On 31 August 2023 we’re going to have a blue supermoon, which will be neither particularly blue, nor particularly super, though to read some of the media coverage of these events, you might expect to see something like the image above. So I thought I might write a bit about blue moons (this post) and supermoons (to follow).
So: a blue moon, in current usage, is the second of two full moons falling in the same calendar month—the full moon of 31 August is a blue moon because there has already been a full moon on 1 August.
The phrase once in a blue moon, meaning “very occasionally”, has been around since the nineteenth century. The current astronomical usage is more recent—it originated in 1946, with a notorious error in a Sky and Telescope article, and became popular only during the 1980s, when it was included as a question in the game of Trivial Pursuit. (I’ll write a little more about all that at the end of this post, if you’re still with me.)
To show how blue moons work, I need to introduce the idea of a lunation, which (for our purposes) is the time between two successive full moons. Although the moon looks full to a casual observer for a couple of nights each month, in astronomical terms there is a precise time at which the full moon occurs—the moment at which it’s on exactly the opposite side of the sky from the sun. The time between two of those full moons averages around 29.53 days, but can vary by six or so hours in either direction, as the moon responds to the gravitational tug-of-war between the Earth and the sun. And, given that the astronomical timing is precise, I need to specify the time zone I’m using in what follows—a full moon that occurs before midnight in one part of the world can easily occur after midnight (and perhaps therefore in a different month) somewhere else. But it seems to be customary to figure the blue moon calendar according to Greenwich Mean Time, or its astronomical equivalent, Coordinated Universal Time.
So there’s room, in any calendar month of 30 or 31 days, for an entire lunation to take place. If there’s a full moon very early in the month, there can be another at the end of the month. For a 31-day month, there’s a window of about 1½ days at the start, during which a full moon can occur while still leaving room for another one in the same month, 29½ days later. For a 30-day month, the window is just half a day—meaning that blue moons will be about three times more common in 31-day months than in 30-day months. (When Steve Holmes of the British Astronomical Association crunched the numbers, taking into account the variable duration of a lunation, he determined that any given 31-daymonth has a 1:250 chance of hosting a blue moon; for 30-day months it’s just 1:835.) But February, even in a leap year, is too short to accommodate even the shortest lunation, and is therefore the only month that never has a blue moon.
A “normal” year plays host to twelve full moons. But twelve lunations add up to only 354.36 days, which is almost eleven days less than an average calendar year of 365.25 years, and those “missing” eleven days have a couple of consequences.
Firstly, the date of the full moon in any given month will drift earlier in the month, by about eleven days, with each successive year. The first full moon of 2021 came on 28 January; in 2022 it fell on 17 January; in 2023 on 6 January. In 2024 it will leap back towards the end of the month, 25 January—which we can think of as the 5 February full moon of 2023 coming eleven days earlier in 2024, as a January full moon.
Secondly, if a full moon falls in the first eleven (or so) days of January, that allows room in the calendar year for a thirteenth full moon before the end of December. Thirteen full moons in a twelve-month year means there will be a blue moon in some month during such a year. So the late-January full moons of 2021 and 2022 “prevented” a blue moon; whereas the early January full moon of 2023 provided the opportunity for the blue moon of August. But there will be no blue moon in 2024, when the January full moon skips back towards the end of the month.
It seems like there’s a cycle here, with 13-moon years interspersed with 12-moon years. How often do we get a blue-moon year? We can make a rough-and-ready estimate by realizing that there are eleven days at the start of January during which a full moon will be associated with a blue-moon year, and twenty days in the rest of January when a full moon will prohibit a blue-moon year. So that suggests that a blue moon will appear in 11/31 of years, or about 35%. (Steve Holmes’s detailed calculations came up with a figure of 36.3%, so my quick estimate turns out not to be too shabby.)
