Tonight’s full moon is getting some notoriety in the press as what is commonly referred to as a “Blue Moon”. If blue moons aren’t the color blue, and they are not blue, why are they called blue, and why do we have them?
No one knows for sure how blue moons came to be called blue, but history suggests it first started with King Henry VIII’s infamous adviser Cardinal Wolsey. Wolsey reportedly claimed his enemies would have you believe the moon is blue. From that the term somehow turned into meaning a long unspecified period of time.
Why we have blue moons is somewhat more complicated. The answer is more cultural than a scientific one. Most cultures, including our own, have unique full moon names for each calendar month. There are twelve months in a year and typically thirteen full moons a year. The extra full moon has come to be known as a blue moon. Blue moons keep the names of the full moons in sync with the calendar months.
Today blue moons have two definitions. The most accepted is the second full moon within a calendar month is called a blue moon, not that rare. The next blue moons under this definition occur on January 31, 2018 and March 31, 2018. It is curious to note 2018 will have two blue moons and 2017 has none.
The other, which has been around much longer, defines a blue moon as the third of four consecutive full moons within a season. This kind of blue moon will occur on May 21, 2016 and again on May 18, 2019.
We astronomers do not celebrate full moons or blue moons for that matter. Dark moonless nights are much preferred. Observatories are shut down and telescopes are packed way unless there is a lunar eclipse which will happen in September. Black moons are another story.
I, like most amateur astronomers, use full moons to catch up on reading and other non-observing activities like sleep. I do enjoy watching a full moon rising and here in the Denver area watching a full moon setting over the mountains in the west with a spectacular Colorado sunrise in the east. Priceless!
Wishing you clear skies