipse, essentially all of the light would be coming from the more distant parts of the corona.
While there is a portion of the corona close to the sun that is very hot, the further the corona extends from the sun, the cooler it would be (I think). And, thus, I would expect the spectrum to be shifted redder.
One other thought...
Is the reason the sky is blue is that our atmosphere absorbs/blocks blue.
That might also give an effect of red-shifting (less blue getting though) some of the light that passes through the atmosphere.
Perhaps the color of the moon is a combination of the earth's atmosphere, as well as light being generated from the cooler parts of the corona.
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Just a weird thought. Is there any way for a shadow to reflect light :)
Don't think so myself?
So how can a shadow become red?
That the sun can is quite understandable though..
So either it's not shadowed at all, as we see it become red, or it must have to do with the atmosphere creating a red touch, as a shadow won't reflect any light, well, does it? :)
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Just a weird thought. Is there any way for a shadow to reflect light :)
Don't think so myself?
A shadow is just a region in which there isn't much light, because you're blocking it. Light can't reflect off of "a lack of light," so a shadow can't reflect light.
So how can a shadow become red?
That the sun can is quite understandable though..
Easy. You just block all the light except for red light and the shadow becomes red.
So either it's not shadowed at all, as we see it become red, or it must have to do with the atmosphere creating a red touch, as a shadow won't reflect any light, well, does it? :)
The atmosphere blocks (by scattering) most of the non-red light, so only the red light reaches the shadow.
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Yeah, that's the easiest understandable option for me, assuming that it is light reflected from the 'shadow'. And that should mean that there is light from the sun, getting bent by our atmosphere, as well as filtered to only leave the red spectrum', to then be reflected back from what we call the earths 'shadow'.
I was alternatively wondering if it could be some sort of effect of the atmosphere itself actually :) Not that I see how it should work as I write, but then there would be no need for any reflected light from the shadow as the effect would be an 'optical illusion'. I know those effects exist otherwise.
But the explanation you gave is the one I think too as most reasonable.