Institute of Astronomy

The colour of the sky

Published on 28/02/2011 
Question: 

Why is the sky blue during the day then red at sunset?

Light from the Sun contains all colours of the rainbow. When we look directly at the Sun, we see all of the light from it, so it appears white or a faint yellow. Particles in the Earth's atmosphere scatter shorter wavelength (bluer) light more than the longer wavelength (redder) light due to a process called Rayleigh scattering. During the day, the entire sky looks blue, as we are seeing the light that has been scattered towards us from all directions. However, when the Sun's rays travel through more of the denser part of our atmosphere (at sunrise and sunset) it appears red, as more of the blue light is scattered.

Page last updated: 2 May 2011 at 11:10