Laura Dean Bennett
When you talk in astronomical terms about the moon, you’re heading for a morass of confusion.
As Shakespeare gave Juliet to say in the famous balcony scene in Romeo and Juliet, “O, swear not by the moon, the inconstant moon, that monthly changes her circled orb, lest that thy love prove likewise variable.”
Talking about any Blue Moon involves a lot of technical Blue Moon information because Blue Moons are calculated two different ways – seasonally and monthly.
The next seasonal Blue Moon will occur Saturday night, May 18.
The next monthly Blue Moon will occur on October 31, 2020.
A “Blue Moon” is the infrequent phenomenon of an additional full moon within a given period.
Traditionally, Blue Moons were calculated according to the seasons.
Usually, each of the four seasons contains three full moons, but sometimes a season will have four.
A seasonal Blue Moon is the third full moon in a season that has four full moons, like this year, when spring in the Northern Hemisphere runs from March 20 to June 21, in which time there will be four full moons.
Experts can’t tell us why the third, and not the fourth full moon is called the Blue Moon – that’s just the way it has always been.
A seasonal Blue Moon, according to NASA, occurs about every 2.5 years.
But, more recently, we’ve also started calculating monthly Blue Moons – when there is a second full moon within a single calendar month.
Whichever way one calculates Blue Moons, there are typically 29.5 days between full moons, so it’s unusual for two full moons to fit into a 30- or 31-day month.
Based on this calculation, of course, February never has a Blue Moon.
But which definition of Blue Moon is correct?
Well, astronomers say that they both are.
But, whether it’s a seasonal or a monthly Blue Moon, the moon is never really blue.
We believe Blue Moons got their name from the Old English word belewe, meaning both “blue” and “to betray.”
The extra moon was a “betrayal” of the normal 29-day cycle for full moons.
For thousands of years, English speaking people have used the expression, “once in a blue moon,” to describe something that rarely happens.
If the sky is clear Saturday night, we’ll be able to witness May’s “Flower Moon,” a seasonal “Blue Moon.”