A rare blue moon will appear this Friday

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Friday's Lunar Event Happens Once in a Blue Moon

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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 color 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 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.

SEE ALSO: Photos of blue moons:

17 PHOTOS
July blue moon
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A rare blue moon will appear this Friday
A picture taken on July 31, 2015 in Kuwait City shows the full moon. This is the second time in July that a full moon is seen, a phenomenon known as the 'blue moon' is the the first one since 2012 and will be the last until 2018. (Photo credit should read YASSER AL-ZAYYAT/AFP/Getty Images)
CHICAGO, IL - JULY 31: A blue moon rises over the scoreboard during the sixth inning in a game between the Chicago White Sox and the New York Yankees on July 31, 2015 at U.S. Cellular Field in Chicago, Illinois. (Photo by David Banks/Getty Images)
People are silhouetted against a rising blue moon as they ride an attraction at Worlds of Fun amusement park camp Friday, July 31, 2015, in Kansas City, Mo. A blue moon happens when the moon rises in its full stage twice during the same month. (AP Photo/Charlie Riedel)
Fireworks explode as the blue moon rises in the distance at Worlds of Fun amusement park Friday, July 31, 2015, in Kansas City, Mo. A blue moon happens when the moon rises in its full stage twice during the same month. (AP Photo/Charlie Riedel)
The 'blue moon' is seen above Washington on July 31, 2015. The blue moon is the second appearance of a full moon in the same calendar month. The next blue moon will not be seen until January 2018. AFP PHOTO/MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
A Blue Moon rises over the skyline of New York City from West Orange in New Jersey on July 31, 2015. (Photo credit: KENA BETANCUR/AFP/Getty Images)
A blue moon rises behind the torch of the Statue of Liberty seen from Liberty State Park in Jersey City, N.J., Friday, July 31, 2015. A blue moon happens when the moon rises in its full stage twice during the same month. (AP Photo/Julio Cortez)
A blue moon rises behind Brooklyn seen from Liberty State Park in Jersey City, N.J., Friday, July 31, 2015. The blue moon happens when the moon rises in its full stage twice during the same month. (AP Photo/Julio Cortez)
A Blue Moon rises Friday, July 31, 2015 as seen from Taguig city east of Manila, Philippines. A Blue Moon occurs when two full moons fall on the same month. The next Blue Moon will be in January 2018. (AP Photo/Bullit Marquez)
People are silhouetted against a rising full moon as they ride an attraction at Worlds of Fun amusement park Thursday, July 30, 2015, in Kansas City, Mo. When the full moon appears at 6:43 a.m. EDT (1043 GMT) in the U.S. Friday, it will become the second full moon of July. Geoff Chester of the U.S. Naval Observatory said the traditional definition of a blue moon is two full moons in a month. (AP Photo/Charlie Riedel)
A dandelion is silhouetted against a rising full moon at Worlds of Fun amusement park Thursday, July 30, 2015, in Kansas City, Mo. When the full moon appears at 6:43 a.m. EDT (1043 GMT) in the U.S. Friday, it will become the second full moon of July. Geoff Chester of the U.S. Naval Observatory said the traditional definition of a blue moon is two full moons in a month. (AP Photo/Charlie Riedel)
Light from the setting sun reflects off the One World Trade Center tower as the moon rises in its waxing stage seen from The Heights neighborhood of Jersey City, N.J., Wednesday, July 29, 2015. The moon will reach its full stage Friday in a phenomenon known as the blue moon. (AP Photo/Julio Cortez)
FOLKESTONE, ENGLAND - JULY 31: The Eurotunnel terminal is illuminated by a rare blue moon on July 31, 2015 in Folkestone, England. Hundreds of migrants are continuing to attempt to enter the Channel Tunnel in Calais, France and onto trains heading to the United Kingdom. (Photo by Peter Macdiarmid/Getty Images)
GLASTONBURY, ENGLAND - JULY 30: The moon rises over people gathered on Glastonbury Tor ahead of tomorrow's Blue Moon on July 30, 2015 in Somerset, England. The full moon appearing on July 31 will be what's called a Blue Moon, which refers to the second of two full moons appearing in the same calendar month. The last time this happened was in 2012 and there isn't due another until 2018. (Photo by Matt Cardy/Getty Images)
The moon sets behind the gilded statue of the angel Moroni atop the Indianapolis Indiana Temple of The Church of Jesus Christ of Latter-day Saints in Carmel, Ind., Friday, July 31, 2015. (AP Photo/Michael Conroy)
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