Blue Moon in 2019: What is a Blue Moon?

The "Blue Flower Moon" Rises on May 18, 2019

May 16, 2019
Full Blue Moon

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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.

What is a Blue Moon?

We get a lot of letters about the term “Blue Moon.” Some of the questions are:

  • Is a Blue Moon the color blue or bluish in any way?
  • Did this term originate with Native American folklore, like a number of the other full Moon names?
  • Is this a scientific term used in astronomy?

In fact, none of these is true!

The modern understanding of “Blue Moon” only took off in the 1980s as a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine. Since then, the term has gone viral in the media.  

The Two Types of Blue Moons

There are two definitions commonly used today:

  • Seasonal Blue Moon: The extra full Moon that occurs within 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 in the season may be called a Blue Moon. 
  • Calendrical 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), so if a full Moon occurs on the first of a month, there will be a second full Moon at the end of the month, too (except in February).

Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s gave way to the calendrical definition, which was later popularized in the 80s and has stuck around to today!


When is the Next Blue Moon?

As of this writing, the next Blue Moon according to either definition will occur on May 18, 2019, at 5:11 P.M. EDT

Why? It’s the 3rd of 4 full Moons to occur in a season, specifically Spring 2019, defined by the Spring Equinox and the Summer Solstice.

The four full Moons are:

  1. Full Worm Moon: March 20 at 9:43 P.M. EDT
  2. Full Pink Moon: April 19 7:12 A.M. EDT
  3. Full Flower Moon: May 18 at 5:11 P.M. EDT <-– Blue Moon
  4. Full Strawberry Moon: June 17 at 4:31 A.M. EDT

See the May Full Moon Guide for more details.

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, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “once in a Blue Moon” isn’t so rare after all! 

After this 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 occurred in January and March 2018.)

This won’t happen again until 2037, when we’ll again have another Blue Moon in January and March.

Learn More

Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe