IT MAY only happen once in a blue moon, but take a moment to look at the sky tomorrow evening for an unusual astronomical event.

Australasian Science magazine's Dave Reneke said blue moons were real, and had happened several times before.

"The moon doesn't actually turn blue, it just looks that way, but it does have a very real cause," Mr Reneke said.

Pollution from bush fires or volcanoes in the Earth's atmosphere can make the moon look particularly bluish.

Mr Reneke said the extra dust scattered blue light.

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

In 1927, the Indian monsoons were late arriving and the extra long dry season blew up enough dust for a blue coloured moon.

In 1951 the moon appeared blue in North America when huge forest fires in Canada threw smoke particles up into the sky.

But astronomically, a blue moon is simply the second full moon in a calendar month.

"Ancient cultures around the world considered the second full moon to be spiritually significant," Mr Reneke said.

"Even in music there's a connection.

"Songs that use blue moon do so as a symbol of sadness and loneliness."

Mr Reneke said it was once thought that to sleep under direct moonlight would cause a person to go mad or blind.

It was from this lore that the word "lunatic", originating from "luna", or moon, and "tic", meaning stricken arose.

"Blue moons happen every two and a half years, on average and an interesting fact is that February is the only calendar month that can never have a blue moon, with just 28 days in total," Mr Reneke said.

"The Indonesian volcano erupting at the moment may add a slight colouring this time, but we'll have to wait and see."

Mr Reneke said for most of Australia, the full moon rose in the east just as the sun was setting in the west tomorrow night.

"Telescopes won't help, in fact the worst time to view the moon is when it's full, there's simply too much light for you to see any crater details, mountain ranges or valleys."