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What is a Blue Moon?
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12-01-2009 - astronomy essentials
Don’t be misled by the photo above. Although certain-sized particles of dust or smoke can cause a moon to look blue in color, most Blue Moons are not really blue.
Instead, Blue Moon is just a name, much like other full moon names.
Every month has a full moon, and, most of the time, the names coincide with particular months of the year. The Blue Moon is different. In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.
The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so blue moons come about that often. The next blue moon will come on December 31, 2009 if you live in the Americas, Europe or Africa. If you’re in Australia, New Zealand or Asia, your clocks and calendars will show that same moon falling on January 1, 2010. So your blue moon will come at the end of January, 2010.
Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had blue moons.
The next year of double blue moons is coming up in 2018.
By the way, there’s been a controversy about the use of the term Blue Moon. The Farmer’s Almanac defined a Blue Moon as an extra full moon that occurred in a season. One season typically had three full moons. If a season had four full moons, then the third full moon was named a Blue Moon.
The modern use of the term Blue Moon to describe two full moons in a single calendar month has been called a “mistake” made in the March 1946 Sky and Telescope article “Once in a Blue Moon” by James Hugh Pruett. Apparently, Pruett misinterpreted the 1937 Maine Farmer’s Almanac. He wrote: “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.”
EarthSky’s Deborah Byrd happened upon a copy of the this old 1946 issue of Sky and Telescope in the Peridier Library at the University of Texas Astronomy Department in the late 1970s. Afterwards, she began using the term Blue Moon to describe the second full moon in a calendar month on the radio. Later, this definition of Blue Moon was also popularized by a children’s book (Margot McLoon-Basta and Alice Sigel, “Kids’ World Almanac of Records and Facts,” published in New York by World Almanac Publications, in 1985), and the board game Trivial Pursuit.
Since then, purists have argued that the definition of Blue Moon as the second full moon of a month is “wrong.”
But as the folklorist Phillip Hiscock wrote in his comprehensive article Folklore of the Blue Moon: Old folklore it is not, but real folklore it is.
Written by earthsky
Comments (17)
at 10.49 pm on 12-04-2009 Deborah
I found this a very interestng article. I used to think a blue moon was really blue. Thank you for the clarification
Replyat 03.58 am on 12-15-2009 yooki
In my opinion, there are 13 definitions of a blue moon, because there are 13 full moons in a blue moon year. The definition of blue moon as the second full moon in a month was, and still is, not a mistake or wrong!
Replyat 08.02 am on 12-15-2009 Deborah Byrd
Hi Yooki, ha! Yes, I have to smile a little at the people who call using the name Blue Moon for the second full moon in a month "wrong." It's folklore. The folk get to determine what's right and what's wrong!
at 1.15 pm on 12-28-2009 yosiah
I am trying to find a source that defines a "blue moon" prior to the pre-calculated modern calendars that use the term "month" instead of "moon" like all ancient lunar based calendars did. Can anyone help me?
Replyat 11.05 am on 12-29-2009 admin
Yosiah,
If you use a lunar calendar or a lunar/solar calendar whereby the month is based on the phases of the moon and not the calendar month, then you would have no blue moon by the modern definition of the term - the second full moon in a single calendar month. Since a calendar month (except for February) is longer than a lunar month, it is possible for two full moons to fall in a single calendar month. A blue moon by this definition happens about 8 times every 19 years.
Another definition of blue moon - the 3rd of 4 full moons to fall in one season - is the older definition of the term.
Bruce
at 2.24 pm on 12-30-2009 Donna
Since hearing about the blue moon for 12/31/09, i have been reading some explanations of a blue moon. Some i've read said that it is caused by dirt/smoke particles in the air making the moon look blue i.e., volcano eruptions, etc. If this is the fact, then how can the blue moons be predicted in advance not knowing what the dirt/smoke particles would be like at that time? Secondly, the 2nd full moon in a month explanation, is the 2nd full moon always blue?
Replyat 4.01 pm on 12-30-2009 admin
Donna,
The "blue moons" which can be predicted in advance do not refer to the color of the moon, but to calendar oddities. The second full moon to fall in a single calendar month is said to be a "blue moon" in the sense that it is relatively rare phenomenon. A blue moon by this definition happens some 8 times every 19 years.
