Light from the horizon crosses up to 38 times more air than overhead
Why is the sky deep blue straight up but pale near the horizon? Light arriving from low angles has pushed through as much as 38 times more atmosphere than light from directly above, so its colours get scattered and washed out. The same physics paints sunsets red and leaves the Moon's sky black.
Apart from direct sunlight, daytime brightness comes from scattering by air molecules, a process called Rayleigh scattering. It affects every visible colour but works hardest on short, blue wavelengths, so the scattered glow is bluer than the Sun, and the Sun itself looks slightly less blue for having lost some. Scattering is strongest toward and away from the light source and weakest at right angles, and it partly polarises skylight, an effect most noticeable 90 degrees from the Sun. Clouds scatter even more, and thick ones blend the coloured rings from individual droplets into plain white.
When sunlight takes a long slanting path at dawn or dusk, reds and oranges get scattered too, tinting whole stretches of sky. Higher up, thinner air scatters less and the sky darkens; on the airless Moon it stays black even with the Sun overhead. Twilight is split by the Sun's depth below the horizon in steps of 6 degrees, civil, nautical and astronomical, and beyond 18 degrees the sky reaches its dimmest.
Twilight has its own displays. Opposite the setting Sun, Earth's shadow shows as a dull bluish band above the horizon, topped by a pink glow called the Belt of Venus, with no sharp line between them. A green flash, a spot or ray of green above the Sun lasting a second or two, usually appears when a temperature inversion bends the light, and can even be seen alongside Venus or Jupiter.
Even a moonless night is never fully dark, thanks to airglow, starlight and scattered sunlight, which is why silhouettes remain visible. Farmers once timed planting by the night sky, and forecasters still read it: high thin cirrostratus that throws a halo around the Moon warns of an approaching warm front and rain. The English word comes from Old Norse sky, meaning cloud, while in Old English the visible expanse overhead was called heaven, a term that took on its religious sense in Middle English.
Source: Sky
