I have been impressing a new girlfriend with my knowledge of the night sky. “Why is the Sky Blue?” she asked. Halfway through my explanation Margaret stopped me; “You’re making it up!” rang her accusation. Caught red-handed, and red faced I confessed, but promised to find out;

Blue Skies
Light from the Sun contains all the colours of the rainbow. However to reach us on earth it has to go through our atmosphere. The shorter blue wavelengths of light are the perfect size to interact with the main components of air – oxygen and nitrogen.

So although every colour is being reflected in the air, the majority of collisions are with oxygen and nitrogen particles, meaning that we see a blue sky.  This was discovered by Lord Rayleigh,  and if you really want to impress YOUR girlfriend, the technical term for a blue sky is Rayleigh Scattering. 

Red Sunsets
There is a lot more in the atmosphere than just air. There’s sand from sandstorms, volcanic ash from volcanos, dust, water and man-made pollution. We can bunch all this together and simply call them particles. It’s these particles that can affect other colours.

When the sun is high above, it has a relatively short amount of air to travel through, and Rayleigh Scattering favours short wave-length colours – hence the blue sky. However as the sun gets lower, its light has to travel through more and more atmosphere before it reaches our eyes.  This means that its light collides with far more particles in the atmosphere.

If there are plenty of particles in the air, maybe after a long dry period, almost all of the blue can be scattered away. Long wavelengths however will still reach you, and long wavelengths are red in colour. Hence red sunsets - which are more frequent in dry, hot countries.

Red Moons
Sunlight reflected by the moon behaves in exactly the same way, so when it’s low down on a hot dry day, you may see a red Moon.

I hope you understand this now, (I think I do)!

John McNally