Once in a Blue Moon

Once in a Blue Moon

"Once in a Blue Moon ..." is a common way of saying not very often,
or every now and then.

"Blue moon" appears to have been a colloquial expression long before
it developed its calendrical senses. According to the Oxford English
Dictionary, the first reference to a blue moon comes from a proverb
recorded in 1528:

"If they say the moon is blue,
We must believe that it is true."

But what exactly is a Blue Moon ?

According to the popular definition, it is the second Full Moon to
occur in a single calendar month. The average interval between
Full Moons is about 29.5 days (lunar month), whilst the length of
an average Gregorian calendar month is roughly 30.5 days.
This makes it very unlikely that any given calendar month will
contain two Full Moons, though it does happen sometimes.

On average, there will be 41 months that have two Full Moons
in every century, so we could say that "once in a Blue Moon" actually
means once every two-and-a-half years. There are two Full Moons
this month (March 1 and 30) in certain time zone, which means you
may have the opportunity to see the second Full Moon of the month
tonight, if you are in the right time zone and your weather permits.

Two full moons in one month (From Wikipedia):

•2009: December 2, December 31 (partial lunar eclipse visible in
some parts of the world),
•2010: January 1 (partial lunar eclipse), January 30,
•2010: March 1, March 30,
•2012: August 2, August 31,
•2015: July 2, July 31.
. . . . . . . . . .

Yeh Tze