A full moon is coming Friday night that is unlike anything we've seen since 2012 and that we will not see again until early 2018. It's called a blue moon.
Despite the name, the color of the moon will not change. This full moon is special because it is the second one to occur this month — that's what a "blue moon" is. The first full moon happened on July 2.
Seeing two full moons in the same month is a rare phenomenon that takes place only about once every 2 1/2 years. The next blue moon will not come until January 31, 2018.
The reason we have blue moons in the first place is that the length of a calendar month on Earth is not equal to a lunar month, or the time it takes a phase of the moon to repeat (e.g., the time between two full moons or two new moons).
A lunar month is about 29 days long, whereas most of our calendar months are 30 or 31 days long. That is why the full moon doesn't happen on the same date every month and why we sometimes see an overlap in the number of full moons within a single month.
In theory, we can see a blue moon during any month of the year except February — even on a leap year — according to Larry Ciupik, an astronomer at the Adler Planetarium who spoke with NBC about blue moons.
The term has changed over the years, so if you thought blue moon meant something other than the second full moon in a month, you're not imagining things. It used to refer to the fourth moon in a single season.
Either way you slice it, a blue moon is an uncommon anomaly in our night sky.
You don't have to do anything special to see Friday's blue moon. Just look up and enjoy the view.
