The Moon will be full tonight for the second time this month. It last was full on May 2. That relatively rare occurrence has been defined as a blue Moon. However, it’s actually not an official blue Moon. A mistake by an amateur astronomer who wrote an article for “Sky and Telescope” magazine in 1946 led to the misnomer. A blue Moon, as originally described in old issues of the “Maine Farmers Almanac,” is the third full Moon during a season that experiences four full Moons.
Although the mistake occurred more than six decades ago, the magazine didn’t discover the error until eight years ago. It tracked down multiple copies of the old almanacs and ferreted out the true pattern of blue Moons:
Although the idea of a seasonal pattern suggested itself to us immediately, verifying the details required a lot of detective work. We found that the Blue-Moon definition employed in the Maine Farmers’ Almanac is indeed based on the seasons, but with some subtle twists.
Instead of the calendar year running from January 1st through December 31st, the almanac relies on the tropical year, defined as extending from one winter solstice (“Yule”) to the next.
However, the term and its incorrect definition already have entered our lexicon. As “Sky and Telescope” noted, at least one dictionary defines blue Moon as the second full Moon in a month.
For a closer look at the Moon, try Google Moon. Make sure you zoom in all the way to learn about the curious substance that forms Earth’s only satellite. What does that look like to you?