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Boo! A Halloween blue moon

The last blue moon of the decade this weekend will round off October 2020.

Halloween. Not only is it a great time to dress up in spooky gowns from house to house, but it's also a great time for some (socially distant in 2020) street astronomy. This year it also features a special trick-or-treat event as the second full moon in October falls on the last day of the month.

Why the blue moon?

Of course, the moon does not appear "blue" on Halloween evening, October 31st. This strange astronomy meme comes to us over the years through a series of astronomical errors that have worked their way into the lexicon of the modern observer.

The error goes back to an article in the March 1946 issue of Sky & Telescope magazine, in which a "blue moon" was described as the second full moon in a month with two. Well, an old-fashioned archaic definition for a blue moon is the much more complicated definition of "the third full moon in an astronomical season (such as from an equinox to the solstice and vice versa) at four". I know. It's a bizarre rule. Legend has it that this rule arose from the now defunct Maine Farmer & # 39; s Almanac, which labeled this third false moon in blue instead of black ink. This claim remains unconfirmed. If someone has stacks of the Maine Farmer & # 39; s Almanac lying around in the attic, it may be worth checking them out to see if they actually are.

Sky and Telescope wrote an article about the story behind the blue moon in 2006, but by that point the damage was already done. Like so many modern memes with little or no foundation in reality, the internet has breathed new life into the idea that a blue moon is the second full moon in a month with two. This, of course, is to the dismay of professional astronomers who never sit in observatories at night discussing the blue moon.

Most of the time the rising "blue" moon takes on a distinctly orange hue. Photo credit and copyright: Andrew Symes.

How rare is "once in a blue moon" not very … a synodal period (the length of time from one similar phase to the next) is 29.5 days and is shorter than any month except February. On average, we get a blue moon every 2-3 years. A blue Halloween moon is a little rarer: the last one occurred in 1974 and the next in 2039.

If this period of 19 years (2039-2020) sounds familiar, it is because it is also what is known as the metonic cycle. 235 synodic periods correspond to almost 19 years, which means that if the full moon falls on a specific calendar date, it will almost always fall on the same date in 19 years. We say "almost always" because a metonic period is only 2 hours, 3 minutes and 50 seconds away from 19 years. Therefore, the 2001 full moon missed Halloween and fell on November 1st instead.

Can a moon really appear blue? Yes, and it doesn't even have to be full to do this. On September 23, 1950, due to musk fires in western Canada, the moon assumed such a strange occupation as could be seen in eastern North America.

A gibbous moon with a bluish cast dated December 25, 2012. Photo credit: Wikimedia Commons / Public Domian / crefollet

Another related term that is gaining traction on the internet is a black moon or the 2nd new moon in a month, which is also sometimes used for February without a new moon or full moon.

This is also the last blue moon of the decade … Here is a list of moons (black and blue) for the coming decade:

Black and blue moons old and new styles for the next decade, 2021-2030. Photo credit: Dave Dickinson.

The growing gibbous moon also passes 2.7 degrees south of −2.3 Mars on the evening of October 29th and is still shining brightly after its subtle opposition earlier this month. You can see Mars during the day before sunset using the Use moon as a guide?

Looking east at sunset on the evening of October 29th. Image credit: Stellarium.

The moon will peak on October 30th, just 20 hours before Full. This is also the most distant full moon for 2020 and heralds the antithesis of the supermoon, the "minimoon". After all, this weekend's full moon is also the hunter's moon for 2020, as the first full moon in October fell closest to the September equinox took the title of Harvest Moon for 2020.

What is really rare is a total lunar eclipse on Halloween. The last was back in 1175 AD, and the next won't be until – 2897 AD. Mark your calendar. Should we instead say "once in a blue blood moon"? "

Halloween itself also has an astronomical bond, as it is a quarterly bond day, roughly midway between the solstice and the equinox. In 2020, the technical midpoint between the September 22nd equinox and the December solstice actually falls on November 6th on December 21st, almost a full week after Halloween. Halloween is derived from the older Celtic festival of Samhain and continues on November 1st from All Saints' Day. The other three quarterly festival ties are Groundhog & # 39; s Day (February 2nd), May Day (May 1st) and the darkest day of modern times, Lammas Day (August 1st).

… And when you talk about archaic rituals if you live in North America (except for most of Arizona, Saskatchewan, and parts of British Columbia and Nunavut in Canada), don't forget to fall back an hour on Sunday November 1st . Since the first Sunday in November falls on the 1st in 2020, we cannot fall back on standard time at the earliest.

Filter out a blue moon. Photo credit: Dave Dickinson.

Does it really matter Of course, black and blue moons are mere abstract concepts, anomalies created by two calendar cycles that lead to a mathematical curiosity with little observational or scientific importance. But the strange story of how the meme came about is a strange historical story for modern astronomy.

I say let's take advantage of the non-event of a Halloween blue moon to watch the eyes from the sky and see a beautiful full moon weekend.

Main image: More fun with white balance to produce a "Warhol Moon" composition. Photo credit: Dave Dickinson.

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