Halloween Blue Moon

For the first time since 2001, the Moon will be full on Halloween (October 31). And because this is the second Full Moon in October, it's also considered a Blue Moon. (It won't literally be blue, though.)

On average, the Moon is full on Halloween every 19 years, a period known as the Metonic cycle, used for centuries to construct luni-solar calendars and to calculate the date of Easter. Between 1900 and 2100, the Moon is full on Halloween (in at least one time zone) in the following years:

1906, 1925, 1944, 1955, 1974, 1993,
2001, 2020, 2039, 2058, 2077, 2096

Furthermore, these are all Blue Moons. In fact, every Halloween Full Moon is also a Blue Moon. The cycle of lunar phases (the synodic month) is 29.5 days long, plus or minus a few hours, so any Full Moon on October 31 must be preceded by a Full Moon on October 1 or 2.

Blue Moons occur about every two and a half years. The synodic month is one day shorter than the average calendar month, so it takes 30 months for the two to get back in sync. But because of the short month of February, this isn't always true. Before 2020, there were Blue Moons in both January and March of 2018. The last one before then was in July of 2015. After 2020, there are Blue Moons in August 2023, May 2026, and December 2028.

(Note that the currently popular definition of Blue Moonisn't the only one.)

The animation on this page shows the phase and libration of the Moon during (and slightly before) October 2020, pausing at each of the two Full Moons.