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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.

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 .

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.

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. Byrd now writes for the radio program Earth
& Sky, whose Web site contains a note giving her perspective on this
modern contribution to lunar folklore.

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."

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Don Olson works on historical applications of astronomy at Texas State
University. Rick Fienberg joined Sky & Telescope after earning his
Ph.D. in astronomy and is now the magazine's editor in chief. Senior
editor Roger Sinnott edits (among other things) S&T's Celestial
Calendar.


Once in a Blue Moon
Fact and fantasy about blue Moons.
By Philip Hiscock

In 2004 we're having a blue Moon. If you live in North America or Europe,
a pair of full Moons occurs in July. "According to old folklore," some
people say, the second full Moon in a calendar month is called a "blue
Moon." They go on to explain that this is the origin of the expression
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