We have a blue moon coming this week to the Butler night sky.

It’s the second full moon we’ve had this month. The last time we had two full moons in one month was in December of 2009, and what was wild about the blue moon that time was that it happened on New Year’s Eve. There won’t be another New Year’s blue moon until 2028.

After this coming Friday, our next blue moon won’t be until July of 2015.

Astronomically, a blue moon is when you have two full moons in a given calendar month. That doesn’t happen all that often.

It really all comes down to a number — 29.5 days. That’s the interval between two full moons, something astronomers call a synodic month. Because the average month has 30 days, you can see why the odds are against it, even if the month has 31 days.

In fact, it can never happen in February even in leap years when February sports 29 days, because the synodic month is a half-day longer. On the average, any given century will experience 27 blue moons. We’re on our fifth one of this century.

Very rarely, a moon will actually sport a bluish tinge. About the only way that can happen is when a volcanic eruption or a forest fire causes atmospheric conditions to make the moon look blue, and that phenomena is even rare in those conditions.

That did happen, though, in many sections of Eastern North America in September of 1950, due to smoke from widespread forest fires in western Canada.  It also happened after the massive eruption of Mount Pinatubo in the Philippines in June 1991, when there were many sightings of a physically blue moon all around the world. Fortunately, these calamities don’t happen all that often — really, once in a blue moon!

The term blue moon has been in use for hundreds of years. It even shows up in some of Shakespeare’s writings in the 16th century.

But why the term blue moon? The truth is that no one really knows. Back then, it had more of a frightening tone to it and referred to those very rare times I’ve already described when the moon actually did turn blue.

Many believed it was a bad omen of global calamities in the near future. Even back then there were the Y2K and 2012 Mayan-end-of-the-world kind of informed and misinformed hucksters that tried to scare the populous … and they didn’t even have the Internet to spread their misinformation!

I guess I just gave you my opinion about the Mayan forecast for the end of the world this coming December!

Anyway, back then a blue moon’s real or imagined appearance even interfered with a scheduling of church festivals and feast days.

Over the last century or so, a blue moon was also seen as a symbol of sadness and loneliness. Even music of the last century reflected this in songs like “Blue Moon” written in 1934 by Richard Rodgers and Lorenz Hart and recorded by tons of artists. My favorite version of the song was recorded in 1961 by the Marcels, a doo-wop band that really added some kick to the old standby.

Blue moon or not, the stargazing will be interfered with, but don’t be blue backyard astronomers. Darker and longer nights are coming along late next week once we totally get the bright moon out of the night sky and get a chance to take advantage of the longer nights of September.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, “Pennsylvania Starwatch,” available at bookstores and at his website www.lynchandthestars.com.