The aphorism “once in a blue moon” speaks about the irregularity of the blue moon’s appearance. But what exactly is a blue moon?
Ordinarily one full moon occurs in the span of a month. Or, if one looks at the full year and divides it into quarters to correspond with the seasons – with three months for each season – then there would typically be three full moons every season.
However, the orbital period the moon takes to revolve around the Earth spans roughly 27 days. In other words, there will come a time when two full moons will occur during a calendar month (or four full moons would occur in a season). In contemporary times, the second full moon of a calendar month has thereby been termed the blue moon.
And yet the archival records show a slightly different understanding of the term ‘blue moon.’ Historically, the Farmer’s Almanac – first published in the United States back in 1792 – was utilized by early Americans as a reference guide about astronomical events, tides, and seasonal weather. This resource collated traditional names for the different types of moons, with each moon having its special name – like Hunter’s Moon and Harvest Moon. Almanac compilers of old were well aware that the lunar cycle did not always coincide with a 12-full-moon year. Rather, there were occasions when a year would have 13 full moons. Hence, one season would have four full moons instead of the customary three. The extra full moon had to be given a new name. By default, the third full moon of a four-full-moon season was traditionally called a ‘blue moon.’
Skywatchers who expect to see the moon appear blue might be disappointed, since the moon will be its normal hue. So why call it blue?
Etymologists believe that the moon’s association with the color blue first appeared in a 1528 couplet penned by William Barlow, the Bishop of Chichester. Barlow’s use of the ‘blue moon’ expression, however, held the medieval connotation of impossibility:
Yf they say the mone is blewe,
We must believe that it is true.
Over the course of several centuries the meaning changed to signify the rarity of an event. The earliest example of the modified connotation surfaced in 1821 when Pierce Egan penned Real Life in London, wherein he wrote:
How’s Harry and Ben? – haven’t seen you this blue moon.
But why was the moon associated with the color blue, even for those who lived over 400 years ago? The year 1883 likely provides a clue, for that was the year the moon did in fact look blue to viewers around the globe – thanks to the dust in the atmosphere from the Krakatoa volcano’s eruption.
It has therefore been surmised that atmospheric dust – whether from volcanic eruption or even forest fires – likely gave the illusion of a bluish moon. Yet such an appearance by the moon would only happen from time to time. Thus, the ‘blue moon’ idiom came to signify an event that takes place once in a long while.
In any event, a blue moon will grace our skies on July 31st. Many are excited to see it because the blue moon is associated with folkloric superstitions. For instance, gathering flowers and fruits by the light of a blue moon gives bountiful promise. Likewise, beginning any endeavor towards a well-intentioned goal will prove prosperous, when commenced under a blue moon.
Then, too, some even believe true love will be revealed during a blue moon. That’s because the color blue is sometimes associated with love. After all, a bride is encouraged to wear something blue on her wedding day. How then did the color blue become symbolic of love? The reason may stem from the etymology of the term ‘true blue’ – which has roots that date back to the late 1660s and is associated with unwavering loyalty and fidelity.
So, if you’re planning to catch this year’s celestial event, keep your eyes peeled for moonrise this Friday, July 31st. While you may not see a blue-colored moon, you still are in good standing to see July’s second full moon. Perhaps by its light you can fortuitously begin a new venture, or even be blessed with love’s fidelity, or at least become a budding astronomy buff. In either case, this year’s blue moon is sure to capture the public’s imagination.
If you miss the blue moon this Friday, July 31st, then you’ll have to wait another three years, until January 2018, for the next one.