I can put all this together in a chart:
The years run from 2011 to 2030, from top to bottom. The grey dots marking full moons form up into long diagonals, sweeping down and left through the years, illustrating the 11-day-per-year mismatch between twelve lunations and the calendar year. Blue moon are marked with larger, blue dots. Notice the interesting effect in 2018, when we had two blue moons—an early first full moon in January left room for a blue moon at the end of that month, which was so close to the end of the month that the lunation skipped February entirely and put the next full moon in early March, leaving room for another blue moon.
In blue-moon years, the earlier the first full moon of the year occurs, the earlier the month of the blue moon tends to be. 2018 had its first full moon on 2 January, giving us the January/March blue-moon pair. The first full moon of 2026 falls on 3 January, and brings a May blue moon. Whereas the first full moon of 2028 is on 12 January, pushing the blue moon to 31 December. (And if 2028 wasn’t a leap year, there wouldn’t be room for a blue moon at all.)
Another example of this “leap year effect” occurred in 2012, when the extra day in February moved the blue moon by two months from where it “would otherwise” have been. If the 9 January full moon of 2012 had happened in a non-leap year we’d have seen a full moon on 1 September instead of the blue moon of 31 August, then a full moon on the first day of October (rather than the last day of September), turning the full moon at the end of October into a blue moon.
So there’s a lot to see in my little twenty-year chart. I’ll just point out one final thing—take a look at the positions of the full moons in the top and bottom rows of the chart. They look pretty much the same! This approximate return of lunar phases to the same dates after nineteen years is called the Metonic Cycle. It turns out that 235 lunations are very similar in duration to 19 tropical years—years as measured by the turn of the seasons, which is what our modern Western calendar approximates with its occasional leap years. For instance, if you care to use my little chart to count off 235 full moons after the first full moon of 2011, which fell on 19 January, you’ll find yourself, 19 years later, at the first full moon of 2030, also on 19 January.
To fit 235 lunations into 19 years, we need to have twelve normal years and seven blue-moon years, since (12×12) + (7×13) = 235. Take a look at my chart again and, sure enough, you’ll find seven blue-moon years. (2018 contains two blue moons, to be sure, but the missing full moon in February ensures that the full-moon count is just thirteen.) And 7/19 = 36.8%, another close approximation to Steve Holmes’s more exact calculated figure, above.
The Metonic Cycle gets its name from Meton of Athens, a Greek mathematician of the 5th century BCE, who incorporated the cycle into a lunisolar calendar—that is, a calendar that tracks both the seasons and the lunar phases. People all over the world have used this sort of calendar, counting off the full moons as a guide to how the seasons are progressing, with three full moons in each “normal” season, and a four-moon season cropping up in seven years out of nineteen.
Well into the twentieth century, there were farmers who would keep an eye on the lunar cycle as a guide to their seasonal activities. In the United States they were aided in this practice by the yearly publication of various “farmers’ almanacs”. And one of those, the Maine Farmers’ Almanac, seems to have originated the idea of applying the name “blue moon” to the extra full moon in a four-moon season. So every year the almanac would publish a list of dates of full moons, together with seasonal names from Christian European tradition like Lenten Moon and Moon Before Yule, interspersed with the occasional Blue Moon, but without an explanation of why specific full moons had been chosen to be blue moons.
In 1946, an amateur astronomer called James Pruett tried to figure out the blue-moon rule used by the Maine Farmers’ Almanac, and in a Sky & Telescope article entitled “Once In A Blue Moon” he described the familiar Metonic Cycle:
Seven times in 19 years there were—and still are—13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.
Sky & Telescope went on to use Pruett’s definition, and the rest is history.
But in 2006, Daniel Olson, Richard Tresch Fienberg and Roger Sinnott pored over a lot of copies of the Maine Farmers’ Almanac, and figured out the underlying blue-moon rule that had actually been used, which is very different from Pruett’s version:
At last we have the “Maine rule” for Blue Moons: Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.
Why is the third full Moon identified as the extra one in a season with four? Because only then will the names of the other full Moons, such as the Moon Before Yule and the Moon After Yule, fall at the proper times relative to the solstices and equinoxes.
You can read their full Sky and Telescope article, with much more detail, here.