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.
How Often Do We See a Blue Moon?
Most months have one Full Moon, not two. See the Almanac Moon Phase Calendar.
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.
When is the Next Blue Moon?
As of this writing, the next Blue Moon is March 31, 2018. See the March Full Moon Guide.
Ever heard the expression, “Once in a Blue Moon?” Not so very rare!