What makes a blue moon?

Blue moon

We have all heard the saying “once in a blue moon!” it conjures up thoughts of thoughts of rarity and seclusion. After all, can you think of a time where the moon was ever blue?

When you break up the moon cycle into days it takes 29 per cycle. Since calendar months can contain 30 or 31 days, some moon show their face in the same overlap 2 full moons in one month, this happens every 2 -3 years or so and has been dubbed the Blue moon. It wasn’t always this way, back in 1946 a publication called Sky and Telescope wrote an op-ed piece on the different phases of the moon. This section covered the 1937 Maine Farmer’s Almanac of moon phases. Written by James Hugh Pruett titled Once in a Blue Moon, simplified the definition of what this truly meant.

The almanac followed specific rules for what to call each moon. To give you an example, the last full moon of winter had to fall during Lent; it was called the Lenten Moon. The first full moon of spring was called the Egg Moon, or Easter Moon, or Paschal Moon (and had to fall within the week before Easter) This can be thought of as the reason why we so strongly associate eggs with Easter. It’s certainly not because these furry little creatures lay eggs!

  There was also the Moon Before Yule and the Moon After Yule. So when a particular season had four moons, the third was dubbed a Blue Moon, so that the other full moons could occur at the proper times relative to the solstices and equinoxes.

Unfortunately his research failed him and brought to light what we now refer to as a blue moon. Previous to this definition a moon would only be considered blue if it fell twice within a month within 1 calendar year. Unlike the previous definition above this would only take place every 20 years or so.

Much like the levitating moon this rare type of moon is now considered to be a seasonal blue moon since it needs to fall within the same calendar year as the other one. To give you a sense of how far away that is the next one will be in 2048, then again in 2067. 

This chart by space.com shows just how possible a blue moon really is. Mapped out in the chart shows the beginning of the month containing the first full moon then followed up near the end by the second occurrence. 

Blue moon chart

Image credit: Space.com


All that to say blue moons are extremely rare, so the next time you are referring to something that does not happen to often give a nod to the idea that we once lost the true meaning of a blue moon due to a mis-step in the explanation back in 1946. 


Other resources: 


https://earthsky.org/astronomy-essentials/when-is-the-next-blue-moon 

https://www.timeanddate.com/astronomy/moon/blue-moon.html 

https://www.nasa.gov/vision/universe/watchtheskies/07jul_bluemoon.html