Once in a Blue Moon

I am maybe a few hours late, lunalogically speaking, with this blip. However, the full moon occurred when we could not actually see it so it would have been hard to photograph! (it's hard enough anyway!)  so tonight is strictly speaking outside the deadline but near enough for me.

It's an interesting saying...'Once in a Blue Moon' and we all know we use it colloquially to be something that rarely happens.  The saying does, though, stem (as most sayings do) from  real facts and has nothing to do with colour. Instead it refers to the relatively rare occurrence of what is called a ’blue moon’. 

The commonly accepted definition of a Blue Moon is a full moon that rises twice in one calendar month. It happens because sometimes the number of days in a month are greater than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month, near the first and the last days of the same month. It is said that blue moons happen every 33 months or about every 3 years. This relatively rare occurrence has given rise to the saying ‘once in a blue moon’.  

A Blue Moon cannot happen in February because the calendar month never has enough days. In most years, each season contains three full moons; just as the lunar cycle causes some months to have two full moons, some seasons will have an extra full moon.

Future Blue Moon Dates
These dates are based on the more well-known definition of a Blue Moon (the second full moon in a calendar month). Put them in your dairy now so that you don’t miss them!
            2018 January & March 
            2020 October