An older blue moon definition refers to a different sort of calendar oddity: the 3rd of 4 full moons to fall in one season. Usually, a season only has 3 full moons. But every 19 years, 7 seasons harbor 4 full moons. Again, this definition of "blue moon" does not refer to the color of the moon but to a relatively rare calendar oddity.
at 11.39 am on 12-31-2009 farheen
i think thats such an information for young student cause some people cant imagine on solar stars or blue moon . plz search more information about comets,asteroids,and milky ways means glaxies
Replyat 12.01 pm on 12-31-2009 Glenda
Oh, for heaven sakes, all this dicussion about a blue moon. NOW we have been informed and only need to talk about it, ....well, 'Once in a Blue moon' :)
Replyat 4.13 pm on 12-31-2009 Ken Brown
I have a neighbor that swears that the name or term Blue Moon was an accident that happened when the painter of the Blue Boy splatered blue paint on a window. When the painter looked out window through the blue splater he saw a blue moon. He is a conservative. This may explain why he thinks that this is the one and only explanation.
Replyat 4.17 pm on 12-31-2009 helena
I'm so intesting to this article, reading it helped me understand about what is blue moon, opposite with my thought that I had before" blue moon" is the moon has blue color.
Replyat 6.25 pm on 12-31-2009 Ken-Stuart, Fl
Of all I have read about blue moons I find nothing that actually talks about where "blue" came from. I have seen it said it came from "once in a blue moon", but isn't that the chicken and egg sort of thing?
Replyat 10.49 pm on 12-31-2009 Stacy Hebert
I think this is really cool to know that as we will be ringing in the "New Year,"tonight, that there is gonna be a "Blue Moon" out in the sky! This to me is really Awesome and really Romantic...It's just too sad to me that my husband will not be able to be right beside me:-( when the clock turns 12 a.m! Anyway,I will Love and Kiss him in my mind as if he were here! XOXO to you, MY CODY BEAR! I LOVE YOU ALWAYS-STACY "Red" AKA..."MRS.FIRE RED HEBERT!" "HAPPY NEW YEAR"... MY LOVE! "2010!"
Replyat 12.26 am on 01-25-2010 yooki
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."
Where is the note?
Replyat 04.02 am on 01-25-2010 yooki
According to Phillip Hiscock, the term "blue Moon" has been around a long time, well over 400 years. In fact, the very earliest uses of the term were remarkably like saying the Moon is made of green cheese. There are seven meanings for blue moon.
Meanings number one and two: 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.
Meanings number three and four: That's the third meaning ¡ª (by the mid-19th century) the Moon appearing blue in the sky, though rare, did happen from time to time ¡ª 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.
Meaning number five: But meaning is a slippery substance, and a half dozen songs use "blue Moon" as a symbol of sadness and loneliness. That's meaning number five.
Meaning number six: A slinky blue liquid in a cocktail glass, one that requires curaçao, gin, and perhaps a twist of lemon. That's number six.
Meaning number seven: Finally we arrive at the most recent meaning of all, the second full Moon in a month.
In fact, there is another meaning of blue moon: the 3rd of 4 full moons to fall in one season.
Replyat 04.44 am on 01-25-2010 yooki
Bruce,
"The second full moon to fall in a single calendar month is said to be a "blue moon" in the sense that it is relatively rare phenomenon. A blue moon by this definition happens some 8 times every 19 years. An older blue moon definition refers to a different sort of calendar oddity: the 3rd of 4 full moons to fall in one season. Usually, a season only has 3 full moons. But every 19 years, 7 seasons harbor 4 full moons."
In my opinion, strictly, a blue moon by the most recent definition happens 7 times every 19/20 years just as the older definition. There were two full moons in January and two full moons in March and no full moon in February in a double blue moons year. So one of full moons in January or March belongs to February -- that is, there is a fake blue moon in such a year.
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at 10.49 pm on 12-04-2009 Deborah
I found this a very interestng article. I used to think a blue moon was really blue. Thank you for the clarification
Replyat 03.58 am on 12-15-2009 yooki
In my opinion, there are 13 definitions of a blue moon, because there are 13 full moons in a blue moon year. The definition of blue moon as the second full moon in a month was, and still is, not a mistake or wrong!
Replyat 08.02 am on 12-15-2009 Deborah Byrd
Hi Yooki, ha! Yes, I have to smile a little at the people who call using the name Blue Moon for the second full moon in a month "wrong." It's folklore. The folk get to determine what's right and what's wrong!
at 1.15 pm on 12-28-2009 yosiah
I am trying to find a source that defines a "blue moon" prior to the pre-calculated modern calendars that use the term "month" instead of "moon" like all ancient lunar based calendars did. Can anyone help me?
Replyat 11.05 am on 12-29-2009 admin
Yosiah,
If you use a lunar calendar or a lunar/solar calendar whereby the month is based on the phases of the moon and not the calendar month, then you would have no blue moon by the modern definition of the term - the second full moon in a single calendar month. Since a calendar month (except for February) is longer than a lunar month, it is possible for two full moons to fall in a single calendar month. A blue moon by this definition happens about 8 times every 19 years.
