You know how people say that phrase “once in a blue moon”, meaning something is very rare? That’s an actual thing, and it doesn’t refer to the colour of the moon.
A typical lunar cycle is about 29.53 days, which doesn’t divide cleanly into the 365.24 days in a year on the Gregorian calendar (12.368 times, to be exact). The rotation of the Earth, our orbit of the sun, and the orbit of the moon relative to the sun not being on a clean schedule means that every now and then we’ll get two full moons in one month, which is what we now call a blue moon. That happens roughly once every 2.7 years, and sometimes happens twice in one year which is both cool and confusing.
A “blue moon” originally referred to having four full moons in a season instead of three. But, thanks to some misunderstandings and miscommunications around the Maine Farmer’s Almanac in the 1900s, it’s evolved to the current meaning. There is some dispute about whether the term originated in a pamphlet written in the 1500s by some Greenwich Friars complaining about the Roman clergy, which is the earliest use of the “blue moon” term. But that seems to be more a case of some dude named Jeff mocking the priests for thinking laymen will believe anything, rather than any deep astrological discovery.
With the history lesson out of the way, let’s get down to the spooky fact that there is going to be a blue moon on Halloween this year. What’s more is that it’s going to be a full moon for almost everyone worldwide on October 31, which is super rare.
The moon follows the 19-year Metonic cycle, which is how long it takes for the full moon to start happening on the same dates again. This is actually the cycle around which Greek Orthodox Easter is based, and was the basis for the Greek and Hebrew calendars long before the Gregorian calendar came into the picture. On this calendar, the months follow the moon instead of the sun, so every 2.7 Gregorian years, the Greek and Hebrew calendars have 13 months instead of 12, which is cool.
So, following that, there should be a blue moon on Halloween once every 19 years. Blue moons only happen when there’s a full moon at the beginning of the month and the very end, hence why Halloween gets so many. This means any full moon on Halloween will always be a blue moon. It’ll also be named a Hunter’s moon.
But the Metonic cycle doesn’t exactly line up with the Gregorian calendar, either – it’s 1 hour, 27 minutes and 33 seconds off, necessitating a leap day once every 216 or so years (which is still more accurate than the Gregorian calendar which needs a leap day every four years).
That means while there was a Halloween blue moon for some time zones in 2001, the last Halloween Full Moon that the whole world got to enjoy was 1944.
Unfortunately, while it is a blue moon for the whole world, it won’t be visible to most of Australia or any of New Zealand, which are in the only time zones to miss out. So, Western Australia, please go out and quietly howl at the full moon for all of us. Eastern Australia will likely get to see the next one in 2039, though, so mark your calendars.