Tomorrow it will be a rare - 'Blue Moon'
Although the full moon on July 31, 2015, will look like an ordinary full moon, it's actually a bit extraordinary—It will be a Blue Moon.
A Blue Moon has nothing to do with the colour of the moon - The moon was full on July 2, and it’ll be full again on July 31. The second of two full moons in a calendar month is often called a Blue Moon, hence the saying - 'Once in a blue moon' translates to about once every three years in astronomical terms, but the meaning has shifted over time.
What is a Blue Moon?
The modern day definition of a Blue Moon is when there are 2 full moons in one Month. A full moon occurs roughly every 29.5 days and on the rare occasions when the full moon falls at the very beginning of a month there is a good chance a Blue Moon will occur at the end of the month. According to this definition the next blue moon will occur on July 31, 2015.
The chances of seeing two full moons within the same month is a rare phenomenon that only takes place about once every 2 1/2 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.
