A blue moon happens every two or three years.

You ever said “once in a blue moon”? Who hasn’t but most people don’t know that a blue moon is an actual event.

What The Fact?

Well, there are usually twelve full moons a year,…so roughly monthly but the lunar (synodic) month is slightly shorter than the calendar (solar) month.  What that means is that the calendar year is roughly 11 days longer than the lunar year and because of that, every two or three years (or more precisely 7 times every 19 years) we get an extra full moon which we call a “blue moon”.

So Why Do We Call It Blue?

The origin of the moniker ‘blue’ moon is a little hazy but it’s most likely to do with how the Clergy calculate the dates for Lent and Easter.  Historically, they called an early full moon the ‘betrayer’ moon or ‘belewe’ moon. Honestly, It’s one of those things that too early to tell where it comes from exactly but it seems pretty likely right?  Of course, that doesn’t explain the big literally blue moon in the image at the top.  That’s another thing entirely although it’s another very rare event.

When Is The Moon Visibly Blue?

On the odd occasion, the moon can have a very bluish tinge but it’s nothing to do with the extra full moon.  In fact, a visibly blue full moon is rarer than rocking horse shit.  The reason that happens is usually down to one of three things,.. pollution, forest fires or volcanoes!

When there is an unusually large quantity of smoke and dust in the atmosphere, the oily droplets in the smoke diffuse all the red and yellow light away.  If you remember the lessons in school about subtractive color you’ll know that leaves just one primary light color left,..blue.  It really is a rare thing though since the particles have to be a particular diameter (about 1 micron) and in most cases, the particles thrown up from these things tend to be smaller than 1 micron in diameter and that filters out all the blue light making a ‘red’ moon much more common.