February 27, 2015

Astro-Event: What’s in a Name? Black & Blue Moons through 2020.

The August 2011 Full Moon rising as seen from Astroguyz HQ.

(Photo by Author).

(Note: This week’s lunar-related event is a fitting tribute to the life of astronaut hero and legend Neil Armstrong, who passed away this weekend. As the second Full Moon of the month approaches, don’t forget to look skyward and remember when the first man walked on the Moon in 1969. Next week’s special Astro-Event will be Apollo 11 related as well. This one’s for you, Neil!)

It’s strange what astronomical events “seep through” to the public consciousness sometimes. Early last year, articles breathlessly reported the discovery of the “13th constellation” of the zodiac, and the astrologically-minded wondered whether they may in fact actually be Ophiuchians. Then there’s the yearly round of the “Supermoon,” a Proxigean Full Moon that always seems to fail to wreak havoc for yet another year, all to great fanfare. Again, we science bloggers shake our heads and dust off last year’s post on how “the Moon reaches perigee nearly as close every lunation,” or “the precession of the equinoxes isn’t really a new discovery” etc…

And yet, we’re pulled almost lemming-like to these non-stories that sent our search numbers surging. Hey, it’s a “teachable moment,” right? And we’re just thrilled at the opportunity to get “eyes on the sky,” or at least explain to anyone willing to listen just how cool the universe really is.

Such is the case with this week’s Full Moon, occurring on Friday, August 31st at 9:58 AM EDT/ 13:58 UTC. This is the only “dual Full Moons in one month” for 2012. We get the twin questions of “What is a Blue Moon?” & “When will the next Blue Moon occur?” frequently enough to warrant discussion. Hey, hook ‘em with the mathematically odd, then feed ‘em astronomy, right?


This week’s Full Moon is the 2nd Full Moon of the month, and this occurrence has entered the modern vernacular as a “Blue Moon” over the last few decades. Not that it will appear very blue, or is even very rare, as the term “Once in a Blue Moon” might suggest. OK, seeing the Full Moon appear visually blue is indeed rare, unless you live near a sulfur-spewing factory. It has been historically documented to have occurred worldwide in 1883 after the eruption of Krakatoa. A visually blue Moon was also noted following the delay of the monsoon in 1927 (due to suspended dust) in India, and in North America in 1951 following widespread forest fires in western Canada. OK, I’ve yet to see such an occurrence, or a convincing photograph of such a phenomenon. Surely, in this age of a “mobile phone camera in every pocket,” a non-Photoshoped Blue Moon is awaiting capture somewhere out there?

And we also know of the controversy surrounding the modern definition of a Blue Moon; that stemmed from an error in the March 1946 issue of Sky & Telescope, which was researched and further laid out in their March 1999 edition. The original definition of a Blue Moon as described by the 1937 Maine Farmer’s Almanac is even more byzantine, defined as “the 3rd Full Moon in a season with 4”. Seasons (from equinox to solstice, & vice versa) are on average 91.3 days long, and 3 lunations (equal to 4 Full Moons) are about 29.5 x 3= 88.5 days in length, so you’ve got not quite three days of wiggle room for ye’ ole Blue Moon of your grand-pappy’s day to occur. Legend has it that the “Blue” refers to the ink used to mark the extra superfluous moon on those tables of yore. Any Maine Farmer’s Almanacs circa 1930’s kicking around out there? Check those attics and garages… we’d love to see ‘em!

So, what’s the practical upshot of a Blue Moon? What significance do you assign to this Friday nite to those astronomy neophytes eager for all things indigo? Physically, there is none. The Full Moon will rise in the east at local sunset, “unblued” in appearance just like it did at the beginning of August. The phenomenon of a “Blue Moon” is an artifact of our solar-based Gregorian calendar; all months except February are longer than the lunar synodic period, (29.5 days on average) and thus can contain the same phase twice.

Incidentally, the term “Black Moon” has also come into vogue in recent years, as if we astronomers don’t have enough bizarre non-events in our lives. (Hey, say it long enough, or get enough folks to agree with you on terms such as “dwarf planet” & “Super-Moon,” and it enters the modern lexicon, right?) Think of a Black Moon as a Blue Moon’s New Moon opposite; it can also refer to a February devoid of a Full or New Moon, the only month on which this can occur. So, is a February on a leap year minus a Full or New Moon a “Super-Black Moon?” do we even need to go there?

Love ‘em or hate ‘em, these colorfully named lunations are here to stay. We’ve even designed this nifty table of future Black & Blue Moons for you to plan the next decade around;

Black & Blue Moons thru 2020. (As reckoned in Universal Time).

*Note: “3rd of 4” Is the third New or Full Moon in an astronomical season, a definition established by the Old Maine Farmers Almanac.

**Also note that ye ole Maine Farmer’s Almanac method of reckoning seasons used mean versus apparent solar time; under the old convention, the Black & Blue Moons in 2019 & 2020 would not fall on August 30th & October 31st, but February 19th, 2019 (Blue) & May 22nd, 2020(Black)!

Save the above table, post it on your refrigerator or observatory wall, confound your friends with all that is esoteric in the realm of the synodic. Note that the total solar eclipse of August 21st, 2017 that crosses the U.S. fits the definition of a Black Moon that is “The 3rd in a season with 4!” Likewise, the next “Blue Moon Lunar Eclipse” occurs on January 31st, 2018. The last one occurred during a partial in 2009) A Blue Moon can never occur in February, not even on a leap year, because the shortest synodic period (the length between identical lunar phases) is about 29 days & 6 hours, just 0.25 days too long.

Our thanks to Ed Kotapish for keeping us straight on seasonal dates and times, as well as running predictions for the coming decades. Be sure to raise a Blue Moon beer to all things lunar this Friday, or perhaps you’ll want to mix a Blue Moon cocktail;

-2 oz of Gin

-˝ oz of Crčme de Violette (A new one on us, too!)

-˝ oz of lemon juice

(Our extensive research reveals that a shot of Blue Curacao will deepen the neon Blue of the drink!)

Be sure to raise a glass to this Friday’s esoteric Full Moon & make Neil proud… As always, we welcome all pics, theories and cultural sightings in the wild of the Moon, both Black, Blue and otherwise.

(Next week; as promised, we’ll journey to the Sea of Tranquility to track down the Apollo 11 craters!)


  1. ed Kotapish says:

    Just saw the video Dave – lookin’ good!


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