Apparently, the phrase “once in a blue moon”, in the sense of something that occurs very rarely, dates back to 1824. I will check this out as soon as I have time…
However, the astronomical phenomenon of “blue moon” is different.
What this is Really About
The Moon can sometimes appear reddish, especially during eclipses. But what we call a ‘blue moon’ has nothing to do with its colour.
A “blue moon” isn’t really all that blue. Although the moon can appear somewhat blue if there has been a major volcanic eruption that emits tons of particles in the air, blue moons refer to multiple full moons within certain time frames.
A picture speaks a minimum of 1000 words, so…
The particular circumstances leading to a blue moon event are the following.
Normally there are 12 fully lit, of full, moons per year. A season of three months should therefore contain three full moons.
On occasions where there are four full moons in a season instead of three, the third of the full moon is traditionally called a ‘blue moon’.
A blue moon happens on average about once in 2.7 years.
Occasionally, two full moons will fall within the same month. Some people call the second full moon a ‘blue moon’ but this is not the term’s original meaning”.
Next Blue Moon
The next blue moon is due to occur on 21st August 2013. So, sit back and relax… No need to rush out and look up at the sky just yet. But beware the next one is not until 21st May 2016!
Check out the excellent Moongiant website for all moon-related things. 😉