Whenever there’s a full Moon, things get pretty exciting at the Farmers’ Almanac office. Our staff and readers alike enjoy all of the Moon’s phases, but we get especially enthusiastic about the folklore surrounding each full Moon. But what if there was no monthly full Moon? In February 2018 that’s exactly what’s happening—we won’t have a full Moon. But to make up for it, January and March will be “double full Moon months,” meaning that during both of these months, there will be two times that the Moon reaches its full phase. When we have two full Moons in a single month the second of these is called a “Blue Moon.”
Is it Rare?
We often describe an unusual event as happening “once in a Blue Moon.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare. But what about a month with no Moon at all? That is a bit more rare than a Blue Moon (which happens on average every 3 1/2 years), and a “No-Moon” month happens about once every 19 years. The last time February didn’t have a full Moon was in 1999 and then again in 1980.
The timing of the full Moon is related to the “Metonic Cycle,” named by the Greek astronomer, Meton, who discovered this phenomenon around 500 B.C. He noted that a given phase of the Moon usually falls on the same date at intervals of 19 years.
Why “Blue” Moon?
Whenever two full Moons appear in a single month, the second is called a “Blue Moon,” however, the Moon will not look “blue” in color. For the longest time, nobody knew exactly where the term “blue” came from. One explanation is that on past occasions, after vast forest fires or major volcanic eruptions, the Moon can take on a bluish hue due to soot and ash particles propelled high into the Earth’s atmosphere (but that should not be the case for this year’s Blue Moons).
Another explanation connects it with the word belewe, from the Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of witnessing only one full Moon month.
However, in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed that perhaps the reason we call the second full Moon in a month a “Blue Moon” is the result of an article published in the March 1946 Sky & Telescope. The article was “Once in a Blue Moon,” written by James Hugh Pruett. Pruett interpreted what he read in a publication known as the Maine Farmers’ Almanac (no relation to this Farmers’ Almanac, first published in Lewiston, Maine in 1955), and declared that a second full Moon in a calendar month be called a “Blue Moon.”
But, after reviewing the Maine Farmer’s Almanac, Hiscock found that editor Henry Porter Trefethen (1932–1957) made occasional reference to a Blue Moon, but derived it from a completely different (and rather convoluted) seasonal rule. According to Trefethen’s almanac, there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, the third full Moon of that season is referred to as a Blue Moon.
A Month With No Full Moon?
This brings us back to February 2018. There doesn’t seem to be name for a month that lacks a full Moon, but the only month where this can happen is in February, the shortest month. The other 11 months must have at least one full Moon.
The reason February is a “No-Moon” month is really quite simple: our Gregorian calendar isn’t synching up with the lunar calendar, which is 29.53 days long. It usually synchs up nicely, giving us at least one full Moon each month, but with February being a short month, sometimes things don’t line up. So, of course there will be a Moon in the sky in February — and it might even appear full — it just won’t reach the moment where it’s 100% astronomically “full” until March 1 (at 7:51 pm EST) on our calendar.
As for the two months with two full Moons this year, there is no real name for when this happens either, but if you read our article on full Moons on page 112 of the 2018 Farmers’ Almanac, you may notice that January and March seem a bit “crazier” than normal.