Idiom of the week: Once in a blue moon

Certain things only happen once in a blue moon. 

What does that mean? Well, "a blue moon" is the second full moon in a calendar month (in Spanish, we call it "segunda luna llena"). If you grab a calendar and have a look at it, you will see that, as a rule, in each month there is a full moon, a waxing moon, a new moon, and a waning moon. As you may be guessing, a second full moon is rather unusual -and hence the explanation for this idiom: when something happens once in a blue moon, it happens rarely.

Surprised? Don't be. It's a rare but not impossible phenomenon. In fact, a blue moon happens every 2 to 3 years. For a blue moon to take place, the first full moon must occur at the beginning of the month, usually on day 1 or 2. Therefore, the second full moon will occur right at the end of said month, usually on day 30 or 31.

Check these links for the last blue moon (in August 2012) and the next (in July 2015):

• August 2012: http://www.calendario-365.es/calendario-lunar/2012/agosto.html
• July 2015: http://www.calendario-365.es/calendario-lunar/2015/julio.html

Now, what about the colour? Can a moon actually look blue? Yes, it can -as a result of volcanic eruptions or fires, for example. How come? This effect can be caused by huge amounts of smoke and dust particles in the air. In fact, after the Krakatoa erupted late in the 19th century, the moon is said to have had a bluish tinge to it for months! 

The choice of the color blue isn't just coincidental. Fortunately, we ony get volcanic eruptions and big fires once in a blue moon.

I still marvel at how language unravels itself...