and oxygen) are much smaller than the wavelength of light. When light

encounters particles much smaller than its wavelength, the scattered

intensity is inversely proportional to the 4’th power of the

wavelength. This is called “Rayleigh scattering,” and it means that

half the wavelength is scattered with 2**4 = 16 times more intensity.

That’s why the sky appears blue: the blue light is scattered some 16

times more strongly than the red light. Rayleigh scattering is also

the reason why the setting Sun appears red: the blue light has been

scattered away from the direct sunlight.

Thus, if the atmosphere of another planet is composed of a transparent

gas or gases whose molecules are much smaller than the wavelength of

light, we would, in general, also expect the sky on that planet to

have a blue color.

If you want another color of the sky, you need bigger particles in the

air. You need something bigger than molecules in the air—dust.

Dust particles can be many times larger than air molecules but still

small enough to not fall out to the ground. If the dust particles are

much larger than the wavelength of light, the scattered light will be

neutral in color (i.e., white or gray)—this also happens in clouds

here on Earth, which consist of water droplets. If the dust particles

are of approximately the same size as the wavelength of light, the

situation gets complex, and all sorts of interesting scattering

phenomena may happen. This happens here on Earth from time to time,

particularly in desert areas, where the sky may appear white, brown,

or some other color. Dust is also responsible for the pinkish sky on

Mars, as seen in the photographs returned from the Viking landers.

If the atmosphere contains lots of dust, the direct light from the Sun

or Moon may occasionally get some quite unusual color. Sometimes,

green and blue moons have been reported. These phenomena are quite

rare though—they happen only “once in a blue moon….” 🙂 The dust

responsible for these unusual color phenomena is most often volcanic

in origin. When El Chicon erupted in 1982, this caused unusually

strongly colored sunsets in equatorial areas for more than one year.

The much bigger volcanic explosion at Krakatoa, some 110 years ago,

caused green and blue moons worldwide for a few years.

One possible exception to the above discussion is if the clouds on the

planet are composed of a strongly colored chemical. This might occur

on Jupiter, where the clouds are thought to contain sulfur, phosphorus,

and/or various organic chemicals.

It’s also worth pointing out that the light of the planet’s primary is

quite insignificant. Our eyes are highly adaptable to the dominating

illumination and perceive it as “white,” within a quite wide range of

possible colors. During daytime, we perceive the light from the Sun

(6000 K) as white, and at night we perceive the light from our

incandescent lamps (2800 K, like a late, cool M star) as white. Only

if we put these two lights side-by-side, at comparable intensities,

will we perceive a clear color difference.

If the Sun was a hot star (say of spectral type B), it’s likely we

still would perceive its light as “white” and the sky’s color as blue

thats what I think haha

yeah, yeah, I know… It’s God’s favorite color sounds so much nicer, but until science can prove there is a god & we can ask him what his favorite color is, this is the best science can do.

thats my theory

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think about that for a while…..