Are Blue Moons Rare?

Well yes, they are, but not as rare as you might think.

And I say that because we can see two Blue Moons in one calendar year.

– ⋅ o ♥ o ⋅ –

This does seem to be in direct contradiction with the phrase “once in a Blue Moon” which usually indicates something that happens ‘very rarely’ and that, at least in recent years, Blue Moons seem to happen rather frequently.  At least that’s my observation, and yet, that observation I’m making is due to the occurrence of another set of unique circumstances.

When it comes to the modern interpretation of a ‘regular’ Blue Moon – that is, the second of two full Moons in one calendar month – this only occurs when a calendar year contains 13 full Moons, and as I said in my post Blue Moon Squares in July, (quoting James Pruett) “. . . seven times in 19 years we get 13 full Moons a year“.

Digging a little further, I’ve discovered there’s something special about that stated 19 year period.  This is the Metonic cycle.  Greek astronomer Meton of Athens observed (back in fifth century BC) that a period of 19 years is almost exactly equal to 235 lunar months with the difference between the two periods being only a few hours.

Consequently, any given day of a lunar month will often occur on the same day of the solar year as it did 19 years previously.  And this is how long it takes for a single calendar year to experience two Blue Moons, because . . . every 19 years, there is no full Moon in February.

In 2018, February was devoid of a full moon, while January and March had two full moons each giving us two Blue Moons appearing within a three month period, and I caught both of them.

This Blue Moon was featured in my post The Super Blue (No So Blood) Moon.

(This is also the image I chose for the featured image on this post.)

And this Blue Moon was featured in my post Another Blue Moon.

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Actually witnessing two Blue Moons in such quick succession is the reason behind me thinking that Blue Moons are not so rare, and yet, our next Blue Moon – in accordance with modern interpretation – won’t occur until October 31, 2020.  I guess that makes it rare enough and I will have my camera at the ready.

The second of two Blue Moons in one calendar year won’t happen again until March 31, 2037, and I still plan to have my camera at the ready.

– ⋅ o ♥ o ⋅ –

This is my next entry in my Blue Moon Squares as part of Becky’s ‘Blue’ Square in July photo challenge.