Can the Moon Appear Blue?

Most people will say ‘No!’

However, that’s not true and, although it is a very rare sight indeed, a Moon that actually looks blue can happen.

That’s right 🌕  Full or otherwise, the Moon can appear blue when the atmosphere is filled with dust or smoke particles slightly wider than 900 nm (nm = nanometre which is one thousand-millionth of a metre).  These minuscule particles act like a colour-filter making the Moon appear blueish, or even slightly lavender.

Oh – a lavender Moon.  Now, I would like to see that, though not the destruction that causes it.

Dust particles in our atmosphere are normally the perfect size to diffract blue light – this makes the Moon appear reddish at sunset – but when the particles are larger, they diffract red light making the Moon appear bluish.

This phenomenon is called Mie scattering, and in much the same way that Rayleigh scattering scatters sunlight off (even smaller) molecules making the sky appear blue, Mie scattering can happen after a dust storm, a forest fire, or a volcanic eruption.  And apparently, even clouds of water droplets, ice crystals or fine-grained sand can do the same thing.

Volcanic eruptions like the ones on Mt. Krakatoa in Indonesia (1883), Mt. St. Helens in the US (1980), El Chichon in Mexico (1983), and Mount Pinatubo in the Philippines (1991) are all known to have made the Moon look blue.  Mt. Krakatoa’s dust turned sunsets green and the Moon blue all around the world for the best part of two years.  Now that’s both interesting, and scary.

But there are other examples.  In 1927, the Indian monsoons were late arriving and the extra-long dry season blew up enough larger dust particles for a Blue Moon.  And in northeastern parts of North America, the Moon turned blue in 1951 due to smoke particles from huge forest fires in western Canada.

So it is possible that the Moon can appear blue from time to time, though it happens rarely, and hence we get the phrase ‘once in a Blue Moon’ meaning:

“Something that happens rarely.”

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There are other reasons we might see the Moon looking odd notes atmospheric optics expert Les Cowley:

“Our eyes have automatic ‘white balances’ just like digital cameras.  Go outdoors from a cosy cabin lit by an oil lamp (yellow light) and the moon will appear blue until your eyes adjust.”

Time to get out a yellow oil lamp and see if I can fool my vision.

– ⋅ o ♥ o ⋅ –

This is my next entry in my Blue Moon Squares as part of Becky’s ‘Blue’ Square in July photo challenge.

You can read more Blue Moon stories here.