Moonrise from behind Gray Mountain, six miles northwest of Lyons, on October 16th, 2016. The tree is about one mile distant; the Moon is roughly 240,000 times more distant. (All images by the author)
Halloween conjures up images of a crisp autumn evening with Linus awaiting the arrival of the Great Pumpkin which ultimately becomes a giant orange full moon rising up out of the pumpkin patch. This month it will actually happen: the Full Moon does come on Halloween. Although the Moon will have officially become full at 8:48 a.m. in the morning of the 31st, it will still be 99.7 percent illuminated by the time it rises for evening trick-or-treaters.
Not only will the Moon be full, it will also be “blue”. And therein lies our story.
We’re used to thinking of the Moon as rather yellow in color, sometimes orange as it breaks the horizon, and even copper-colored or red during times of a total lunar eclipse (giving rise to the much over-used “blood moon” moniker). And while on rare occasions it has been reported to appear blue-tinged due to atmospheric ash from volcanic eruptions, we generally we think of a Blue Moon as an absurdity … and that, in fact, was the original intent of the name. The first known use of the phrase appeared in a 1528 pamphlet critical of the Roman clergy, which proclaimed that “Church Men are Wily Foxes, they say the Moon is Blue”. Eventually the phrase “once in a blue moon” became an equivalent to saying an event is either rare or unlikely, rather than being a reference to its color.
The concept of a “month” is derived from the time it takes for the Moon to go from one full phase to the next, about 29.5 days on average. Thanks to the Julian Calendar Reform of 46 B.C., we have 12 months in a year and usually 12 full moons, each month containing one full moon. However, there are an extra 11 days of lunar phases left over every year, which means that about once every 2.7 years we’ll have a 13th full moon in a calendar year; and that obviously implies that one of the twelve months must contain a second full moon. Hence the modern meaning of a Blue Moon: the second full moon in a calendar month.
The full Moon closest to the autumnal equinox is termed the Harvest Moon because the extra light it provided upon rising allowed farmers to continue their harvest labors further into the evening. Image taken on September 24th, 2018.
Oddly enough, that simple and logical definition was not the original meaning. Farmers of the 17th through the 19th centuries paid far closer attention to the seasons (defined by the solar equinoxes and solstices) and the lunar phases therein, than we do today, since those celestial phenomena were what governed the cycle of planting and harvesting. The old Farmer’s Almanac’s original idea of a Blue Moon still referred to an “extra” lunar cycle, but one that results in four full phases in a single season instead of the normal three. And (for reasons nobody seems to be quite sure of) it wasn’t even the fourth full moon that was considered the oddball, it was the third, or “second-middle-moon”, that was pronounced to be Blue.
But by the 20th century most of humanity had drifted away from its agrarian roots, so that the seasonal meaning had become lost on modern man. It wasn’t until 1946 that a writer for Sky and Telescope magazine attempted to decipher the Almanac’s ancient rationale behind the naming of Blue Moons, and mistakenly concluded that it meant the second full moon in a calendar month. And we’ve been living with that new definition ever since.
It’s not possible to get a full Moon that is any “fuller” than one in total eclipse, since it must be exactly opposite the Sun in the sky to pass through the Earth’s shadow. The eclipsed Moon shown is about 10,000 times dimmer than it was a few hours earlier before it entered the shadow; note that stars can be seen next to it that would otherwise be drowned out by the glare.
Which brings us back to this coming Halloween. This will be the first full moon falling on Halloween in 19 years, and by the modern definition, it will be a Blue Moon as well. (According to the original definition, it is not … in fact, the current year doesn’t even contain a seasonal Blue Moon, the next one will be on August 22nd, as the third full moon of Summer 2021).
So how rare is the Halloween Blue Moon? By the original definition, it’s not even possible … any full moon falling on October 31st can only be the second of the season, never the third.
But by our modern definition, every full moon on Halloween must be a Blue Moon, since it’s not possible without the previous one occurring on either October 1st or 2nd.
But of course, since the second definition was only invented in 1946 … and since there has been only one full moon on Halloween (in 2001) since then … in the Mountain Time Zone, at least … it can rightfully be claimed that this Halloween Blue Moon is only the second one ever, in all of recorded history!