Many people are aware that the full moons have names. But where did these names come from? What do they mean? Just who would bother to name a moon? And not one, but 12 names - all for the same moon? What craziness is this?

Many cultures have named the full moons over the course of history, but it seems the most prominent set of names originates with the Native American Algonquin tribes of the northeast US. The Algonquins had a name, indeed sometimes several, for each recurring full moon. The names reflected the seasonal cycle of the year, and the entire month in which any given moon appeared was known by the name of the moon.

While the Algonquins inhabited what is now the northeast US, hundreds of other tribes and nations were spread across the rest of North America. Many Native Americans used a system of naming moons; some used the same names but applied them to different months. Over time though, it is mostly the Algonquin names that persevere, and these are known across the US and parts of Europe.

In an average year there is one full moon per calendar month. In order, they are:

1. The Wolf Moon in January, so called for the cries of hungry wolves that would prowl the edges of a tribe's territory in the winter snows. Sometimes the moon in January is called the Old Moon, the Ice Moon, or the Moon after Yule.
2. The Snow Moon is in February when the heaviest snow usually falls. This might also be called the Full Hunger Moon or the Little Famine Moon because of the difficulty of hunting at this time of year.
3. The Worm Moon marks the time when the worms begin to appear during the thaw in March, heralding springtime. This can also be the Crust Moon for the crust that forms over the snow when it melts by day then refreezes overnight, or the Windy Moon. Colonial settlers called March's moon the Lenten Moon because it falls during Lent. It is the last full moon of winter.
4. The Pink Moon in April acknowledges the first early blooms of spring. Some coastal tribes called this the Fish Moon.
5. The Flower Moon occurs in May due to the abundance of flowers. This is sometimes called the Milk Moon or the Rose Moon, but depending on the source, the Rose Moon can also be in June or July.
6. The Strawberry Moon coincides with the relatively brief strawberry season in June. The name Strawberry Moon is more or less universal among the tribes. The Honey Moon ('keep a-shining in June') is celebrated in the words of the song 'By the Light of the Silvery Moon' and probably has connections to honeymooning couples after a June wedding.
7. The Buck Moon in July denotes the time when new antlers appear on the deer. July's moon might also be called the Hay Moon, the Crane Moon or the Summer Moon.
8. The Sturgeon Moon in August is so named for the plentiful sturgeon to be fished in the Great Lakes. Sometimes this is called the Red Moon for the appearance of the rising moon through heat haze, or the Fruit Moon.
9. The Harvest Moon is, of course, the one everyone has heard of. While most people expect it to occur in October, and by some accounts it does, it is most commonly the September moon that is called Harvest. The Full Harvest Moon allows farmers to work the fields late into the night bringing in crops.
10. The Hunter's Moon rises in October when, after the fields have been reaped, hunters can easily spot foxes and other prey. October's is sometimes called the Blood Moon, which has nothing to do with vampires or Hallowe'en.
11. The Beaver Moon in November either refers to the time to trap beavers to stock up on furs for winter, or the time that the beavers are preparing themselves for winter. The November moon might also be called the Frosty Moon, the Trading Moon or the Sassafras1 Moon.
12. The Cold Moon of December acknowledges that it isn't even remotely warm anymore. This can also be known as the Long Nights Moon, either because the winter solstice is the longest night of the year, or because this moon has more hang time than any of the other full moons. Settlers called this the Moon before Yule.

On average a 'blue moon' occurs once every two-and-a-half to three years. Very rarely a blue moon will occur twice in a calendar year; in these years February will have no full moon at all - this is because the average month has 30.5 days, and the lunar cycle is only about 29.5 days.

A blue moon is the second full moon in any calendar month ... sort of. That's currently the widely accepted definition, but it is derived from a mistake in interpreting an older definition of a blue moon.

The formula for calculating the older blue moon was derived from using a calendar with fixed seasons of equal length as opposed to seasons that began based on the position of the sun: from winter solstice to winter solstice. In any season with four full moons, it was the third that was labelled the blue moon so as not to disrupt the designations of the other full moons.

The lunar blue moon was defined in the Maine Farmer's Almanac2 in 1937 as being the third full moon in a seasonal quarter that contained four full moons. Since it was the third full moon in a quarter that was 'blue', this blue moon never fell as the second blue moon in a month, but always on the 21, 22 or 23 of February, May, August or December. The Maine Farmer's Almanac identified more than a dozen such blue moons between the 1820s and the 1960s.

The modern blue moon is defined based on a misinterpretation of the original rules, publicised in a magazine article in 1940. Ironically the magazine, Sky and Telescope, was referencing its own earlier article as the source in the story.

The term 'blue moon' has been a part of the English language for centuries, but its meaning has changed several times over the years. Earliest references to a 'blue moon' indicate something off-the-wall or absurd: saying the moon was blue was akin to saying it was made of green cheese. From there it evolved to a phrase meaning never, something like 'when the moon turns blue' would translate to something similar, eg 'when pigs fly' or 'when Hell freezes over'.

Since it has been documented that the moon has, on occasion, actually turned blue, at times 'blue moon' means just that. Atmospheric conditions can result in the moon appearing blue, but this is an optical illusion. Most often this would be caused by ash from a volcanic eruption, or a forest or brush fire. Forest fires can be somewhat rare, volcanic eruptions even more so, and this is probably where the modern meaning behind the phrase 'once in a blue moon' came from, to mean something that happens rarely, or very infrequently.

Musically, 'blue moon' denotes sadness or loneliness, and can be found in at least half a dozen instances, eg 'Blue Moon of Kentucky' sung by Elvis Presley, and 'Blue Moon' (... you saw me standing alone ...) which has been covered by many artists and is a popular instrumental.

The most recent meaning of blue moon is that of the second full moon in a month. Although, as noted above, this originated from the error printed in a 1940 magazine article, this meaning didn't become widely acknowledged until sometime in the 1980s when it was published in a children's fact book, then consequently used as a question in the original Trivial Pursuit game. It's also a staple question in pub quizzes.