There's going to be a blue moon. Um, what?

It's worth taking a good look at the night sky later as this is your last chance to see a blue moon until 2018.

Blue moons are pretty rare. We last saw one in 2012.

The term is normally used to describe the second of two full moons appearing in the same calendar month.

The kicker? They aren't actually blue.

The moon's colour has nothing to do with whether it appears twice in a calendar month.

Moons only have a bluish tinge if there is specific-sized particles in the air, like dust or water droplets. So blue-coloured moons have been seen after wildfires and volcanoes, when smoke in the air scatters red wavelengths of light, acting like a blue filter.

One time there was said to be a blue moon was after the eruption of Mount Krakatoa in Indonesia in 1883. The dust in the atmosphere caused spectacular sunsets around the world - some think it's the inspiration for Edvard Munch's The Scream.

In reality a blue moon is more likely to be red, according to Nasa, because minute particles called aerosols in our atmosphere filter out the blue light.

One quite interesting thing about blue moons is that while the phrase has existed in English for a long time (the first recorded use of blue moon appears to be in the 15th century) it didn't used to have a scientific meaning. It just used to mean something rare.

There's lots of confusion about how and why the astrological definition for the blue moon arose. There is a different "seasonal" definition which says a blue moon is the third of four full moons in one season, which is based roughly on the Christian calendar.

So now you know.

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