Look out for the blue moon tonight!

NASA

Look up at the sky tonight (August 31) and you will be able to see a rare celestial quirk that only ever occurs once in a blue moon - literally.

Ok, so the "blue moon" which will grace the sky won't look blue, unless there happens to be a volcanic eruption to fill the air with ash.

But it's still a rare event, because term blue moon signifies a full moon that occurs for a second time in the same month. The next one isn't due for nearly three years.

And tonight's is blue moon is particularly timely, as it coincides with the funeral of the first man to walk on the moon, astronaut Neil Armstrong, who died this week at the age of 82.

PA

Full moons occur every 29 days, but blue moons only come along every 36 months.

No one quite knows why it's called a blue moon, but there have been times when the moon really has turned blue. Volcanic eruptions which send up ash particles can give tints of blue and even green.

PA

The next blue moon will be in July 2015.

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