We’ve all heard about it, but the meaning of the ‘once in a blue moon’ idiom is more elusive than you may think — so much so that researchers are still debating it to this day.

“Once in a blue moon” refers to events that only happen very rarely, at least in the current form of the idiom. It doesn’t refer to a moon that’s actually blue, although as we’ll see shortly, the moon can also appear blue under certain conditions and that probably also shaped the saying or cemented it into popular consciousness.

A blue moon is a real occurrence and, you might be surprised to hear, isn’t actually that all that rare or unpredictable. Technically speaking, 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 cycles. The lunar cycle varies between a full moon and a new moon. The lunar cycle is 29.5 days, but the month is a bit longer. So instead of having 12 full moons, some years have 13 full moons. This typically happens every couple of years.

But the phrase was first used in different ways, sometimes 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 doesn’t emit light. It’s not red, it’s not blue, it’s not white. The moon reflects solar light, and in order for us to see it, that light passes through the Earth’s atmosphere. This interaction can make it appear as if the moon was colored.

Sometimes, 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 appears as a whiteish gray on a dark background, this effect causes it to look blue.

This type of blue moon is possibly 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, and the impact that this had on the population at the time must have been significant.

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

There are other events linked to the emergence of this idiom. In 1528, two friars from England published an anti-clerical pamphlet, writing: “O churche men are wyly foxes […] Yf they say the mone is blewe / We must beleve that it is true /“

This is the first time the use of the term “blue moon” was used, but it’s not clear if the idiom sprung from this or emerged separately.

At any rate, 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”.

As it’s so often the case with sayings, their origins are blurred through history, but by the 20th century, the “once in a blue moon” idiom had become established in the English language.

“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, they usually pop up commonly (there were two in 2018 and two in 2020), but the next one won’t be for a few more years, in 2028.