When we think of “All Hallow’s Eve,” many of us conjure up terrifying images of werewolves, goblins, zombies, and other scary things that go “bump” in the night, all lit up by a spooky full Moon. But how common is a Halloween full Moon? Apparently, not very.

In 2001, ghosts and goblins in Central and Pacific time zones trick-or-treated by the light of a full Moon, but a Halloween full Moon hadn’t appeared for everyone in all time zones since 1944!

Blue Moon Definition

For more than half a century, whenever two full Moons appear in a single month (which happens on average every 2 1/2 to 3 years), the second full Moon is christened a “Blue Moon.”

When you look at the full Moon on Halloween night, it won’t appear blue in color but you’ll be looking at something pretty uncommon. A full Moon on Halloween occurs roughly once every 19 years—a pattern known as the Metonic Cycle. 

This well-known lunar cycle was discovered in 432 BC by the Greek, Meton, of Athens. He determined that after 19 years have elapsed, the phase of the Moon will repeat on the same date.Well . . . not always. Because of slight variations in the Moon’s orbital period, and the number of leap days that intervene over a 19-year time span, the Metonic Cycle can be accurate only to within a day.

For a Halloween full Moon, the Metonic Cycle worked well early in the 20th Century—in the years 1925 and 1944.  But thereafter, using the cycle, the date of full Moon shifted a day to November 1st (in 1963, 1982, and 2001). But then, in 2020, it returned to October 31st. Making it a rarer sight, indeed.

Fun fact: Any time the Moon is technically “full” on October 31st (as it will be this year), it would also have to be a Blue Moon because the lunar cycle is only 29.5 days long.

There is an alternate definition of a “Blue Moon” —when there are 4 full Moons in a single season, the third is considered a “Blue Moon.”

When’s The Next Halloween Full Moon?

According to astronomers, we will all see a 100%-illuminated Halloween full Moon (after 2020) in the years  2039, 2058, 2077, and 2096 (note the 19-year pattern). The good news is that even if the Moon is a day or two away from 100% full on any particular Halloween, it can still serve the purpose for a spooky backdrop since most people can’t tell the difference between a 98% illuminated Moon and a 100% “full” Moon (Cases in point: November 2, 2029, and October 30, 2031). Plan your costumes accordingly!