Why is the sky blue?

Why is the sky blue? This one of those questions that you will ask at some point in time and is something really interesting to think about.  Let us start at one of the main reasons you ask the question.  If you look at the sky at night the sky is black, full of stars and the moon all acting as just little bright points or dots of light in the sky.  Why is it during the day when the sun is up, the sky isn’t black with just one really large point of light? If you are on the moon, you see a black sky with the sun shining brightly at you, why does it not do the same on earth…..

When the Sun’s light reaches the Earth’s atmosphere it is scattered, or deflected, by the tiny molecules of gas (mostly nitrogen and oxygen) in the air. These molecules are much smaller than the wavelength of visible light, with a diameter of 1/10 of the size of that of the wavelength or colour of the light. This effect is called Rayleigh scattering, named after Lord Rayleigh who first discovered it. 

Light from the Sun appears white but it actually consists of many different colours. These colours are Red, Orange, Yellow, Green, Blue, Indigo and Violet and the will always appear in the same order. The different colours or wavelengths of light are scattered in all directions as they pass through the atmosphere. Blue light (shorter wavelengths) is scattered more than red light (longer wavelength). The shorter wavelengths (violet and blue) are scattered the most efficiently, so more of the blue light is scattered towards our eyes than the other colours. You might wonder why the sky doesn’t actually look purple since the violet light is scattered even more strongly than blue. Because of the lack of violet light in sunlight, our eyes are much more sensitive to blue which is more prominent.

So basically when you look up at the sky on a clear day you will see the sun shining brightly and the blueness we see in the sky is all of the atoms in the atmosphere scatter blue light towards us. This blue light is so bright that it blocks out the vision of the stars you see in the night sky which are dull in comparison. All other colors such as red, green, and yellow are just not scattered nearly as well but are still there.

You might also notice that the sky tends to be the most vibrant overhead and fades to pale as it reaches the horizon. This is because the light from the horizon has had further to travel through the air and so has been scattered and re-scattered.