Once in a Blue Moon ... is a common way of saying not very often, but what exactly is a Blue Moon? It is the second Full Moon to occur in a single calendar month. The average interval between Full Moons is about 29.53 days, whilst the length of an average month is roughly 30.4 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen. The well-known Metonic Cycle of lunar phases (whereby the phases of the moon occur on the same dates of the year) is 19 years long. During this time, there are 235 lunar months, and hence 236 Full Moons. There are also 228 calendar months, so at least 8 of those months must have seen two Full Moons. So, we can define Once in a Blue Moon as a mathematical probability: 8 chances in 228, or about 3.5 per cent!
Double Blue Moons
by Deborah Byrd
We don't know for sure what the New Year will bring, but we do know that 1999 will be a banner year for Blue Moons! For the first time since 1961, there will be two Blue Moons this year, one in January and one in March. Meanwhile, the month of February won't have a full moon.
We're not talking here about a blue-colored moon. Some observers have reported seeing blue-colored moons, such as that seen from Hawaii in August, 1991, not long after the eruption of Mt. Pinatubo in the Philippines. Volcanic dust in the air -- or smoke from a forest fire -- can make a moon appear blue in color. But these blue-colored moons are rare. They might have inspired the expression "once in a blue moon."
On the other hand, in recent years, Blue Moons have come to be associated with the calendar. The idea here is that there is a full moon every month, because the moon takes a month to complete one orbit around the Earth. That's where the word "month" or "moonth" came from. Each of the full moons have names, which correspond to the months of the year. The January full moon, for example, is called the Old Moon or Moon after Yule. But, once every two or three years, there are two full moons in a single month. That was the case in in August of 1993, and again in June of 1996. And it will be the case in both January and March of 1999.
The first full moon for this month comes on January 2 at 2:49 Universal Time. That's January 1 at 9:49 p.m. Eastern Standard Time in the United States. The second full moon comes on January 31 at 11:06 a.m. Eastern Standard Time. By rights, the January 1 full moon will be called the Old Moon or Moon After Yule. And the January 31 full moon will be the Blue Moon.
Likewise, the March full moon is typically called the Sap Moon, Crow Moon or Lenten Moon. Those will be the names for the full moon that'll come during the night of March 1. The second full moon of that month -- the so-called Blue Moon -- will come on March 31.
The time between full moons is 29.53 days. Thus the month of February is shorter than the lunar cycle. If the first full moon of a year falls on January 1st or 2nd, there will always be two full moons in January, and two in March -- but none in February. That's the case this year.
The last time we had two blue moons in one year was in 1961. The last time before that was in 1885. The next times that we will have two blue moons in a year will be 2018 -- and again in 2037.
The recent use of the word Blue Moon to describe the second full moon of a month can be traced to J. Hugh Pruett's April, 1946, article in Sky and Telescope magazine entitled Once in A Blue Moon.
In his article, Hugh mentioned an old Maine Farmers' Almanac for the year 1937. He wrote, "In effect . . . at one time the various full moons of the year were given names according to the order in which they occurred -- provided there was only one per month. These names were as follows: Moon after Yule, Wolf Moon, Lenten Moon, Egg Moon, Milk Moon, Flower Moon, Hay Moon, Grain Moon, Fruit Moon, Harvest Moon, Hunters' Moon and Moon Before Yule. But 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, and was considered unlucky and a real nuisance as it occurred at various times of the year and upset scheduling of church festivals."
If you pay attention to the sky you'll know that tonight there will be a full moon. Something to howl at while you're down on the waterfront, if you need it. Modern folklore has it that full moons make for better parties and higher booking rates at mental hospitals, but the studies I've heard about seem to deny the relationship.
At least once this week you have probably heard through the media that the old year (for purists, the decade of the eighties) is going out on a blue moon. People have been saying that "according to folklore" a second moon in a calendar month is a "blue moon." So, they say, this is the origin of the phrase "once in a blue moon." Don't believe them! "Once in a blue moon" is old, about 150 years old, but the age of the two-full-moons-in-a-month meaning of "blue moon" is less than ten years. The older meaning may be wishy-washy and the newer one solid and technical, but don't let anyone tell you they have replaced one with the other.
It's not rare to see two full moons in a month. Because the moon and our calendar are not in sync and all the months but February are longer than the moon's synodical cycle, it happens about seven times in every nineteen years. That's every thirty-three months on average. Months have different lengths, so the phenomenon moves around a bit. In 1999 there will even be two "blue" moons. If you think about it, it's a little like getting paid every second Friday and finding some months you get paid three times instead of twice.
