There's a blue moon tonight.

It's the second full moon of the month -- the first was Aug. 2.

It will rise in the east at 7:08 p.m. with clear skies forecast

Blue moons come along every once in a while -- once every three years, to be exact. The last one was on New Year's Eve in 2009. The next will be in July 2015.

For sky watchers, it's a fine reason to go out and be one with the heavens.

For Sky & Telescope magazine, it's a chance to acknowledge a 66-year-old mistake that is now part of the common culture.

"We'll say that we take credit for it," Senior Editor Alan MacRobert said about the term we now use for a month's extra full moon.

"It's a case of the English language needing a word to describe something," he said. "We provided that."

MacRobert said even going back to medieval times, people used the term "blue moon" to describe something rare, unusual, and maybe a hoax.

"It was like saying `When pigs fly,' " he said.

There are, in fact, real blue moons, when the atmosphere gets full of particulate matter that acts as a filter. After the Earth-shaking volcanic eruption of the island of Krakatoa in 1883, the moon shone blue for two months.

In 1946, Sky & Telescope magazine printed an article by James Hunt Pruett describing a blue moon as the second full moon in a month. Pruett thought he was using the term as the Maine Farmer's Almanac had used it.

But the almanac defined it differently.

It considered the third full moon in a season with four full moons to be a blue moon. By that reckoning, Friday's moon isn't blue.

This summer will have only three full moons, July 3 and Aug. 2 and this week.

In 1999, Sky & Telescope conceded the mistake. By then, it was too late. People liked it and stuck to it, MacRobert said,.

MacRobert said the term blue moon fills a need. The Algonquian Indians named each month's moon, and the early Colonists took up those names: harvest moon, hunter's moon, snow moon, wolf moon.

"There's no name for an extra moon," MacRobert said. "Now it's a blue moon."

It's become a part of the territory where folklore, names and the night sky meet.