Once in a blue moon is about to happen.

On Thursday, a second full moon in a calendar month will appear in the night sky, an occurrence known as a blue moon.

There has not been a month with two full moons since 2007, when sky gazers enjoyed one on June 1 and again on June 30 of that year. The first full moon this month occurred on Dec. 2.

The phrase "blue moon" has nothing to do with the color of the sphere, explained Conrad Jung, a staff astronomer at the Chabot Space & Science Center in Oakland. "It's just a colloquial term, which means very, very rare," he said.

"It's not so much a phenomenon but an event of timing," Jung added. "It's a bit esoteric, to say the least. I don't get a lot of questions about blue moons."

Early in the last century, a blue moon was considered the third full moon in a three-month season with four full moons, regardless of when it appeared, he said. Agrarian cultures paid more attention to full moons because their nighttime illumination could extend the working day during planting and harvest seasons, Jung explained. The September full moon, for example, is known as the harvest moon in the United States.

A blue moon, or any other moon, can have a blue tint if it is viewed through dust in the atmosphere from volcanic ash. However, Jung does not believe the recent eruptions of the Mayon volcano in the Philippines will have any effect on the color of Thursday's blue moon.

This year's blue moon, though, will give New Year's Eve something special — a full moon to which wackier-than-usual-behavior among revelers is often attributed.

"We joke about it," said a San Jose Police Department dispatcher. But, she added, "it's an urban myth."