Meaning is a slippery substance. The phrase "blue moon" has been around a long time, well over 400 years, but during that time its meaning has shifted around a lot. I have counted six different meanings which have been carried by the term, and at least four of them are still current today. So that makes discussion of the term a little complicated.
The earliest references to the term are in a phrase remarkably like early references to "green cheese." Both were used as examples of obvious absurdities about which there could be no argument. Four hundred years ago, if someone said, "He would argue the moon was blue," the average Sixteenth Century man would take it the way we take, "He'd argue that black is white." This understanding of a blue moon being absurd (the first meaning) led eventually to a second meaning, that of "never." To say that something would happen when the moon turned blue was like saying that it would happen on Tib's Eve (at least before Tib got a day near Christmas assigned to her).
But of course, there are examples of the moon actually turning blue; that's the third meaning - the moon visually appearing blue. When the Indonesian volcano Krakatoa exploded in 1883, its dust turned sunsets green and the moon blue all around the world for the best part of two years. In 1927 a late monsoon in India set up conditions for a blue moon. And the moon here in Newfoundland was turned blue in 1951 when huge forest fires in Alberta threw smoke particles up into the sky. Even by the nineteenth century it was clear that although visually blue moons were rare, they did happen from time to time. So the phrase "once in a blue moon" came about. It meant then exactly what it means today - that an event was fairly infrequent, but not quite regular enough to pinpoint. That's meaning number four, and today it is still the main one.
I know of six songs which use "blue moon" as a symbol of sadness and loneliness. In half of them the poor crooner's moon turns to gold when he gets his love at the end of the song. That's meaning number five: check your old Elvis Presley or Bill Monroe records for more information.
Finally, in the 1980s, comes the most recent meaning of blue moon - the second full moon in a month. I first became aware of the new meaning of the term in late May 1988 when it seemed all the radio stations and newspapers were carrying an item on this interesting bit of "old folklore." At the MUN Folklore Language Archive we get calls from all over, from people wondering about bits of folklore, and in that month I got calls about blue moons. You see there were two full moons that month. There hasn't been such a month since then, until this month. December 1990 has full moons on the 2nd and the 31st.
In 1988 I searched high and low for a reference to the term having this meaning, or for any other term used to describe two moons in a single calendar month. But it was all in vain. There just seemed to be no history to this term. Through that research I uncovered the information on other meanings of "blue moon." But not this blue moon, meaning number six.
This month, with the new "blue moon" coming on, I started getting calls again and I searched harder this time. I had already exhausted all the usual sources of historical and astronomical dictionaries, indexes of proverbial sayings and the like. A brand new edition of the huge Oxford English Dictionary had come out in the meantime, but even that seemed to have nothing on this new usage. A new tack was called for. Almost every day I use computer networks to contact other folklorists around the world (in fact I send this column all around the world each week on one of the networks), so I started with them. But no one could give me an earlier use of the term than the 1988 wire stories. I then turned to other computer networks, for scientists and especially astronomers. Still no luck. "Blue moon" seemed to be a truly modern piece of folklore, masquerading as something old.
Then I remembered that the term was a question in one of the Trivial Pursuit boxes, the "Genus II edition," which was published in 1986. Trivial Pursuit is a fine company for scholars - they keep all their files and they can tell you the source of any bit of information in their games. Yes, they told me, that question came from a certain children's "Facts and Records" book, published in 1985. Where the authors of that book got it, no one seems to know.
The term, used this way, must have been very, very local before the publication of the children's book, so local that it was never written down by amateur or professional astronomers, or by the newspapers which might have been searched by dictionary makers. It certainly was very rare. Perhaps it was even made up by the authors of the children's book as a safeguard against plagiarism. This is sometimes done in order to be able to prove in a court of law that a later work has stolen from your own - how else would they have gotten something which you invented? Well, if this is what the authors did, they have lost out because the term immediately entered the folklore of the modern world and it has become as living a meaning of the term "blue moon" as any of the earlier ones. Since it has a kind of technical meaning which most of the earlier meanings lacked, it will probably last a whole lot longer, too. "Old folklore" it is not, but real folklore it is.
Philip Hiscock is Archivist at the Memorial University of Newfoundland Folklore and Language Archive.
Once in a Blue Moon
Fact and fantasy about blue Moons.
By Philip Hiscock
In 1999 we're having blue Moons. Two, in fact. If you live in North America or Europe, a pair of full Moons occurs in January and then another pair in March. In other parts of the world the phenomenon happens in April or May. While everybody experiences the Moon's fullness at the same time, our local clocks differ, and this sometimes pushes the event into the previous or next month.
