This Halloween it’s going to be a ‘Blue Moon’. Unfortunately no, I’m sorry to tell you that doesn’t mean the Moon is actually going to turn blue. But once you get over your disappointment you will see that it’s still a pretty sweet treat.
A Blue Moon is when there’s two full Moons in a month. Because this seldom happens (only once every two or three years) it’s long become an expression for something that you only do, or treat yourself to, occasionally, or indeed ‘once in a Blue Moon’. (But since Blue Moons are when there’s two full Moons in a month, maybe the expression should be ‘twice in a Blue Moon’? That’s a good excuse to do something nice - twice.)
The Moon is something you imagine being synonymous with Halloween. You probably have a picture in your mind of a big full Moon providing the eerie backdrop of a scene filled with pumpkins with gruesome expressions, witches and bats flying wildly across the sky, and maybe you even catch a glimpse of a Werewolf’s tail disappearing as he chases a black cat into the forest. And while all this commotion is going on, if you pay close attention you might just see, lit up in the moonlight, a Goblin - gobbling up all the candy in the corner.
But despite how you might imagine the Moon always being there to illuminate this spooky spectacle, the last official full Moon on Halloween was actually 19 years ago, and the last Blue Moon was even rarer, and this will be the first one in 76 years.
The average Moon cycle takes around 29 days to complete - about a month.
This isn’t a crazy cosmic coincidence; we set up our timetables with the Moon in mind. In fact, Moon and month come from the same root word. By this schedule the Moon takes 354 days to complete 12 full cycles. Since this falls short of the 365 days it takes Earth to go around the Sun (a year), it means that roughly every two and a half years a 13th full Moon is seen and is called a Blue Moon.
Why exactly we called it a Blue Moon isn’t clear, one theory has it that it was a mispronunciation of an unused word ‘belewe’, which means to betray. And maybe we did feel a little betrayed after we thought we had the cosmos figured out, and that there would be 12 months and Moons in our calendar year - only for an impromptu 13th Moon to appear about 3 years later.
The Moon sure is difficult to keep track of. Just trying to get my head around it enough to write this story is proving difficult. However, the Ancient Greek Meton of Athens in 432 BC had the mind for it and worked out that after 19 years have elapsed, the phase of the Moon will repeat on the same date. A pattern that became known as the ‘Metonic Cycle’. Of course the Moon can’t be pinned down to stick to even Meton’s carefully observed schedule, and what with slight variations in its orbital period and with leap days that intervene over a 19 year time-span, the Metonic Cycle can only be accurate to within a day.
This is why the last Blue Moon on Halloween was in 1944, and the following 19 year cycles in 1963, 1982 and 2001 were slightly off, and although they may have been spooky full Moons they weren’t in fact ‘Blue’. But now in 2020 Metons Metonic Cycle seems to have reset and we should see the return of the spooky Blue Moon phenomenon in 2039, 2058 and 2077.
So, it’s clear the Moon sure is full of tricks, but this really is a once in a Blue Moon treat. So, even if this Halloween is an unusual one in many ways, at the very least we can channel our inner Werewolf and go outside and have a little howl at the spooky bright Blue Moon. It’ll be awhile before we get the chance again.