Once in a Blue Moon

Look to the night sky this Thursday and you’ll see what is referred to as a “Blue Moon”. No, it doesn't mean the moon is actually blue, if that's what you were wondering. As we had a Full Moon on August 1st already, this second Full Moon within the same calendar month is called a Blue Moon, just simply because of its rare occurrence. The next time we see this phenomenon is in May 2026.

A full Lunar cycle is 29.53 days, where we go from no Moon visible in the sky (New Moon) through the waxing phases, to 100% illumination (Full Moon) back to New Moon through the waning phases. "Waxing" meaning getting bigger, and "waning" getting smaller. As 29.53 days is relatively close to the lengths of our calendar months, we usually only have one Full Moon each month.

Our calendar is based off our motion around the sun though, not phases of the moon, hence why the periods don't fully match up. As a result, every 2.7 years two Full Moons are squeezed into one calendar month.

February, being the shortest month, can never have two Full Moons. Sometimes February doesn't even have a Full Moon at all. The last time that happened was in 2018, and the next February without a Full Moon is not lined up until 2037.

Everyone will have heard of the phrase "once in a Blue Moon", meaning very rarely, but how many people know what it actually means? Since 1819, The Maine Farmers' Almanac has listed the dates of forthcoming blue moons. The compilers of the almanac had their own definition of what blue moons are. It wasn't when there was a 100% illumination twice a month, but four times rather than the usual three Full Moons per season.

Amateur Astrologer James Pruett read an edition of the Maine Farmers' Almanac, but misinterpreted the system and printed the 'second full moon in a month' version in a 1946 edition of the Sky & Telescope Magazine. For some reason, probably because it ended up being printed as an answer in an early version of Trivial Pursuit, Pruett's version has gained currency and has become the widely accepted definition of "Once in a Blue Moon".

On a separate note, on very rare occasions, the moon actually can appear blue. If there’s been a recent forest fire or volcanic eruption that pumped significant smoke or ash into the upper atmosphere, it is possible for the moon to take on a bluish hue.