This Friday marks the occurrence of a blue moon. However, the saying and the name are often misunderstood.
When someone says “once in a blue moon,” it usually means something rarely happens. But when the second full moon of July appears this Friday, it will be neither blue nor all that rare.
“Within any particular year there are usually 12 full moons, but sometimes there are 13," said Steve Desch, ASU astrophysics professor . "In fact, seven times out of 19 years, there has to be 13 full moons in a year and so there is going to be one extra moon.”
That means each blue moon shows up about two and a half years after the last one.
The common understanding of a blue moon is when there are two full moons in a calendar month. However, the technical blue moon definition is the third full moon in a season, like this summer, with four full moons.
