The moon orbits around the earth every 29.5 days and since our months typically have 30-31 days that leaves some leftover time (0.5-1.5 days). Eventually that adds up and we get an extra cycle in one month, and that means an extra full moon. Typically, months only get one full moon and so having two is pretty rare, happening every 2.7 years.
Hence, the phrase, “once in a blue moon,” which is a lot like saying “once in about 3 years”.
I love this stuff (orbits, planets, stars) and have been a star-gazer since I was a kid. If you are too, then you can enjoy the “blue moon” this Friday, August 31, 2012.
Yep, it’s been around three years since the last one (2.67 years to be exact, the last blue moon was on New Year’s Eve – Dec 31, 2009). I also think you should try, at whatever the cost, to say “once in a blue moon” about something. Think of something you know you won’t do or won’t happen until July 2015 (the next blue moon).
Also, the moons aren’t actually blue.
**Source: Space.com & Infoplease