Look skyward for a blue moon

WHEN you hear someone say "once in a blue moon", you know what they mean.

They're usually talking about something rare, silly, or even absurd.

After all, when was the last time you saw the moon turn blue?

Well, rare or not, according to astronomer David Reneke, writer and publicist for Australasian Science magazine, a blue moon is slated for the last day of this month, which happens to be tonight. The other full moon was on August 2.

It's not at all clear where the term blue moon comes from. According to modern folklore it dates back at least 400 years. 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," Mr Reneke 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.

February is the only month that can never have a blue moon.

Does the blue moon actually turn blue? No.

Blue moons are rare, and that's where the phrase "once in a blue moon" comes from. 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 bluish green across the entire earth for about two years after the eruption of Krakatoa in 1883.



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