# Do the gases in the Earth's atmosphere affect the color of a lunar eclipse?

As the sun's rays pass through the Earth's atmosphere only the red light gets through. Is this the result of specific gases in the Earth's atmosphere filtering the red light? Does all of the nitrogen and oxygen affect the sunlight shining through it?

Let's say the Earth (or just about any earth-like planet) has an atmosphere made mostly of green chlorine. When the sun shines through the chlorine atmosphere and hits the moon during a lunar eclipse, would the moon appear green? Would different gases result in the moon becoming different colors?

• Possible duplicate of Red Moon a Characteristic of all Total Lunar Eclipses? – Carl Witthoft Feb 1 '18 at 14:05
• Not really. With some overlap. But the link you suggested can complement my reference to Wikipedia. – Alchimista Feb 1 '18 at 15:20
• @uhoh that it why is a different answer to a different question. It assumes RS as the only process occurring in a lit atmosphere. RS will take place alongside a most important absorption in, e.g., a chlorine atmosphere. – Alchimista Feb 2 '18 at 9:01
• And RS is not agnostic to the the molecular nature at all. As mentioned in my answer. – Alchimista Feb 2 '18 at 9:03
• Gliese you might be interested to this space.stackexchange.com/questions/24108/… – Alchimista Feb 3 '18 at 11:53

From the way you have formulated your question, it seems to me that you are aware of the light diffusive process named Rayleigh Scattering and why it explains the various tints and colours of the sky depending on the Sun position. Other readers can refer to https://en.m.wikipedia.org/wiki/Rayleigh_scattering.

We are used to our atmosphere and as such often discuss the RS merely on geometrical bases, as I shortly did just above.

However, the intensity of RS does not only depends on the inverse of the fourth power of wavelength, but also on the size of the scattering particles and specifically on the square of the molecular polarisability.

As such, changing the nature and composition of the atmosphere does effect the intensity of RS. An example is SO2, which tends to intensify the yellow tint of sunsets in polluted area.

This should answer the first part of your questions. Let us move to the second part, specifically using the example atmosphere that came to your mind, chlorine as a green gas, i.e. molecular chlorine Cl2.

In this case, the gas is not colourless. Different respect to nitrogen and oxygen, chlorine does absorb in the visible part of the electromagnetic spectrum. It absorbs the colours opposite to green in the colours wheel, to keep things simple as possible.

In an inherently coloured atmosphere the relative importance of scattering will be less or at least variable depending on parameters such as atmosphere thickness, density etc. Let me ignore RS in such a toxic atmosphere :)

What is important now is that when you point your eyes up to the sky, the green colour you see is not that one whose propagation is hindered, but rather the one that can propagate (given that we are ignoring RS). In other words, the light will be always richer of green colour.

To make the final answer simple let us look at the moon when the only light that hit it is that coming throughout the planet atmosphere.

With Earth, the Moon is bloody as for RS has removed the blue violet part of the solar spectrum.

A hypothetical change on Earth atmosphere composition will also have a minor impact, as we have seen that the polarisability of the molecules counts.

About Chloro, the planet having chlorine atmosphere. During the eclipse, its moon will have a greenish tint, or likely appear green. In this case the moon is lit by sunlight depleted of the wavelengths absorbed by chlorine.

To resume, the colour of the moon is always that less affected by the planet atmosphere but the mechanisms and the final result differ.

When the atmosphere is intrinsically colourless, the colour of the moon is somehow "complementary or so" respect to that of the daily planet skydome. When the atmosphere absorbs, the moon and the sky dome will tend to the same colour.

Note: I did assume that sunlight can pass the intrinsically coloured atmosphere. Again depending on various parameters, this is of course not given. Most likely we won't see the Moon. We shall really care of our atmosphere.

Also the intrinsic properties of the moon do not change. But this is even more trivial.

• +1 Very nice answer! At first I said (in a since-deleted comment) that the particular species would be color-agnostic as far as Rayleigh scattering. I don't think it has an impact on the wavelength dependence, but "color" is a complex perceived phenomenon, and light blue and deep blue (for example) are different colors! Even if the shape of the absorption spectrum were the same, the perceived color of the light is pretty subjective and would be different. And then there's the chlorine which I missed completely – uhoh Feb 2 '18 at 15:23
• – uhoh Feb 2 '18 at 15:26
• @uhoh. Meantime I have qualitatively solved the Mars issue and I am rather glad of it. However while looking for corroborating data or possible interpretation I have found a page that does the work. I am not going to write a page just to get likes. How can I signal the link but put emphasis on it so it can serve others and not only the OP? – Alchimista Feb 2 '18 at 20:43
• I'm all for neatness and comment cleanups. :-) – uhoh Feb 2 '18 at 23:36
• Link-only answers are strongly discouraged. There should be enough of an explanation so that if/when the link breaks, future readers can still get at least a partial understanding from your summary of the important points from the link that apply. – uhoh Feb 3 '18 at 11:23