Blue Moon…Is It Real?

What is a Blue Moon?

I am pretty sure you’ve heard the phrase, “until a blue moon” or “once a blue moon”. It  developed in the 19th century, meaning ‘never’, or at least ‘extremely unlikely’. It’s been now and then used in titles of many songs and UF novels among other things because let’s face it, the idea is visually attractive. Is a blue moon just a  fiction or a metaphor? You might be surprised but, after all, blue moons do occur still the whole matter is a bit tricky.

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). The full Moon on Aug. 31, 2012, was an example of this type of blue moon – it was the second full moon in one month.

The Other Kind of Blue Moon

The older definition, which is recorded in early issues of the Maine Farmer’s Almanac, states that 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.

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.

Some years have an extra full moon—13 instead of 12. 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 13th moon skewed the calendar, since there were names for only 12 moons. By identifying the extra, 13th moon as a blue moon, the ecclesiastical calendar was able to stay on track.

How Often Does a Blue Moon Occur?

Over the next 20 years there will be about 15 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 2014, and 2017.
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 in May 2008 and  on Nov. 21, 2010. Since this type of blue moon is reckoned according to the seasons, it can only happen in certain months – February, May, August, or November, about a month before the equinox or the solstice.

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.

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.
(Information Please® Database, © 2007 Pearson Education, Inc. All rights reserved.)

Can the Moon really turn blue?

Yes it can turn blue, in the eye of the beholder of course, but not everywhere and it can be seldom witnessed.

The key to a literal blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)–and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires.

For example in 1883, an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth’s atmosphere and it was the reason why the moon temporarily turned blue. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide – the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

So, will you ever see a blue moon? In astronomical terms, it is quite possible. If you hope to see a full moon which is the actual color blue, that is less likely unless you live in a country with an active volcano or with a forest fire season.

(Info from Nick Greene at

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6 Responses to Blue Moon…Is It Real?

  1. Blodeuedd says:

    No real blue moon? That makes me sad :/

  2. Reality is so disappointing sometimes 😉

  3. Aurian says:

    Very interesting post, thank you Bridget.

  4. As always you are welcome Aurian!

  5. red witch says:

    Very nicely done. I was going to mention the Krakatoa incident but you covered that too. 🙂

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