The blue color of the sky is explained by the diffusion of light by the air molecules contained in the atmosphere. Most colors, red, orange and yellow, go through the atmosphere without being too disturbed by the air. On the other hand, the blue light is absorbed by the air molecules that re-emit this radiation in all directions. The blue light is thus scattered throughout the sky. This is the reason why, no matter where you look in the sky, the sky looks blue to you. We can thus conclude that on a star without atmosphere like the moon, the light will not be diffused and will reach the ground directly without being disturbed. Since the bottom of the space is black, the moon’s sky will also be desperately black.

Why is the sky darker at altitude? It is also due to the phenomenon of diffusion. At the seaside the density of the air is greater than at altitude and the atmosphere also contains much more dust and water vapor raised from the ground or transported by the winds. It is therefore normal that the blue light is more diffused at the edge of the sea and that the sky becomes clearer because the light encounters many more obstacles than at altitude where the air becomes more rare, it influences less the blue light.

Why does the sun turn red in the morning and evening? At sunset, the light must pass through a greater thickness of atmosphere, the diffusion of light is more important, which is why the sun is much less bright and can be observed without dazzling. Its color gradually changes from orange to red because this time it is not only the blue radiation but also the green rays that are diffused and only the red light finally crosses the atmosphere and reaches you.

Sometimes, when the sun is close to the horizon, at dawn or dusk, we can observe the red disc of the sun then suddenly a green flash as seen above right. This rather rare phenomenon is an effect of the refraction of the light and appears more easily when one observes the sea since a raised site (a hill or a mountain) or a clear horizon towards the rising or the setting of the sun from a site of altitude (high mountain).

Finally, sometimes the sky turns red-mauve in the evening; it is said that it bathes in a purple light. This phenomenon is caused by dust suspended in the air. This often happens after a volcanic eruption or a catastrophic explosion that has raised a lot of dust, with winds transporting them up to several kilometers of altitude. It is necessary to wait several days so that the sky becomes limpid again. During this period the sky is sometimes so bright that it is possible to read a magazine in the middle of the night outside without extra lighting.

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