A Blue(ish) Moon
Published on May 31, 2007 at 12:02 pm.
6 Comments.
Filed under astronomy, moon, sky lore, skywatching.
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Some people will regard tonight’s Full Moon as a Blue Moon. Others will think of the Full Moon of June 30 as the next Blue Moon. And, some won’t regard either as a Blue Moon. So, just what is going on? And, for that matter, just what is a blue moon?Â
According to popular sky lore here in the US, and now spreading across the world, the second full moon in a calendar month is called a blue moon. The last Full Moon was May 2, 2007, at 10:09 UT. The Full Moon tonight will occur at 01:04 UT on June 1, 2007. The next Full Moon after that will be at 13:49 UT on June 30, 2007. Huh? But didn’t I just say that it was the second Full Moon in a calendar month that was the Blue Moon? Yes, I did. But, wouldn’t that make the June 30 Full Moon the Blue Moon. Yes, it would. If you live in Europe, Asia, or Africa. Note that the Full Moon isn’t just when the Moon looks pretty much fully illuminated. To the naked eye, the Moon appears basically full for two to three days. The actual Full Moon occurs when the Moon is most nearly on the opposite side of the Earth as the Sun. That happens at a well defined moment in time. Hence, I can give the exact time of the Full Moon here. But, note that each of these times has the letters UT after it. That stands for Universal Time, which is the local mean time at 0° longitude. This is also called Greenwich Mean Time for those outside of the astronomical community. Here in Texas, the May 2 Full Moon occured at 5:09 am, and the Full Moon tonight will occur at 8:04 pm on May 31, and so that makes it the second Full Moon of May, and thus the Blue Moon.
But, that isn’t really the full story of the Blue Moon. It turns out that the definition that the blue moon is the second full moon of the month, though popular, originated with a mistake! That definition has only been around for a bit over half a century, and it only has become popular in the last three decades.  So, how did this happen, and what was a blue moon prior to the mistake?
The term “blue moon” has apparently been around for at least hundreds of years. But, saying that something happens “once in a blue moon” used to mean about the same thing as saying that something will happen “when pigs fly.” In other words, the Moon isn’t generally blue, so “once in a blue moon” meant basically “almost never.” Now, in certain rare circumstances, aerosols in the atmosphere can scatter light and make the Moon appear sort of bluish on very rare occasions, but that is unpredictable and exceedingly rare. It normally takes something like a powerful volcanic eruption to do that. And, of course pigs can fly if they are shipped on an airplane.
The “second Full Moon in a month” definition became popular after it was used in 1980 on StarDate, an astronomy radio show that played on PBS radio stations for many years (it has been replace by Earth and Sky). Deborah Byrd, who wrote the script for the show apparently used James Hugh Pruett’s 1946 article in Sky and Telescope as her source for that definition. In Pruett’s article he refers to an earlier Sky and Telescope piece by Lawrence Lafleur from 1943, in which Lafleur discusses the term “blue moon”, referencing a 1937 Maine Farmer’s Almanac that lists blue moons. Unfortunately, in each step down the road from the original sources, errors can creep in. Pruett notes that blue moons come in years in which there are 13 Full Moons in the year. The synodic period of the Moon, the length of time that it takes the phases to go through one complete cycle (for example, the time between one Full Moon and the next) is about 29.5 days. Twelve cycles is 354 days. That is a bit shy of the 365 or 366 days in a calendar year. Most years, you only get twelve full moons, but sometimes you get thirteen. Here is where Pruett made a mistake. Since there are only 12 calendar months, and you need 13 Full Moons in the year to get a blue moon, then he reasoned that means that you’d have to have at least one month with more than one Full Moon. He stated his supposition that it was that extra Full Moon in the month that was the Blue Moon. And that is where the idea got started. But, he didn’t go and look at the Maine Farmer’s Almanac, or else he’d have seen that the Blue Moon listed was not the second Full Moon of it’s month! In fact, very few of the Blue Moon’s listed in the Almanac, going back many years, were the second Full Moon of their month. They were using a more complicated way of doing things.
