They’re usually talking about something rare, silly, and even absurd. After all, when was the last time you saw the moon turn blue?

Well, rare or not, we’re having one this month, and according to astronomer David Reneke from Australasian Science magazine, it’s going to happen on New Year’s Eve.

It’s not at all clear where the term ‘blue moon’ comes from. It dates back at least 400 years.

According to modern folklore, a blue moon is the second full moon in a calendar month.

“Usually months have only one full moon, but occasionally a second one sneaks in,” David said.

“Ancient cultures around the world considered the second full moon to be spiritually significant.”

Full moons are separated by 29 days, while most months are 30 or 31 days long, so it is possible to fit two full moons in a single month.

This happens every two and a half years, on average.

By the way, February is the only month that can never have a blue moon by this definition.

Does the blue moon actually turn blue? No. Blue moons are rare, and that’s where the phrase comes from, ‘once in a blue moon’.

There are occasions, though, when pollution in the Earth’s atmosphere can make the moon look particularly bluish. The extra dust scatters blue light.

For example, the moon appeared blue across the entire Earth for about two years after the eruption of Krakatoa in 1883.

There were also reports of blue moons caused by Mt St Helens in 1980 and Mount Pinatubo in 1991.