Once in a blue moon

The blue moon from NASA

I've wondered about this expression once, sometime back. Blue moon? why blue? were there any instances where the moon appeared blue?

Today, I came across the answer in an unexpected manner. I was reading through a Physics book called "Absorption and scattering of light by small particles", which is quite nicely written I have to say.

Anyhow, the part about the blue moon....I think we are quite familiar with the reddening effect at sunrise and sunset, which is partly the result of light scattering from airborn particles. Ok I'll spare the scientific details......

Similarly there's a 'bluing' effect, but for this to take place the airborn particles must be within a narrow range of particle sizes, which is very hard to come by, hence the term 'once in a blue moon'.

However, there have been instances recorded in history where the sun and moon were observed to be blue, specially after the giant eruptions of the volcano Krakatoa and following huge forest fires in Canada.

If you'd like a more scientific explanation see here & here.