What is a "Blue Moon"?

24 April 2014

THE SHORT ANSWER (TSA)

A “Blue Moon” can be 1 of 3 things.

First, a “Blue Moon” means “a really rare event.”  The phrase, “once in a blue moon,” refers to a rare event that happens only once in a great while.

Second, a “Blue Moon” is a somewhat rare event involving the actual Moon.  I’ll call the "Blue Moon," the “extra” moon. 

Rarely, there can be 2 Full Moons in the same calendar month.  When this happens, the 2nd Full Moon in that month is called a “Blue Moon.” But that extra Full Moon “throws everything off,” and we get more "Blue Moons."

If 2 Full Moons fall in the same month, then, there will 4 Full Moons (instead of 3) in that season of the year, Spring, Summer, Autumn or Winter.  The 4th and last (or extra) Full Moon in that calendar season is also called a “Blue Moon.” 

But wait, there’s more.

If 2 Full Moons fall in the same month, then, there will be 13 Full Moons (instead of 12) in that year.  What do you think they call that last, the 13th, Full Moon in December?  You guessed it.  That last Full Moon of the year is also called a "Blue Moon."  

So, if we’re talking about the actual Moon, there are 3 kinds of "Blue Moons." A "Blue Moon" is:

(1) the second Full Moon falling in a particular calendar month.  (There’s usually only 1)

(2) the fourth Full Moon falling in a particular calendar season of the year.  (There’s usually only 3)

(3) the thirteenth Full Moon falling in a single calendar year.  (There’s usually only 12)

The next time there will be 2 full moons in a single month will be in July of 2015 with Full Moons on July 2nd and July 31st.   The 2nd Full Moon in July, the Full Moon on July 31st, is a “Blue Moon.” 

July is part of the summer season.  So, in the summer of 2015, there will be 4 Full Moons instead of 3.  The 4th (and last) Full Moon of the summer falls on September 13, 2015 and is another "Blue Moon."  

Since this particular July falls in the calendar year 2015, there will 13 Full Moons in 2015 instead of the usual 12.  That last, 13th, Full Moon on December 11, 2015 is also a “Blue Moon.” 

Again, I think of the “Blue Moon” as the extra “Full Moon” at the end of the line in a particular month, season or year.

Oh!  I almost forgot . . .

Third, the phrase “Blue Moon” also describes a Full Moon that’s actually the color blue.  Major volcanic eruptions or forest fires can fill the atmosphere with so much grey dust that Moon will seem to have a blue-ish tinge:  A blue moon.

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
24 April 2014
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