is a Blue Moon?
There are in fact two definitions for a blue moon. According to the more recent
definition, a blue moon is the second full moon in a calendar month. For a
blue moon to occur, the first of the full moons must appear at or near the
beginning of the month so that the second will fall within the same month (the
average span between two moons is 29.5 days). In July 2004 there were two full
moons: the first on July 2, the second on July 31—that second full moon
is called the blue moon.
An older definition for the blue moon is recorded in early
issues of the Maine Farmer's Almanac. According to this definition,
the blue moon is the third full moon in a season that has
four full moons. Why would one want to identify the third
full moon in a season of four full moons? The answer is complex,
and has to do with the Christian ecclesiastical calendar.
Some years have an extra full moon—thirteen instead
of twelve. Since the identity of the moons was important
in the ecclesiastical calendar (the Paschal Moon, for example,
used to be crucial for determining the date of Easter), a
year with a thirteenth moon skewed the calendar, since there
were names for only twelve moons. By identifying the extra,
thirteenth moon as a blue moon, the ecclesiastical calendar
was able to stay on track.
A Star Rating for the Modern Blue
Although some astronomical media sourses call the modern blue moon definition"trendy" and
a "mistake," the fact that there is an older, preexisting (and more
complicated) definition does not necessarily make it the more interesting or
Charting the "third full moon in four full moons" in
a season isn't everyone's idea of an fascinating enterprise. The modern, "trendy" definition,
however, points to an intriguing astronomical phenomenon—every so often
two moons can manage to position themselves in the same month. Given that full
moons occur once every 29.5 days, this is quite an accomplishment!
How Often Does a Blue Moon Occur?
Over the next twenty years there will be a total of 17 blue moons, with an
almost equal number of both types of blue moons occurring. No blue moon of
any kind will occur in the years 2006, 2011, 2014, and 2017.
The more recent phenomenon, where the blue moon is considered
to be the second full moon in a calendar month, last occurred
on Nov. 30, 2001. Two full moons in one month may occur in
any month out of the year except for February, which is shorter
than the lunar cycle.
The other, older blue moon event, which happens when there
are four full moons in a season, last occurred on Nov. 20,
2002. Since this type of blue moon is reckoned according
to the seasons, it can only occur in February, May, August,
or November, about a month before the equinox or the solstice.
Twice in a Blue Moon
The rare phenomenon of two blue moons (using the more recent definition) occurring
in the same year happens approximately once every 19 years. 1999 was the
last time a blue moon appeared twice, in January and March.
The months of the double blue moons are almost always January
and March. That is because the short month that falls in
between them, February, is a key ingredient in this once-every-nineteen-year
For January and March to each have two full moons,
it's necessary for February to have none at all. Since
February is usually 28 days long, and the average span between
moons is 29.5 days, if a full moon occurs at the end of
January, it's possible for the next full moon to skip February
and fall in the beginning of March.
Once in a Blue Moon
"Blue moon" appears to have been a colloquial expression long before
it developed its calendrical senses. According to the Oxford English Dictionary,
the first reference to a blue moon comes from a proverb recorded in 1528: If
they say the moon is blue, We must believe that it is true.
Saying the moon was blue was equivalent to saying the moon
was made of green (or cream) cheese; it indicated an obvious
absurdity. In the 19th century, the phrase until a blue moon
developed, meaning "never." The phrase, once in
a blue moon today has come to mean "every now and then" or "rarely"—whether
it gained that meaning through association with the lunar
event remains uncertain.