The term “Blue Moon” is most often associated with a month containing 2 full moons. Not happening this month, and yet 18 May brings a Blue Moon according to rules drawn up by an Almanac editor nearly a 100 yrs ago.
This year, the Vernal Equinox occurred at 5:58p EDT (2158 GMT) on 20 March followed less than 4 hours later by a full moon, officially the 1st full moon of Spring.
Ecclesiastical rules do not recognize Spring arriving any earlier than 21 March. So Easter, a feast day that is celebrated on the 1st Sunday after the 1st full moon of Spring, was delayed until after the following full moon, on 19 April.
Now comes another unusual artifact of that near coincidence of the Vernal Equinox and the March full moon: This Saturday brings us the May full moon, and it will be a Blue Moon.
Hang on, isn’t a Blue Moon defined as the 2nd full moon that occurs during a calendar month?
The Big Q: The full moon on the 18th is the only full moon this month, so, how can we call it a Blue Moon?
The Big A: It is a Blue Moon if we follow an obscure rule. This rule was actually the original definition for branding a full moon as Blue.
The Almanac Rule
In the July 1943 issue of Sky & Telescope magazine, a Question & Answer-column referenced the term Blue Moon. The unusual term was cited from a copy of the 1937 Edition of the once Maine Farmers’ Almanac.
The Almanac page for August 1937 gave a calendrical meaning for the term Blue Moon. That explanation said that the moon “usually comes full 12X in a year, 3X for each season.”
Occasionally there will come a year when there are 13 full moons during a year, not the usual 12.
“This was considered a very unfortunate circumstance,” the almanac continued, “and it upset the regular arrangement of church festivals. For this reason, 13 came to be considered an unlucky number.”
And that extra full moon also meant that 1 of the 4 seasons would contain 4 full moons instead of the usual 3. When a particular season has 4 moons, the 3rd was apparently called a Blue Moon so that the 4th and final 1 could continue to be called the late moon.
The Big Q2: Where did we get the “2 full moons in a month rule” that is popular today?
The Big A2: Back to the pages of Sky & Telescope. This time, on Page 3 of the March 1946 issue, James Hugh Pruett wrote an article, “Once in a Blue Moon,” in which he referenced the term Blue Moon and the S&T article from July 1943.
Mr. Pruett unfortunately came to this conclusion: “Seven times in 19 years there were and still are 13 full moons in a year. This gives 11 months with 1 full moon each and 1 with 2. This 2nd in a month, so I interpret it, was called Blue Moon.”
Mr Pruett’s Y 1946 explanation was wrong, and it might have been completely forgotten were it not for Deborah Byrd’s use of it on her popular NPR program “StarDate” on 31 January 1980.
Over the next 10 years, Mr. Pruett’s misinterpretation gained traction with a new audience of baby boomers, and today, his “2 full moons in a month rule” is recognized worldwide.
Meanwhile, the original Maine Farmers’ Almanac rule was forgotten.
As was noted earlier, that publication has long since gone out of business.
Do not confuse it with the Farmers’ Almanac, which has been publishing continuously since Y 1818.
If going by the old “Maine Almanac rule,” for 1spring 2019, we have 4 full moons: on 20 March, 19 April, 18 May and 17 June. The 3rd full moon, the Blue Moon according to the original rule set down by the Maine Farmers’ Almanac comes Saturday, with the moon officially turning full at 5:11p EDT (2211 GMT).
That means that for much of North America, the moon will be just past full phase when it comes over the horizon at nightfall. This moon will still look full to everyone who sees it, even though it will be slightly past perfect roundness.
Oh, and do not expect to see the moon shine with a bluish tint. In the past, very unusual atmospheric circumstances have caused the moon and the Sun to appear bluish, caused by aerosols injected into the atmosphere, such as volcanic ash and dust after the eruption of the Krakatoa volcano in August 1883 or airborne soot from forest fires (such as from western Canada in September 1950. Nothing out of the ordinary has occurred in recent days or weeks, so this weekend’s moon should look pretty much the color we are accustomed to seeing.
At Space.com, there are many stories regarding the differences between the 2 Blue Moon definitions over the years. And as it turns out, the last time we had a Blue Moon based on the “4 full moons in a season” rule was also in May of 2016.
If you like you can meditate with this Blue Moon.
Have a terrific weekend.