Blue moon just for lovers

By Deborah Gough
August 30, 2004

Astronomers might be standing alone tonight as lovers and amateur star gazers watch an event that happens, well, once in a blue moon.

A blue moon will rise over Sydney at 5.42pm, the first time since 2001.

While the term "once in a blue moon" conjures up impressions of an event that happens rarely, if ever, blue moons occur about 41 times a century, or an average of every two years.

A blue moon is no bluer than any other moon (the phrase was coined in medieval times when debate raged about the satellite's colour) and occurs when a full moon occurs twice in the same calendar month.

Blue moons occur only on the 30th or 31st day of the month, and never in a February because a second full moon needs 29.5 days to reappear. Even in a leap year, February has just 29 days.

Romantics and songwriters might thrill to the sight, but full or blue moon skies frustrate astronomers.

The Astronomical Society of Victoria's vice-president, Perry Vlahos, said they produce so much bright light that any real observations are impossible - as happens with light pollution in big cities.

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Astronomers might be tempted to take the night off, but couples should not miss the blue moon, Mr Vlahos said.

"It is a beautiful to watch a full moon, to watch it rise in the east," he said.

"And if you have never seen a full moon rise on a horizon it is something to see."

He suggested that after viewing the blue moon in the eastern sky, star gazers should look west, where - using a set of binoculars - they would see the moons of Jupiter, the second brightest sphere in the sky.