Side Trip: Why is the Sky Blue?

• If you look AT the Sun in the sky, it appears white-yellow.

• If you look AWAY from the Sun, since there is no light source in that direction, you might expect the sky to be black.

• The reason it is not is because of scattering of photons in the upper atmosphere, mostly by molecules.

  Why does the Sun look red at sunset?

  

• The long pathlength through the atmosphere when the sun is low give means lots of blue and green photons get scattered out of the line of sight leaving only the red light.

One more interesting sidelight:

Because of the gradient in atmosphere's density, it acts like a prism

• Red light is less ``bent'' than green or blue light so the ``red'' Sun sets first, followed by the ``green'' Sun, then the ``blue'' Sun. When the Sun is low on the horizon, all the blue light gets scattered out, so, the last thing you see is a ``green flash''. I

• When you see the Sun hit the horizon, it is already a full solar diameter below the horizon.