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Updated: July 27, 2015

Once in a Blue Moon is this Friday

By Eric Elwell, StormCenter 7 Chief Meteorologist

WHIO - Dayton, Ohio

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Have you ever told someone that “it only happens once in a blue moon?”

Well, that blue moon is about to happen! Look for it on Friday, July 31.

>> READ: 10 things to know about Friday's blue moon

The moon won't actually be blue.  So what is it?

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There are two definitions of a Blue Moon in astronomy; both are a type of full moon. (If the moon does look blue, it’s caused by a rare type of dust in the atmosphere.)

In astronomy, a Blue Moon is a full moon that doesn’t quite fit in with the months in our calendar.

There are two different ways of calculating which full moon is a Blue Moon.

  • Blue Moon = The third full moon in an astronomical season with 4 full moons (versus the normal 3)
  • Blue Moon = The second full moon in a month with two full moons.

That being said, the correct, original definition is that a “Blue Moon” is the third full moon in an astronomical season with four full moons.

A normal year has four astronomical seasons — spring, summer, fall and winter — with three months and normally three full moons each. When one of the astronomical seasons has four full moons, instead of the normal three, the third full moon is called a Blue Moon.

So now you know! (Or are you more confused?)

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