If you want to impress your co-workers mention that this Friday will be a “blue moon”, but don’t be surprised if the office know-it-all begs to differ. Turns out there are two definitions for this lunar rarity, and according to earthsky.org, they’re both acceptable.

The first definition of a blue moon is the second full moon occurring in a calendar month. The moon cycles through its phases in approximately 28 days, so now and again you’re going to see a full moon at the beginning of the month and again near the end of the month. In a really rare occurrence (I am trying very hard to resist the cliché “once in a blue moon”), there were two blue moons in 1999 – in January and again in March. There was no full moon in February that year.

 The second  definition of a blue moon is attributed to the Old Farmer’s Almanac, and is a little more complicated. It has to do with the seasons. There are four seasons in a year and each season usually has three full moons. When a season has four full moons, the third is called a blue moon.

 So, by the first definition, this Friday’s full moon is a blue moon. By the second definition, the next blue moon won’t occur until August 21st in 2013. However, that blue moon will match only the second definition, not the first.

Blue moons are rare (hence the afforementioned cliche) but not that rare. By either definition they usually occur every 2-3 years.

 Oh, and don’t be surprised if the blue moon Friday night actually looks blue. A lot of smoke in the air will do that.

 If you’d like to read a full article on blue moons from earthsky.org, click HERE.