Blue Moon (Full Moon May 2008)

Finally, we come to the infamous Blue Moon: the subject of songs, sayings and speculations … or maybe we don’t. It all falls back to a 1946 publishing error.

According to the Farmer’s Almanac article “What is a ‘Blue Moon’?“:

For more than half a century, whenever two full Moons appeared in a single month (which happens on average every 2 1/2 to 3 years), the second has been christened a “Blue Moon.” In our lexicon, we describe an unusual event as happening “Once in a Blue Moon.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.

But according to Sky & Telescope magazine, “The trendy definition of “blue Moon” as the second full Moon in a month is a mistake.” And moreover, the mistake started in their magazine. (Read the full article for a full explanation of how this mistake came about.)

Before 1946, almanacs used a completely different system for determining whether to call a moon “blue” or not. some folks (like myself) still use this system today. As I explained in Full Moon Names: Rewilding your calendar, you basically have two systems to choose from for determining how to name the full moons: the monthly method and the seasonal method. The monthly method gives a moon its name based on what month it falls in. The seasonal method gives a moon its name based on the order it appears in (1st, 2nd or last) during the season.

Each method practically begs for its own definition of determining whether to call a moon “blue” or not. If each month has one moon and the moons name derives from that month (as in the monthly method), then what do you do on the rare occasion when you have two moons in a month? Likewise, in the seasonal method, what do you do when you have more than three moons in a season? In each method, you would call that extra moon “blue”. To make it a little more convoluted, the seasonal method of naming moons depends on the moon’s order during the season: First, Second or Last. (Notice that I didn’t say “third”.) Thus, the extra moon in season gets inserted between the second and last moon.

Since our spring season started on the Spring Equinox (March 20th) and the first full moon of the season occurred the next day, we had plenty of time to fit four full moons into this season before the Summer Solstice rolls around on June 21st. That makes this month’s moon the third of four and therefore a Blue Moon according to this method. Furthermore, it pushes the moon names back into alignment between the two methods. For the rest of the year, the seasonal method and the monthly method of naming the moons will not differ, thanks to this extra moon.

Speaking of the monthly method, this month’s moon goes by the name of the Full Flower Moon. As the Farmer’s Almanac points out:

In most areas, flowers are abundant everywhere during this time. Thus, the name of this Moon. Other names include the Full Corn Planting Moon, or the Milk Moon.

And speaking of the Farmer’s Almanac, click here to see their video about Blue Moons.