When looking at the day sky, you see a solid blue, with no stars, Moon only rarely, barely visible.

But photos of Earth show clear colors of the continents, yellow Sahara, green amazon forests. There's the same amount of air for the light to pass through when looking "Up" as "down", so why aren't all photos of Earth from space just of featureless blue ball with clouds - or at least strongly tinted blue?


Well we do see the day lit part of the moon if we actually look. If we knew where to look we'd see some brighter planets and stars as well. See 10 surprising space objects to see in the daytime sky and also read through Naked-Eye Venus Apparitions, Conjunctions & Elongations 2010 to 2020.

Raleigh scattering of visible light in Earth's atmosphere is wide angle. Photons that get kicked out of their initial direction go all over the place

If you are looking at the moon, most of the photons from the moon traveling towards your eye make it there un-scattered, so no definition is lost, and the ones that are scattered go in very different directions, so the moon is lightly dimmer but just as clear and crisp as it would be at night. However the scattered light from the Sun brightens the scene, so the dimly lit part of the moon (lit only by Earthshine) is not easily noticed.

If you put a large black spot on the Earth and viewed it from LEO, it would appear sky blue to you as well! However because the reflected light from the Earth is so bright it would be hard for you to see it, your eyes would be overwhelmed by the bright reflected light from the Earth.

However if you used a straw or hole in a piece of paper or whatever you had available on the ISS and blocked the excess light out, you'd see the blue sky above the (now mysterious) large back spot easily.

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above: Venus and the Moon during the day, from Earthsky.org, photo cretid nakedeyeplanets.com.

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above: Venus seen through a telescope during the day, from here.

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above: The Moon seen through a telescope during the day, from here.


Why doesn't Rayleigh Scattering obstruct Earth from space?

It does, to some extent. The Earth isn't called the "Blue Marble" for nothing, and it's not because the oceans are blue. It's called the Blue Marble because the sky is blue. A true color image from space, shows this:

"Blue Marble" image from the HIMAWARI satellite

Note the fuzziness of Asia in the image. That's mostly because of Rayleigh scattering. The blue oceans? The deep oceans are a very dim blue as seen from space, also thanks to Rayleigh scattering. However, the much more intense Rayleigh scattering from the atmosphere makes the oceans appear to be blue as seen from space. If the Earth's atmosphere was predominantly helium instead of nitrogen and oxygen, and if the Earth was still covered with 70% water, the Earth would be better called the Almost Black Marble.

  • $\begingroup$ Helium = lower electron density and molecular polarizability, and therefore lower dielectric constant? $\endgroup$
    – uhoh
    Feb 10 '17 at 19:26

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