Once In A Blue Moon

There's a harvest moon, a strawberry moon, a blood moon--and the list goes on. One of those on the list is a blue moon. To take that literally you might think that the moon would be blue then but that is not the case. It has to do with the number of full moons in a year or in some cases, a month.

There are actually two definitions for blue moon. One was created by an amateur astronomer named James Hugh Pruett who referred to the blue moon as the second full moon in a month. His article appeared in a magazine, Sky and Telescope, in 1946 and the definition stuck but it actually had nothing to do with the seasonal blue moon that was originally defined.

The name blue moon was given to the full moon that was actually not one of the original twelve named full moons in the year like the harvest moon, wolf moon, etc. This blue moon only occurs in the years when there are thirteen instead of twelve full moons. Apparently that only happens every two or three years.

This year, 2018, there were two blue moons (in January and in March) when we had two full moons in a month. On May 18, 2019, there will be a seasonal blue moon meaning it is the thirteenth moon of the year or a moon without a traditional name for its season.

So, after all that, you can see that once in a blue moon could mean a long wait for something to happen. Oh, and don't look to see if the moon is actually blue. That all depends on atmospheric conditions and happens even more rarely when a blue moon occurs.