Dang da Dang Dang
I keep seeing stuff about the “blue moon” today and how it “coincides with Neil Armstrong‘s funeral.”
Someone is WRONG on the internet, and I’m here to set the record straight.
The thing that’s wrong is the idea that a “blue moon” is any second full moon in a calendar month. It’s not. That’s the result of an error printed in an old magazine (“Sky and Telescope” if I recall correctly) from the mid-1940s.
To fully explain this, though, there’s a bit of background. First, the calendar we use today, officially called the “Gregorian” calendar after some monk who revised the Julian calendar (named after Caesar). For our purposes, the differences between Gregorian and Julian don’t really matter much; all that matters is the knowledge that our calendar is a solar calendar that long ago divorced itself from the natural cycles of the moon. Now, there are good reasons to use a solar calendar, but lunar cycles and solar cycles just don’t coincide; therefore, what we call a “month” isn’t truly a “month” (same root as the word “moon”) at all but an arbitrary number of solar days that’s usually a day or three longer than a lunar month, which I’ll call a “lunation” to avoid confusion.
So what happens is a situation where these mostly-arbitrary collections of days, called “months,” can easily hold two full moons (or, conversely, two new moons, or two whatever-phase-you-want-to-name moons). But the important point is that the number of days in a month is artificial, while the number of days in a lunation is set by the orbital periods of the moon around Earth and Earth around the sun.
Now, the second part of the background here is that back in the old days, before neon lights and such, people named each full moon based on the season in which it appeared. Most seasons contained three full moons, and the name of the full moon corresponded to its position in the season. These names varied culturally, so you get several different names for the full moon. The only one anyone ever talks about anymore is the Harvest Moon, which is the first full moon of the fall season. People would plant and reap and such based on these moons – not the best system, of course, because once the season changes you have about 29 days in which that first full moon can occur, but since the moon provided extra light at night, these things were important.
And the final bit of background, which most people already know: the seasons themselves are defined by solstices and equinoxes – again, natural cycles, cycles which even early societies could compute to a high degree of accuracy. Essentially, a solstice represents the sun’s minimum and maximum azimuth throughout the year at solar noon, and an equinox is when the sun’s path appears to cross the equator going from north to south or vice-versa. Solstice and equinox days are mostly a function of the Earth’s axial tilt, influenced to some degree by the eccentricity of its elliptical orbit.
Because of these last two bits of background, some rare seasons contain four, not three, full moons – because each season is a bit more than 91 days, you could have a full moon just after a solstice, for example, and then another one just before the equinox. Or vice-versa.
Which brings me to the original definition of “Blue Moon,” which the Wikipedia article of that name sometimes gets wrong:
A blue moon is the third full moon in a season containing four full moons.
(When I started writing this, Wiki said it was the fourth full moon in such a season; it seems to have been corrected again now. As we all know, Wikipedia has its uses and its limitations. Here’s the link to the page so you can see for yourself.)
So, for example, between the summer solstice and the fall equinox, normally you have the Hay Moon, the Grain Moon, and the Fruit Moon (to use the English names listed on this Wikipedia page). But sometimes the Hay Moon falls just after the summer solstice, and there’s a fourth full moon just before the fall equinox. In such a season, the fourth full moon becomes the Fruit Moon, and the one before it, in August, will be called the Blue Moon. It happens only rarely, hence the expression, “once in a blue moon.”
That is exactly what’s going to happen next year, 2013: August will have a Blue Moon.
August does NOT have a Blue Moon this year.
Why is it important to point out this misinformation? After all, now you have the usual trolls insisting that the “twice in a calendar month” definition is the One True Definition, just to annoy purists like me – in much the same way that I sometimes use the Comic Sans font to annoy graphics purists. Well, it’s important because as I mentioned, the calendar we use is highly arbitrary. Using the “third full moon in a season containing four full moons” definition separates “blue moon” from the calendar, so it would be true whether using Julian, Gregorian, or one of the various lunar calendars in existence today – or some new calendar we haven’t used yet.
My personal favorite idea for calendar reform, incidentally, is the Tranquility Calendar, because I maintain that the single most significant event in human history is the moment people first visited a world other than our own, so it should be the beginning of a new Earthly calendar.
And that brings us full circle, as is appropriate for a post on the calendar.