You knew just what I was there for
You heard me saying a prayer for
Someone I really could care for… -Richard Rodgers & Lorenz Hart
Tonight is the last night of 2009, there’s a full Moon, there’s a sliver of a partial lunar eclipse, and it’s the second full Moon this month.
Q: Does having two full Moons in a month make it a Blue Moon?
A: In our modern times, yes, that’s what we use that colloquialism to mean. The phrase “Blue Moon” is much older than that, dating back to at least 1528, where a pamphlet attacking the dogmatism of the church states:
Yf they say the mone is belewe
We must believe that it is true.
Q: Well, so is it true? Is the Moon belewe blue tonight?
A: No. In fact, if you watch it close to Moonrise or Moonset, you’ll see that it gets progressively redder and redder as it gets closer to the horizon, and whiter as it rises higher in the sky. It will never turn blue.
Q: So how did the expression “Blue Moon” come to mean having two full Moons in one month?
A: Each of the 12 Moons in a typical year had a name, some of which still survive to the modern day! For example, harvest moon, hunter’s moon, wolf moon, and snow moon are names you may have heard of. (You can see a couple of lists here.) But there aren’t 12 full Moons in every year; 12 complete lunar cycles, on average, only take up 354 to 355 days, while a year — at 365.24 days — is substantially longer!
So seven times out of every 19 years, there will be 13 full Moons instead of 12. This means that there will be one season in those years that has four full Moons instead of three; the extra one (which they arbitrarily chose to be the third out of four in a season) was called a blue moon.
Q: So why do we call the second full Moon in a month a Blue Moon? Why don’t we still use the old definition of the third full Moon in those rare seasons with four?
A: Because of a misinterpretation dating to 1946, which brought the idea of two full Moons in a month being a Blue Moon into popular culture.
This leads to something incredibly interesting: once every 19 years or so, you will have two blue moons in a year! How’s this possible? If you have a blue Moon in January — one full Moon at the very beginning and one full Moon at the end — the entire (short) month of February can pass by without having a full Moon at all! This way, there will be a full Moon at the very beginning of March, and another one right at the end! This happened in 1999, and will happen again in 2018, so mark your calendars!
Q: Okay, fine, but can the Moon ever actually appear blue, like that Bob Ross painting above?
A: Yes it can! All you need to do is throw particles of the right size up into the atmosphere! You see, the entire spectrum of visible light comes in different wavelengths.
Violet/blue light has the shortest wavelengths, at about 400 nanometers (nm), while red light has the longest, at about 700 nm. Most atmospheric particles are around (or smaller than) the bluest wavelengths in size, which means that blue light gets scattered away (turning the sky blue during the day), and leaving lighted objects appearing red-colored when they’re lower down on the horizon.
However, certain volcanic eruptions (like Krakatoa or, according to some, Mount St. Helens) will throw up particles with an average size of around 1000 nm, preferentially scattering away the red light (turning the sky red during the day) and leaving lighted objects — like the Moon — appearing blue!
So Moons that are actually blue in color do exist, but that isn’t what the expression “blue moon” means! Actual blue-colored moons are even rarer than two full moons in a month. (The latter happens every two-to-three years, which is still pretty rare!)
I can’t think of a better way to close out 2009, so enjoy your New Year’s Eve celebration! I wish you all a Happy New Year, and I’ll see you in 2010!