Another definition of blue moon - the 3rd of 4 full moons to fall in one season - is the older definition of the term.
Bruce
at 2.24 pm on 12-30-2009 Donna
Since hearing about the blue moon for 12/31/09, i have been reading some explanations of a blue moon. Some i've read said that it is caused by dirt/smoke particles in the air making the moon look blue i.e., volcano eruptions, etc. If this is the fact, then how can the blue moons be predicted in advance not knowing what the dirt/smoke particles would be like at that time? Secondly, the 2nd full moon in a month explanation, is the 2nd full moon always blue?
Replyat 4.01 pm on 12-30-2009 admin
Donna,
The "blue moons" which can be predicted in advance do not refer to the color of the moon, but to calendar oddities. The second full moon to fall in a single calendar month is said to be a "blue moon" in the sense that it is relatively rare phenomenon. A blue moon by this definition happens some 8 times every 19 years.
An older blue moon definition refers to a different sort of calendar oddity: the 3rd of 4 full moons to fall in one season. Usually, a season only has 3 full moons. But every 19 years, 7 seasons harbor 4 full moons. Again, this definition of "blue moon" does not refer to the color of the moon but to a relatively rare calendar oddity.
at 11.39 am on 12-31-2009 farheen
i think thats such an information for young student cause some people cant imagine on solar stars or blue moon . plz search more information about comets,asteroids,and milky ways means glaxies
Replyat 12.01 pm on 12-31-2009 Glenda
Oh, for heaven sakes, all this dicussion about a blue moon. NOW we have been informed and only need to talk about it, ....well, 'Once in a Blue moon' :)
Replyat 4.13 pm on 12-31-2009 Ken Brown
I have a neighbor that swears that the name or term Blue Moon was an accident that happened when the painter of the Blue Boy splatered blue paint on a window. When the painter looked out window through the blue splater he saw a blue moon. He is a conservative. This may explain why he thinks that this is the one and only explanation.
Replyat 4.17 pm on 12-31-2009 helena
I'm so intesting to this article, reading it helped me understand about what is blue moon, opposite with my thought that I had before" blue moon" is the moon has blue color.
Replyat 6.25 pm on 12-31-2009 Ken-Stuart, Fl
Of all I have read about blue moons I find nothing that actually talks about where "blue" came from. I have seen it said it came from "once in a blue moon", but isn't that the chicken and egg sort of thing?
Replyat 10.49 pm on 12-31-2009 Stacy Hebert
I think this is really cool to know that as we will be ringing in the "New Year,"tonight, that there is gonna be a "Blue Moon" out in the sky! This to me is really Awesome and really Romantic...It's just too sad to me that my husband will not be able to be right beside me:-( when the clock turns 12 a.m! Anyway,I will Love and Kiss him in my mind as if he were here! XOXO to you, MY CODY BEAR! I LOVE YOU ALWAYS-STACY "Red" AKA..."MRS.FIRE RED HEBERT!" "HAPPY NEW YEAR"... MY LOVE! "2010!"
Replyat 12.26 am on 01-25-2010 yooki
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."
Where is the note?
at 04.02 am on 01-25-2010 yooki
According to Phillip Hiscock, the term "blue Moon" has been around a long time, well over 400 years. In fact, the very earliest uses of the term were remarkably like saying the Moon is made of green cheese. There are seven meanings for blue moon.
Meanings number one and two: 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.
Meanings number three and four: That's the third meaning ¡ª (by the mid-19th century) the Moon appearing blue in the sky, though rare, did happen from time to time ¡ª 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.
Meaning number five: But meaning is a slippery substance, and a half dozen songs use "blue Moon" as a symbol of sadness and loneliness. That's meaning number five.
Meaning number six: A slinky blue liquid in a cocktail glass, one that requires curaçao, gin, and perhaps a twist of lemon. That's number six.
Meaning number seven: Finally we arrive at the most recent meaning of all, the second full Moon in a month.
In fact, there is another meaning of blue moon: the 3rd of 4 full moons to fall in one season.
at 04.44 am on 01-25-2010 yooki
Bruce,
"The second full moon to fall in a single calendar month is said to be a "blue moon" in the sense that it is relatively rare phenomenon. A blue moon by this definition happens some 8 times every 19 years. An older blue moon definition refers to a different sort of calendar oddity: the 3rd of 4 full moons to fall in one season. Usually, a season only has 3 full moons. But every 19 years, 7 seasons harbor 4 full moons."
In my opinion, strictly, a blue moon by the most recent definition happens 7 times every 19/20 years just as the older definition. There were two full moons in January and two full moons in March and no full moon in February in a double blue moons year. So one of full moons in January or March belongs to February -- that is, there is a fake blue moon in such a year.