A BLUE MOON?
Recent decades have seen widespread popular embrace of the idea that
when a calendar month contains two full Moons, the second one is called a
"Blue Moon." The unusual pattern of lunar phases in early 1999 — two
full Moons each in January and March, and none at all in February —
triggered a groundswell of public interest. Countless newspapers and
radio and TV stations ran stories about Blue Moons.
In an article entitled Once in a Blue Moon,
folklorist Philip Hiscock traced the calendrical meaning of the term
"Blue Moon" to the Maine Farmers' Almanac for 1937. But a page from that
almanac belies the second-full-Moon-in-a-month interpretation.
With help from Margaret Vaverek (Southwest Texas State University) and
several other librarians, we obtained more than 40 editions of the Maine
Farmers' Almanac from the period 1819 to 1962. These refer to more than
a dozen Blue Moons, and not one of them is the second full Moon in a
month. What's going on here?
Blue Moons and the Seasons
Several clues point to a strong connection between the almanac's Blue
Moons and the four seasons of the year. All of the listed Blue Moons
fall on the 20th, 21st, 22nd, or 23rd day of November, May, February, or
August. These dates fall about a month before the Northern Hemisphere
winter and summer solstices, and spring and fall equinoxes,
respectively, which occur on similar day numbers.
Although the idea of a seasonal pattern suggested itself to us
immediately, verifying the details required a lot of detective work. We
found that the Blue-Moon definition employed in the Maine Farmers'
Almanac is indeed based on the seasons, but with some subtle twists.
Instead of the calendar year running from January 1st through December
31st, the almanac relies on the tropical year, defined as extending from
one winter solstice ("Yule") to the next. Most tropical years contain
12 full Moons — three each in winter, spring, summer, and fall — and
each is named for an activity appropriate to the time of year (such as
the Harvest Moon in autumn). But occasionally a tropical year contains
13 full Moons, such that one season has four rather than the usual
three.
Today we usually mark the beginning of the seasons when the Sun's
celestial longitude passes 0° (spring), 90° (summer), 180° (autumn), and
270° (winter). The Sun appears to move along the ecliptic at a variable
rate because of the Earth's not-quite-circular orbit, so the seasons
defined this way are not equal in duration. Another approach uses the
dynamical mean Sun or fictitious mean Sun — imaginary bodies that move
along the ecliptic and the celestial equator, respectively, at a
constant rate and produces seasons of equal length. The Maine almanac
defines the seasons using this alternative method.
The almanac also follows certain rules laid down as part of the
Gregorian calendar reform in 1582. The ecclesiastical vernal (spring)
equinox always falls on March 21st, regardless of the position of the
Sun. Lent begins on Ash Wednesday, 46 days before Easter, and must
contain the Lenten Moon, considered to be the last full Moon of winter.
The first full Moon of spring is called the Egg Moon (or Easter Moon, or
Paschal Moon) and must fall within the week before Easter.
At last we have the "Maine rule" for Blue Moons: Seasonal Moon names
are assigned near the spring equinox in accordance with the
ecclesiastical rules for determining the dates of Easter and Lent. The
beginnings of summer, fall, and winter are determined by the dynamical
mean Sun. When a season contains four full Moons, the third is called a
Blue Moon.
Why is the third full Moon identified as the extra one in a season with
four? Because only then will the names of the other full Moons, such as
the Moon Before Yule and the Moon After Yule, fall at the proper times
relative to the solstices and equinoxes.
Questions and Answers
During the period 1932 to 1957, under the editorship of Henry Porter
Trefethen (1887-1957), the Maine Farmers' Almanac consistently listed
Blue Moons derived from the convoluted seasonal rule just described. So
where did the modern convention — that a Blue Moon is the second full
Moon in a calendar month — come from? Sky & Telescope has, and is, the answer!
Laurence J. Lafleur (1907-66) of Antioch College, Ohio, discussed Blue Moons in a question-and-answer column in Sky & Telescope,
July 1943, page 17, citing the 1937 Maine Farmers' Almanac as his
source. It is clear that Lafleur had a copy of the almanac at his side
as he wrote, since he quoted word for word the commentary on the August
1937 calendar page. This commentary notes that the Moon occasionally
"comes full thirteen times in a year," but Lafleur did not judge whether
this referred to a tropical year or a calendar year. More important, he
did not mention the specific dates of any Blue Moons and never said
anything about two full Moons in one calendar month.
Some three years later, in March 1946, an article entitled "Once in a Blue Moon" appeared in Sky & Telescope
(page 3). Its author, James Hugh Pruett (1886-1955), was an amateur
astronomer living in Eugene, Oregon, and a frequent contributor to S&T.
Pruett wrote on a variety of topics, especially fireball meteors. In
his article on Blue Moons, he mentioned the 1937 Maine almanac and
repeated some of Lafleur's earlier comments. Then, unfortunately, he
went on to say, "Seven times in 19 years there were — and still are — 13
full moons in a year. This gives 11 months with one full moon each and
one with two. This second in a month, so I interpret it, was called Blue
Moon."
Pruett must not have had the 1937 almanac handy, or he would have
noticed that the Blue Moon fell on August 21st (obviously not the second
full Moon that month) and that 1937 had only 12 full Moons. But only in
retrospect is his error so obvious.
Modern Folklore
Sky & Telescope adopted Pruett's new definition, using it in a
note entitled "'Blue' Moons in May" on page 176 of the May 1950 issue.
In a bizarre twist, the data on lunar phases for this note came from
none other than H. Porter Trefethen of Winthrop, Maine, editor of the
very almanac Pruett misread four years earlier! But Trefethen himself
never called the second full Moon in a month a Blue Moon. The "'Blue'
Moons" headline was likely added by Sky & Telescope's
founding editor, Charles A. Federer Jr. Federer agreed that he probably
wrote that headline with Pruett's then-recent article in mind and
without consulting Trefethen.
As Hiscock explained in the March issue, widespread adoption of the
second-full-Moon-in-a-month definition followed its use on the popular
radio program StarDate on January 31, 1980. We examined this show's
script, authored by Deborah Byrd, and found that it contains a footnote
not read on the air that cites Pruett's 1946 article as the source for
the information.
With two decades of popular usage behind it, the
second-full-Moon-in-a-month (mis)interpretation is like a genie that
can't be forced back into its bottle. But that's not necessarily a bad
thing. Rather than argue over whether to celebrate the dawn of the new
millennium on January 1st in 2000 or 2001, those with the sunniest
outlooks will celebrate twice. Why not treat Blue Moons the same way,
marking both the second full Moon in a calendar month and the third full
Moon in a season with four? "Even if the calendrical meaning is new,"
said Federer, "I don't see any harm in it. It's something fun to talk
about, and it helps attract people to astronomy."
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