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)