"According to old folklore," some people say, the second full Moon
a calendar month is called a "blue Moon." They go on to explain that
is the origin of the expression "once in a blue Moon." But it isn't
The term "blue Moon" has been around a long time, well over 400 years,
but its calendrical meaning has become widespread only in the last 20
A Variety of Meanings
In fact, the very earliest uses of the term were remarkably like saying the Moon is made of green cheese. Both were obvious absurdities, about which there could be no doubt. "He would argue the Moon was blue" was taken by the average person of the 16th century as we take "He'd argue that black is white."
The concept that a blue Moon was absurd (the first meaning) led eventually to a second meaning, that of "never." The statement "I'll marry you, m'lady, when the Moon is blue!" would not have been taken as a betrothal in the 18th century.
But there are also historical examples of the Moon actually turning blue. That's the third meaning - the Moon appearing blue in the sky. When the Indonesian volcano Krakatoa exploded in 1883, its dust turned sunsets green and the Moon blue all around the world for the best part of two years. In 1927, the Indian monsoons were late arriving and the extra-long dry season blew up enough dust for a blue Moon. And Moons in northeastern North America turned blue in 1951 when huge forest fires in western Canada threw smoke particles up into the sky.
So, by the mid-19th century, it was clear that visibly blue Moons, though rare, did happen from time to time - whence the phrase "once in a blue Moon." It meant then exactly what it means today, a fairly infrequent event, not quite regular enough to pinpoint. That's meaning number four, and today it is still the main one.
But meaning is a slippery substance, and I know of a half dozen songs that use "blue Moon" as a symbol of sadness and loneliness. The poor crooner's Moon often turns to gold when he gets his love at the end of the song. That's meaning number five: check your old Elvis Presley or Bill Monroe records for more information.
And did I mention a slinky blue liquid in a cocktail glass, one that requires curaçao, gin, and perhaps a twist of lemon? That's number six.
Can you have two Blue Moons in a year?
The Synodical Month is 29.53 days long between any two Full Moons. There are 365.25636 days in the year which equals 12.37 Lunar Months. The little bit that is left over means that there can be 13 Full Moons during years that are about 1/.37 = 2.7 years apart or so. The following list is the list of Blue Moons for the next 54 years, and a double Blue Moon will happen in 1999 and 2018!:
January 31 1999 16:07
March 31 1999 22:49
November 30 2001 20:50
July 31 2004 18:06
June 30 2007 13:49
December 31 2009 19:13
August 31 2012 13:59
July 31 2015 10:44
January 31 2018 13:27
March 31 2018 12:37
October 31 2020 14:50
August 31 2022 1:36
May 31 2026 8:46
December 31 2028 16:49
September 30 2031 18:59
July 31 2034 5:55
January 31 2037 14:05
October 31 2039 22:37
August 31 2042 2:03
May 30 2045 17:53
January 31 2048 0:15
September 30 2050 17:33
We see that the only months with 30 or 31 days can have a second
Moon (A Blue Moon in April will not occur during this particular
In this series, there also happen to be more Blue Moons in January,
and August than in the other months, but this is due to the statistics
of sampling over only a limited time span. (UT = Universal Time,
determined in Greenwich, England)
Double Blue Moons
In about 4 years per century, there are two Blue Moons. The first Blue Moon always occurs in January (or sometimes December, depending upon your local timezone). The second occurs predominantly in March. In the years between 1600 and 9999, this is true in 282 out of 331 cases, or 85 per cent of the time. In 32 cases (or 10 per cent), the second Blue Moon is in April. In the remaining 17 cases (5 per cent) it is in May. In order for a second Blue Moon to take place in March, there can be no Full Moon in February, and so non-leap years will inevitably be favoured. However, it is possible to have a double Blue Moon in a leap year. This occurs in 30 of the 331 cases between 1600 and 9999. Clearly, the second Full Moon in January takes place near the very end of the 31st, and the first Full Moon in March is early on the 1st. The extremes of the lengths of the lunar month are 29 days 6 hours and 29 days 20 hours, so it is possible to skip February altogether in this way.
Years with Double Blue Moons and months in which they occurred
(1=January, March=3, etc) 1600-2100:
1608 1 3
1627 1 3
1646 1 3
1665 1 3
1695 12 3
1714 12 3
1741 1 4
1771 1 3
1809 1 3
1847 1 3
1866 1 3
1885 1 3
1915 1 3
1934 12 3
1961 1 4 1
1999 1 3
2018 1 3
2037 1 3
2067 12 3
2094 1 4