How on Earth did we start using “once in a blue moon”?

The moon does turn blue sometimes, but this idiom doesn't always refer to its color.

You’ve heard it, I’ve heard it, but not many people we know have actually seen a blue moon — so what gives?

Am image of the moon captured through a blue filter.
Image credits steviep187 / Flickr.

“Once in a blue moon” refers to events that only happen very rarely, but it’s a tricky idiom. It doesn’t refer to a moon that’s actually blue, although it can appear to be that color under certain conditions and that probably shaped the saying.

A blue moon is a real occurrence and, you might be surprised to hear, isn’t actually that rare or unpredictable. Blue moons are ‘extra’ full moons of the regular gray color that pop up every two or three years due to misalignment in the lunar and solar circle. But the phrase was first used to refer to something being absurd — like someone arguing that the moon is blue.

So let’s take a look at both halves of this idiom and see why they came to represent the quintessential rare occurrence.

The literal blue moon

The moon can naturally appear blue or light-blue in the sky. It’s a rare event caused by the presence of dust or smoke particles in the atmosphere at night which alter the way light is diffracted in the atmosphere. If these particles are of the right size, they can scatter the red part of the light spectrum, leaving the rest untouched.

Because visible light spans from red (low-energy) to blue (high-energy), this scattering makes everything take on a blue tint. Since the moon is a white-ish gray on a dark background, this effect causes it to look blue.

This type of blue moon is probably what spawned the idiom. It’s very rare and very unpredictable, as its appearance relies directly on phenomena such as massive wildfires or volcanic eruptions. The fact that it’s entirely dependent on local phenomena also means blue moons are only visible from relatively small areas at a time, not globally — which compounds their rarity.

Some events that led to blue moons include forest fires in Canada, and the eruptions of Mount St. Helens in 1980 and the El Chichón volcano in Mexico in 1983. The eruption of Krakatoa in 1883 (one of the largest in history) reportedly caused blue moons for nearly two years.

A pretty exciting implication of the mechanism that spawns blue moons is a purple sun. In 1950, as huge fires swept the bogs of Alberta, Canada billowing with smoke, leading to sightings of blue moons from the US to England the following night. Two days later, reports of an indigo sun peering through the smoky skies also started to surface.

So why don’t all volcanic eruptions and wildfires turn the moon blue? Well, the size of ash or oil/tar particles they generate is very important. These have to be wider than the wavelength of red light, which is 0.7 micrometers, to block these rays. At the same time, very few to no particles of smaller sizes should be present, as these would help scatter other colors and destroy the overall effect.

Naturally-occurring ash tends to be a mix of particles of various sizes, with most being smaller than the above threshold. Since smaller particles preferentially scatter (i.e. remove) light towards the high end of the spectrum (blue), natural ash clouds typically give everything a shade of red. Red or blood moons are thus a much more common occurrence than blue moons.

The figurative blue moon

Traditionally a blue moon is an additional full moon that appears every 2 and a half years or so, according to NASA. In recent times it has also come to denote the second full moon to appear within a single calendar month in popular use.

This stems from the way lunar and solar cycles relate to one another. There are 29.5 days between full moons, the agency goes on to explain, so each year will have roughly 12.3 full moons. Another implication of this is that 28-days-long February can’t ever have a blue moon.

Both uses of the phrase are considered valid today.

Over time, the idiom turned from meaning that something is impossible to “never” — think along the lines of “I’ll help you when the pigs fly”.

“The definition of a Blue Moon [as] ‘the second full moon in a calendar month,’ is a curious bit of modern folklore. How it emerged is a long story involving old almanacs, a mistake in Sky and Telescope magazine, and the board game Trivial Pursuit,” wrote Dr. Tony Phillips for NASA.

One of the almanacs Dr. Phillips mentions is the Maine Farmer’s Almanac, more specifically its August 1937 issue. The publication followed certain conventions about how to name each moon depending on the time of year. The first full moon of spring for example was called the Egg Moon, Easter Moon, or Paschal Moon, and had to fall within the week before Easter. If a particular season had four moons, the extra one was called a Blue Moon to maintain the naming conventions.

The definition of the blue moon as being the second full month in a single month came, according to Space, from a mistaken interpretation of the term which was popularized by a nationally syndicated radio program in 1980.

Rarer than Blue Moons are double Blue Moons — when the same calendar year gets two of these events. They’re much rarer, only occurring about 3-5 times every hundred years or so; the next double blue moons are expected in 2037. As for a single blue moon, the next one is expected on October 31, 2020.