Once In A Blue Moon

We come at last to the interpretation of the phrase "once in a blue moon." Often the intended meaning on the part of the speaker as the words are uttered is "seldom, if ever." But it happens that the saying has a long and mixed up history.

Over just the past few decades the astronomically defined meaning of a "blue moon" has altered, due to a mistaken belief about previous usage. This new meaning was based on the second occurrence of a full moon within one calendar month. Because there are about 29.5 days between two lunar oppositions, in a calendar month with 30 or (much better) 31 days there is a small chance that two full moons will occur. The second of these full moons started to be referenced as being a "blue moon" only during the past few decades. That is, it's a new piece of folklore. My lengthy discussion in the Preface was based on that modern meaning. I won't repeat the details here.

It is the earlier usage of the term, which is a little more com plicated, that we need to clarify. As I wrote at the head of this chapter, this has a New England connection. For many years the Maine Farmers' Almanac would indicate the full moons that were to be regarded as being "blue," and the rule had nothing to do with calendar months. Unfortunately, times change and during the 1940s the interpretation of the blue moon rules got a little confused. Let us look further back.

The Church has labels that it attaches to each of Sundays closest to the full moons in a year, because all the moveable feasts are phased against Easter. Similarly, farmers' activity during the year can be somewhat affected by the seasonal dates of full moons. Before the advent of artificial lights on gigantic combine harvesters, having a full moon at the time that the crops were ready to be brought in out of the fields was a huge boon. Therefore it is not surprising that each full moon was given a name. These are:

Spring season: Egg (or Easter) Moon, Milk Moon, Flower Moon Summer season: Hay Moon, Grain Moon, Fruit Moon Fall season: Harvest Moon, Hunter's Moon, Moon preceding Yule Winter season: Moon following Yule, Wolf Moon, Lenten Moon

But how were those seasons defined? Some people think of the seasons beginning with the first day of a calendar month, such that spring begins on March 1. Others (like astronomers) may insist that the equinoxes and solstices mark the season starts, so that spring begins with the vernal or spring equinox around March 20. This leads to seasons that are of differing lengths, because the Earth's speed in its orbit varies during the year.

The farmers' seasons, however, are defined in another way again. In essence the year is split into four equal seasons, each lasting 91 days plus a bit. That's a fairly straightforward way to do things. Three lunar months (synodic months) last for a total of 88.6 days, indicating that although three would be the norm, a fourth full moon could fall within one of those farmers' seasons. To look at it another way, each calendar year contains at least 12 full moons, but in about one year in three there is a 13 th full moon. That means that there is one more to be inserted, beyond the dozen named full moons listed above. And that one is called— you've guessed it—the blue moon. In a season with four full moons it is the third of them that is termed the blue moon, according to these rules.

As a result, the blue moon can only occur in February, May, August, and November: that is, close to one lunar month before the next equinox or solstice, although those points are defined slightly differently in this scheme of equal length farmers' seasons. Consequently the blue moon by this definition can only occur around the 21st or 22nd day of one of those calendar months, and never on the 30th/31st as happens according to the recently evolved version of the meaning of "blue moon."

By this original rule, the next blue moons will occur in November 2002, August 2005, May 2008, and November 2010. There are gaps between two or three years, then, which gives you another handle on what "once in a blue moon" may be taken to imply.

I wrote that this is the "original rule," but in fact it is not so ancient in itself. Tracing through such volumes as the Maine Farmers' Almanac indicates that it sprang into usage in the agricultural community of New England around the middle of the nineteenth century. The couplet with which this chapter began is around three times as old as that, dating back to before Shakespeare's era. If we are to ask, "why blue?" then we need to go back a long way.

The answer to that query is that no one really knows. People may have started saying that "the mone is blewe" in 1528, but the following year a similar phrase appeared, with a different color involved: "They woulde make men beleve that ye Moone is made of grene cheese." Of course everyone knew that the Moon is not made of green cheese. The literal meaning of this piece of doggerel is similar to saying that someone would argue that black is white.

The origin of the blue moon pairing of words is the same, a straightforward example of something that is known not to be true. Yes, under certain atmospheric conditions the Moon in the sky may attain a somewhat bluish tinge, but that is irrelevant. "Once in a blue moon" is a common phrase with ancient roots, and its interpretation in terms of astronomical phenomena has changed over the last century or so. In effect, though, it still means once in two or three years, about the same frequency with which a total solar eclipse occurs in some accessible part of the globe, in fact.

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