When is the next Blue Moon? And exactly what is a Blue Moon? There’s some confusion about this popular phrase. Here’s the scoop.
Why is it a Blue Moon?
We got a lot of letters about this. Some folks imagine that it actually appears the color blue. Others assume it’s a term that originated with Native American folklore. Neither is true. Also, Blue Moon is not an astronomical term. This term only started in the 1940’s as a result of a mistake in an astronomy magazine. But it slowly went viral in the media.
The notion has no venerable pedigree among Native Americans, ancient cultures, or anything in the actual sky. So OK, we’ll play along and call it a Blue Moon. Just so you know, it’s a new idea.
There are two sorts of Blue Moon definitions:
- An extra full Moon that occurs in a season. One season—winter, spring, fall, summer—typically has three full moons. If a season has four full moons, then the third full moon may be called a Blue Moon.
- The second full moon of a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon). Therefore, there will be an extra Moon in a calendar year.
The second definition has become more popular in recent decades.
When is the Next Blue Moon?
As of this writing, the next Blue Moon (as defined as the second full moon of a calendar month) is March 31, 2018.
This March 31 Blue Moon will light up the skies ahead of Easter Sunday. Kind of neat.
How Often Do We See a Blue Moon?
Most months have one Full Moon, not two.
Since the Moon’s period of phases is 29 ½ days, while months usually have 30 or 31 days, it’s obvious that if a full Moon lands on the first day of any month except February it will repeat again at the end.
Turns out, Blue Moons happen every 30 months on average. Two and a half years. Ever heard the expression, “Once in a Blue Moon?” Not so very rare!
After this March 31 Blue Moon, the next one will be on October 31, 2020—Halloween!
Two Blue Moons in a Single Year
But how often do we have two Blue Moons in a single year?—as happened in 2018?
This won’t happen again until 2037, when we’ll again have another Blue Moon in January and March.
See the Almanac Moon Phase Calendar.