[Sky] Why the sky is blue was a genuine mystery for centuries. The answer turns out to be about the size of light waves

[DQ Eric]

Sunlight looks white, but it's actually made up of every color in the rainbow, each one traveling as a wave of a slightly different length. When that light hits Earth's atmosphere, it collides with countless tiny gas molecules, and shorter wavelengths of light, blue and violet, get scattered around in every direction far more than longer wavelengths like red and orange. Scientists call this phenomenon Rayleigh scattering, and it's the reason a clear daytime sky appears blue rather than white.

Violet light actually scatters even more than blue light because its wavelength is shorter still, yet the sky doesn't look violet to us. That's because sunlight itself simply contains less violet light to begin with, and human eyes are also more sensitive to blue than violet, so blue wins out as the color we actually perceive overhead.

The same physics explains sunsets too. When the sun sits low on the horizon, its light has to pass through a much thicker slice of atmosphere to reach your eyes, and by that point nearly all the blue and violet light has already been scattered away in other directions, leaving the reddish and orange hues to dominate what finally reaches you. This is also why the sky on Mars appears reddish during the day, but can look blue near sunset, the exact reverse of Earth's pattern, because of how differently sized dust particles in the Martian atmosphere scatter light.

Why Is the Sky Blue? | NASA Space Place - NASA Science for Kidsspaceplace.nasa.gov