Once in a blue moon… What’s a blue moon again? And is it really blue? While a blue-colored moon is unlikely to happen, a ‘Blue Moon’ is a natural phenomenon that last took place in 2012, and occurred again on July 31, 2015. It describes either the second full moon in a month (e.g. July 2015, with full moons on July 2nd AND on July 31st) or the 3rd of four full moons in a season.

In short: it is an ‘extra’ full moon, called blue. Space observers explain.

.

‘Can a moon be blue in color?

There’s one kind of blue moon that is still rare. 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. So don’t be misled by the photo above. The sorts of moons people commonly call Blue Moons aren’t usually blue.

[…]

Blue Moon can be second of two full moons in a month. Or it can be third of four full moons in a season.

Next Blue Moon is the second full moon of July, 2015.

In recent years, people have been using the name Blue Moon for the second of two full moons in a single calendar month. An older definition of Blue Moon is that it’s the third of four full moons in a single season.

Someday, you might see an actual blue-colored moon. The term once in a blue moon used to mean something rare.’

Source: When is the next Blue Moon? By Deborah Byrd and Bruce McClure, Astronomy Essentials | Space (Jun 14, 2014) @EarthSky.org

Featured image: Favorite photos of August 31, 2012 @EarthSky.org (photo credit: Priya Kumar, Oman)