The term “Blue Moon” was being used as a filler name for an extra Full Moon of a season. By cultural convention, each Full Moon of the year has a name attached to it. For example, the Full Moon closest to the Autumnal Equinox is the Harvest Moon, and the Full Moon closest to the Winter Solstice is the Yule Moon. Each season is about three months long, and so there are normally three Full Moons in each season. When a fourth one happens, then the the sequence of Full Moon names gets messed up. So, the convention that the Maine Farmer’s Almanac used was to call the third Full Moon of each season the “Blue Moon” in order for the full moons to match up with the name that it is supposed to be (the Harvest Moon, the Yule Moon, and others that have specified designations related to the seasons). The most important ones are typically the first or last moons of the season, so letting the third Full Moon of the season be the Blue Moon if it is followed by a fourth Full Moon works. But, Pruett didn’t know that, and he apparently didn’t go check on the Almanac. And even if he had checked the almanac, it isn’t clear that he’d have easily figured out what they were doing. You see, the Maine Farmer’s Almanac computed seasons differently than simply using the actual solstices and equinoxes! Instead, it uses an imaginary Sun that appears to move at constant speed along the ecliptic. This has the advantage of the seasons always starting on the same date, and the seasons always being the same length. Thus, the Vernal Equinox will always occur on March 21 using this system. But, I don’t know how the Maine Farmer’s Almanac came up with this definition of the Blue Moon in the first place. Certainly, it isn’t the first use of the term. But, it does seem to be one of the earlier ways of computing the Blue Moon astronomically.
However, no matter what the original intent of the term “blue moon” may have been, the most common definition is the one that was created by mistake: the blue moon is the second full moon of a calendar month. But, even this definition gives rise to confusion, as I showed in the early part of this posting. For, how do you define “calendar month?” Is it the calendar month where you are, or is it the calendar month at the Prime Meridian? In the former case, this year’s Blue Moon is either this Full Moon or the next depending upon where you live. In the latter case, then it should by rights be the June 30 Full Moon, and not the May 31 Full Moon (which is only a Full Moon for those living a time zone or two west of the Prime Meridian). Oh, and do include Daylight Saving Time (or whatever you call your summer time adjustment where you live if you live somewhere other than in the United States)? Well, that is the sort of thing that happens, I guess, if your definition comes about from happenstance rather than from a well thought out rule.
You can read more about the Blue Moon and how the “second Full Moon in a month” error came about in an article written by Roger Sinnott, Donald Olson, and Richard Feinberg for Sky and Telescope (one of my sources for this posting).Â
-AstroprofÂ
Photo taken, and then color adjusted to appear blue by Astroprof.
Tonight’s Blue Moon on May 31, 2007 at 10:28 pm: 1
[…] That makes it a blue moon. Check out AstroProf’s cool post about the subject, it’s full of lots of good info. Share This […]
A Ler…-- Rastos de Luz on June 1, 2007 at 11:20 am: 2
[…] “A Blue(ish) Moon“, no Astroprof’s Page; […]
Deborah Byrd on June 1, 2007 at 11:17 pm: 3
Hi Astroprof,
Interesting post. The blue moon phenomenon is a very interesting one. Folklore in the making!
All the best,
Deborah Byrd
Earth & Sky
Uncommon Occurrences - Astronomy Cameras Blog on June 5, 2007 at 4:04 am: 4
[…] Astroprof sets the record straight on Blue Moons. He offers a variety of definitions of what a Blue Moon is, when it occurs, as well as a number of misconceptions on what a Blue Moon is not. Furthermore, he looks at the history of the Blue Moon and its development in folklore. […]
Shadow on February 16, 2008 at 4:36 am: 5
Well it’s not a comment more of a question. Why do some full moons last like 2-3day? Is that also called a blue moon?
Astroprof on February 16, 2008 at 1:04 pm: 6
Astronomically, the Full Moon occurs at one moment in time, when the Moon is opposite the Earth from the Sun. The Moon, though, can look pretty full to the untrained eye for two to three days, though.