When is the next Blue Moon? And what is a Blue Moon, exactly? To prepare you for the Blue Moon on Halloween night this year, we’ve got all the details.
A Hunter’s Blue Moon on Halloween Night
In October 2020, we’re in for a special lunar event: a Blue Moon on October 31—Halloween night!
But that’s not the only thing that makes this October special. The month will contain both the Harvest Moon and the Hunter’s Moon, which are unique as far as full Moon names go.
October’s Two Full Moons
Why these names are unique has to do with the fact that the Harvest Moon and the Hunter’s Moon are the only two full Moon names associated with an astronomical event rather than a particular calendar month, which means that the names aren’t tied to a specific month’s full Moon.
Instead, the Harvest Moon is defined as the full Moon that happens nearest to the Autumnal Equinox (September 22, 2020), while the Hunter’s Moon is the full Moon that directly follows the Harvest Moon.
Typically, this means that the Harvest Moon occurs in September, followed by the Hunter’s Moon in October. This year, however, October will contain two full Moons—on the 1st and 31st—which means that both the Harvest Moon AND the Hunter’s Moon will occur in October, and that the Hunter’s Moon will also be a Blue Moon!
So what happens to September’s full Moon name if it’s not called the Harvest Moon? It will take on one of its alternate names: the Full Corn Moon.
When to See the Blue Moon
If you’re not already out trick-or-treating, venture outdoors on the night of October 31, 2020, to catch a glimpse of a bewitching full Moon—the Hunter’s Blue Moon! This full Moon will reach peak illumination (“peak fullness”) at 10:51 A.M. EDT on Halloween morning, but will appear full that night, too.
What Is a Blue Moon?
Need a refresher on what a Blue Moon actually is? We get a lot of letters about the term. Questions include:
- Is “Blue Moon” a scientific term used in astronomy?
- Did this term originate with Native American folklore, like a number of the other full Moon names?
- Does a Blue Moon really look blue?
In truth, the answer to all of these questions is “no.”
The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.
Two Types of Blue Moons
There are two definitions of the term that are commonly used today:
- Seasonal Blue Moon: The extra full Moon that occurs within a season. One season—defined by the dates of the solstices and equinoxes—typically has three full moons occur within it. If a season instead has four full moons, then the third full moon (not the fourth) in the season may be called a Blue Moon.
- Calendrical Blue Moon: The second full moon to occur in 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—a Blue 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 October 31, 2020, as described in detail above.
After that, we won’t actually have to wait too long for another Blue Moon:
- The next seasonal Blue Moon will occur on August 22, 2021, as the third full Moon in a series of four within that season.
- The next calendrical Blue Moon will happen on August 30, 2023, as the second full Moon in that month.
How Often Does a Blue Moon Occur?
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!
Two Blue Moons in a Single Year
But how often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March, and no full Moon in February.)
This won’t happen again until 2037, when we’ll again have another Blue Moon in both January and March.