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It happens once in a blue moon. Why? Because it is a blue moon

A jet passed in front of the moon earlier this year. This month there will be a blue moon, but, alas, it will not actually be blue.
A jet passed in front of the moon earlier this year. This month there will be a blue moon, but, alas, it will not actually be blue.Alejandro Pagni/AFP/Getty Images/AFP/Getty Images

May’s full moon is something that happens only once in a blue moon. And that’s because, well, it is a blue moon.

May 18’s blue moon is an astronomical event that occurs on average every two and a half years, according to the National Aeronautics and Space Administration.

(The moon also falls on the day of the 50th anniversary of the Apollo 10 mission, which set the stage for the moon landing two months later in 1969, NASA noted.)

A full moon is called a blue moon when it is the third in a series of four full moons in a season, as there are usually only three full moons within that time frame, NASA said. The phrase can be spotted all the way back in 1528, NASA said.

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Some writers speculate, NASA said, that “blue” actually came from the Old English word “belewe,” meaning “betrayer,” because an extra moon in a season confused or “betrayed” the dates for Lent and Easter.

“Blue moon” can also refer to a second full moon that occurs within one month; according to NASA, using the term in that manner started as recently as 1946.

That was the year when the magazine Sky & Telescope incorrectly stated that the term meant two full moons in one calendar month, making this definition the widespread usage in the 1980s, according to the Encyclopedia Britannica.

To see May’s blue moon all you have to do is look up — weather permitting. Those in Boston can catch this rare event when the moon starts to rise at 7:52 p.m. on May 18.

And, sorry, folks, it’s just a name. The moon will not turn blue.


Breanne Kovatch can be reached at breanne.kovatch@globe.com. Follow her on Twitter @breannekovatch.