Indeed it may seem surprising but the sky is not blue, it is… black! Half the time the sky is black when night falls (especially in the countryside and without a moon). And from space, the sky also looks black.
The sky is blue only because the sun exists and its light is… white, even if the sun is often represented as yellow when we paint it.
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The sky is blue only because there is an atmosphere around the Earth: the layer of gas that surrounds our planet. This atmosphere diffuses light. Rays of light generally travel in a straight line. However, milk or clouds are only white (or gray) if there is a light source (at night, clouds are also black). Clouds and milk spread light, like the walls of a lighted room, and like the sky.
With a candle placed behind a finger, it appears to be lit, but red. It is our body that is preferentially red. For diffusion by atmosphere, diffusion is preferentially towards blue, and less and less towards green, yellow and red (rainbow order).
An amusing consequence of this scattering through the sky of the blue light emitted by the sun, is that the sun appears less blue than it actually is, and more red and green than it is: the mixture of red and green appears rather yellow to our eyes.
When it sets, the almost horizontal Sun travels a longer distance, until it loses its green: it appears red to us, in a sky of pretty colours. For the same reason (atmospheric cross length) but on the contrary, at high altitude, as the sun rises, the sky is unusually blue, due to less diffusion of the atmosphere.
So why don’t the moon or stars appear to us in yellow? In fact, when it’s over the horizon, we sometimes have a red moon. Above all, our eyes saturate colors differently, and the intensity of the sun is incomparable with that of the stars or the moon.
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