You have likely heard the old saying, "once in a blue moon," have you heard of a black moon?

The blue moon is the second full moon in a calendar month. Now, people know that to be rare since the saying goes, "once in a blue moon." Those occur about once every two-and-a-half years.

The black moon is when we have a second new moon in a calendar month. A new moon is when the moon is between the sun and the earth and we see a dark or black moon. Some refer to this as a "no moon," but the new moon is the complete opposite of the full moon.

Normally, you get one of each in a calendar month but on July 31, in the Eastern U.S. and other parts of the Western Hemisphere, the new moon occurs at 11:13 p.m. EDT, which makes it the second one in July. 

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This is very rare. In fact, it's much rarer than a blue moon. 

On average, a black moon only happens every 32 months, and sometimes only in certain time zones. 

This moon is also going to be what is referred to as a supermoon, though scientifically the term is perigee moon, meaning the closest approach of the moon to the earth in its orbit. 

So if you want to use all the fun lunar terms, you can call it a "Super Black New Moon." With the naked eye, most people will just see black for the Super Black New Moon.