There's a full moon coming this Friday that is unlike anything we've seen since 2012 and won't see again until early 2018 - it's called a blue moon.

The colour of the Moon will not change. This full moon is special because it's the second one to occur this month - that's what a 'blue moon' is. The first full moon happened on July 2.

The chances of seeing two full moons within the same month is a rare phenomenon that only takes place about once every two and a half years. The next blue moon won't come until January 2, 2018.

The reason we have blue moons in the first place is because the length of a calendar month on Earth is not equal to a lunar month - the time elapsed between when we see one phase of the Moon repeated (like the time between two full moons or two new moons).

A lunar month is about 29 days long whereas most of our months on Earth are 30 or 31 days long. That's 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 the span of 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 astronomy 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.

This article was originally published by Business Insider.

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