Blue Moon, May 21, 2016

A filtered 'Blue Moon' from July 31st, 2015. Image credit and copyright: John Chumack

Astronomy, History, Moon, Skywatching

A May Full ‘Blue Moon’ on Tap For This Weekend?

16 May , 2016 by David Dickinson

Brace yourselves. You are about to hear talk this week of an astronomical non-occurrence of the utmost in obscurity. We’re talking about this weekend’s Blue Moon.
Now, I know what you’re thinking. Isn’t a ‘Blue Moon’ the second Full Moon of the month? How can a Blue Moon fall on the 21st? Trust me, we’re both correct… in a sense. The term ‘Blue Moon’ has taken on several meanings over the last few decades, with the ‘the second Full Moon in a calendar month containing two Full Moons’ now in vogue across ye old Internet. It seems the masses just can’t get enough of Super, Blood, Honey and Moons Black and Blue. We point to last month’s rumored ‘Green Moon‘ as evidence. (Spoiler alert: it wasn’t).
No, we’re talking instead of a Blue Moon in an old-timey sense. You’ll be hard pressed to explain source of this week’s Blue Moon for sure, though it has a fascinating origin story.

A Full Moon rising with Saturn and Mars on the night of May 21st, 2016. Image credit: Starry Night Education software.

The term seems to come down to us from the Maine Farmer’s Almanac, which denoted the ‘third Full Moon in an astronomical season with four as blue.’ The lunar synodic period of 29.5 days — the length of time it takes the Moon to return to a like phase, such as New to New, or Full to Full — means that on most years, there are 12 Full Moons. 29.5 times 12 comes out about 11 days short of a 365.25 day solar year at 354 days, meaning that about every three years, we have a year with 13 Full Moons.
Not a big deal, you say? Well, it assures that lunar based forms of reckoning time, such as the Muslim calendar loses 11 days relative to the Gregorian calendar every year.
Here’s how the 2016 Blue Moon breaks down:
March Equinox- March 20^th 4:30 Universal Time (UT)
March Full Moon- March 23rd 12:02 UT
April Full Moon- April 22nd 5:22 UT
May Full Moon- May 21st 21:17 UT (3^rd in an astronomical season, ‘blue’)
June Full Moon- June 20th 11:05 UT
June Solstice- June 20th 22:34 UT
The last time we had a season with four Full Moons was August 21^st, 2013, and the next Blue Moon under this rule is May 18^th, 2019.

Adding a pinch of blue to the Full Moon with a military flashlight filter. Image credit: Dave Dickinson

Of course, a deeper riddle is just why the Maine Farmer’s Almanac termed this occurrence as Blue, and why they picked the 3^rd of a season with 4 specifically… one legend goes that the extra anomalous Full Moon was depicted on the calendar in blue ink to stand out. We’d love to get our hands on a copy of the Old Maine Farmer’s Almanac circa late 19^th early 20^th century era to see if this was indeed the case. This is on our list of research projects, next time we find ourselves back in our home state of Maine.
Types of Blue Moons
We’ve chronicled the tales of Moons, both Black and Blue. Sky and Telescope also explored the role they had in introducing the modern day Blue Moon into common vernacular. We’ll admit, the ‘2^nd in a month with two Full Moons’ is a much easier rule to explain!