There's a blue moon tonight (which won't be blue)

The full moon tonight is a "blue moon" - famous for its rarity - but it won't be blue, or particularly impressive, and, er, it's not rare.

There's a blue moon tonight (which won't be blue)

The full moon tonight is a "blue moon" - famous for its rarity - but it won't be blue, or particularly impressive, and, er, it's not rare.

The term "blue moon" in this context refers to the appearance of a second full moon in a month.

Since the lunar cycle takes 28 days, it doesn't line up precisely with calendar months. Therefore, every three years, we get a second one in the same month.

This month's first full moon was on July 2. So here, 29 days later, we have our second one.

But why the devil is it called a blue moon?

Ah! Well, that's an interesting question - and there isn't really a clear-cut answer.

Science writer Stuart Clark, in a column for The Guardian, traces the use of the term "blue moon" back to a 1946 magazine article by an amateur astronomer, who posited the phrase "once in a blue moon" was used to refer to the rare occurrence of an extra moon in a month.

But Clark cites a nineteenth century almanac which said the "blue moon" was the third full moon in a season which had four (most seasons would have three by "default").

All the same, it looks like the 1946 definition has stuck, despite being neither blue, nor particularly rare.

Hang on - does that mean there's no such thing as a really blue moon?

No! The moon can appear blue - it's just much, much, rarer and completely impossible to predict.

Particles in the air from volcanic eruptions or similar phenomena can scatter the light from the moon and make it appear blue.

The issue is that it's not something we can observe easily. The particles from such eruptions need to be just the right size to scatter the red light wavelength and eliminate it (so that the blue light still reaches us without the red).

That's very rare indeed, and unless you're talking about a massive, dangerous eruption, it's likely to be a localised, subjective appearance.

So now you'll be able to spot the Instagram filters tomorrow morning.

Oh. So I shouldn't bother checking out the moon tonight?

Well, it's the full moon. It's a floating rock weighing 7.3477×10^22 kilograms, spinning around us at a kilometre a second, lighting the night and working the tides, as a constant companion to our little rock.

It's full. It's gorgeous. Why not take the excuse and appreciate it?

After all, you'll only get one each month until early 2018.

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