Once in a blue moon

"Once in a Blue Moon"  I think that everyone has ever heard this phrase but do you know what is a Blue Moon?

There are two more definitions for Blue Moon. It can be the third of four full moons in a single season. Or, someday, you might see an actual blue-colored moon. It’s very rare that you would see a blue-colored moon, although unusual sky conditions – certain-sized particles of dust or smoke – can create them. Blue-colored moons aren’t predictable. Actually, Blue Moon is defined as the second full moon in a calendar month.In this year, the first full moon is August 1 and the second full moon will happen tomorrow(August 31, 2012). The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the Moreover, there are two blue moons in a single calendar year. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.

If you miss this Blue Moon you have to wait it is coming up in 2018.

Thanks for useful information from:
http://earthsky.org/astronomy-essentials/when-is-the-next-blue-moon