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).

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. 

This will be a seasonal Blue Moon, since the May full Moon is the third of four full Moons to occur in 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 <- lue="" nbsp="" oon="" span="">
4. Full Strawberry Moon: June 17 at 4:31 A.M. EDT