Saturday is everyone’s favorite spooky fall holiday: Halloween. And this year, trick-or-treaters will enjoy making their socially distanced rounds under the light of a Full Moon. But there’s more to this Halloween’s Full Moon than a bit of supernatural mystique. Saturday’s Full Moon is also a
Blue Moon — a term that applies to any second Full Moon within a single calendar month. (This skies this month hosted their first Full Moon on October 1.)
That’s not all, either. If you want to get a bit crazy, this Halloween technically sports a
Blue Hunter’s Moon, as a Full Moon in October is typically referred to as a
Hunter’s Moon. But what about the first Full Moon of the Month? Was that a Hunter’s Moon, too?
Actually, no. It was considered a
Harvest Moon, a name reserved for the Full Moon that occurs closest to the autumnal equinox. Normally, the Harvest Moon falls in September. But every few years — this year included — it pops up in October.
Seriously, though, what’s going on with all these Moon names? And how often do such weird confluences of nomenclature occur?
Once in a Blue Moon
For better or worse, a Blue Moon doesn’t mean the Moon turns blue. According to
Smithsonian Magazine, the modern interpretation of Blue Moon (two Full Moons in a single month) first came about in 1946. But that was the result of an erroneous description by amateur astronomer James High Pruett in an article in
Sky & Telescope — one that was later repeated by several other media outlets.
A slightly older version of the term Blue Moon originated in the early 1900s and referred to four Full Moons occurring in a single season (instead of three, which is the norm). But today, most people use the 1946 definition. Whichever meaning you prefer, however, the saying “Once in a Blue Moon” still describes a rare event. But what is it that makes Blue Moons rare?
As the Moon orbits Earth, we see it in phases that run from New to Full, then back to New again. It takes 29.53 days for the entire lunar cycle to unfold. That’s pretty close to the number of days in a calendar month, so most months, naturally, have one Full Moon.
But 29.53 days isn’t
exactly the length of a month. And the cycle doesn’t exactly match up with the calendar (i.e., the Moon isn’t always in the same phase at the beginning of each month). So, over time, the lunar phases drift to different days of the month. However, every once in a while, the 29.53-day cycle begins, ends, and begins again in the same month. This gives us a Blue Moon.
Blue Moons typically occur once every two to three years, and this Halloween’s Blue Moon is the only one in 2020. However, some years the lunar cycle lines up so there isn’t a Full Moon in February, which only has either 28 or 29 days. When that happens, both January and March can have Blue Moons during the same year, as they did in 2018.
