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What Is a Blue Moon and When Is the Next One?

On Saturday, October 31, 2020—Halloween, in many countries—a Blue Moon shone bright in the night sky. This Halloween Blue Moon, which was also a Micro Full Moon, was October's second Full Moon in many time zones.

Full Moon peeking through branches. Blue and foggy background.

A Blue Moon is very rarely blue.

©iStockphoto.com/JamesBrey

What Is a Blue Moon?

There are two different definitions for a Blue Moon. A seasonal Blue Moon is the third Full Moon of an astronomical season that has four Full Moons. A monthly Blue Moon is the second Full Moon in a calendar month with two Full Moons.

Why is it called a Blue Moon?

The historical origins of the term and its two definitions are shrouded in a bit of mystery and, by many accounts, an interpretation error.

Some believe that the term “blue moon” meaning something rare may have originated from when smoke and ashes after a volcanic eruption turned the Moon blue. Others trace the term's origin to over 400 years ago—folklorist Philip Hiscock has suggested that invoking the Blue Moon once meant that something was absurd and would never happen.

Origins of the Seasonal Full Moon

The definition of a seasonal Blue Moon, the third Full Moon in an astronomical season with four Full Moons, can be traced back to the now-defunct Maine Farmer's Almanac. According to the Almanac, the appearance of a 13th Full Moon in a year ‘upset the arrangement of Church festivals.’ The unlucky status of the number 13 and the difficulties of calculating the occurrence of such a Full Moon led to the extra Full Moon being named a Blue Moon.

We can thank the Christian ecclesiastical calendar for the reason why the third Full Moon of the season is called the Blue Moon. The calendar uses the phases of the Moon to determine the exact dates for holidays like Lent and Easter.

The month of Lent contains the final Full Moon of winter, Lenten Moon. The first Full Moon of spring—also known as the Easter Moon or the Paschal Moon—falls just before Easter. Naming the third Moon of the season as the Blue Moon ensured that Lent and Easter coincided with the right Moon phases, and other celebrations and customs would still fall during their “proper” times.

Origins of the Monthly Blue Moon

The more popular definition of the Blue Moon, that of the second Full Moon in a month, owes its existence to a misinterpretation originally made by amateur astronomer James Hugh Pruett (1886–1955) in a 1946 edition of Sky & Telescope magazine. The error took on a life of its own and spread around as fact. It even found its way into the answers of the 1986 version of the board game Trivial Pursuit! Today, this definition is considered a second definition of the Blue Moon rather than a mistake.

At timeanddate.com, you will find the dates and times of both seasonal and monthly Blue Moons in your time zone.

How Rare Is a Blue Moon?

For a monthly Blue Moon to take place, a Full Moon must occur at the beginning of the month. This is because the time between two successive Full Moons is approximately 29.5 days, just short of most months in the Gregorian Calendar.

Seasonal Blue Moons take place slightly less frequently than monthly Blue Moons—in the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons. This means that either type of Blue Moon occurs roughly every two or three years.

Blue Moons that are blue are incredibly rare and have nothing to do with the calendar or the Moon's phases but are instead a result of atmospheric conditions. Volcanic ashes and smoke, water droplets in the air, or certain types of clouds can all contribute to a Full Moon taking on a blue shade on rare occasions.

Double Blue Moon

Because of this, February, which has 28 days in a Common Year and 29 days in a Leap Year, can never have a monthly Blue Moon. Some years, February has no Full Moon at all, which is called a Black Moon. A February with no Full Moon happens when January and March have a Blue Moon each.

Known as a Double Blue Moon, this phenomenon is rather uncommon and takes place only about three to five times in a century. We saw a Double Blue Moon in 2018 in most time zones and will see it again 19 years later, in 2037, in many time zones.

Seasonal and Monthly Blue Moons Together

Seasonal and monthly Blue Moons can also sometimes occur in the same year. Between 1550 and 2650, 20 years have one seasonal and one monthly Blue Moon in many time zones. The last time this happened was in 1934 and the next time will be in 2048.

In the same period, 21 years have Triple Blue Moons—one seasonal and two monthly Blue Moons in the same calendar year. The next is in 2143, while the last time was in 1961.

Two seasonal Blue Moons in a year is an impossibility, as that would require 14 Full Moons in the same year.

Not the Same Worldwide

The Moon reaches its various phases at specific moments. But because of time zones, the local time of a Full Moon can differ from one location to another. For instance, the Blue Moon in October 2020 takes place at 14:49 UTC on October 31. Locations in time zones that are at least 9:15 hours ahead of UTC will have their Full Moon take place on November 1, 2020 and unfortunately, will not be able to enjoy a Blue Moon on Halloween night. The Moon will, however, still look full and bright for them on October 31, 2020. They will also have a Blue Moon in November.

Topics: Astronomy, Moon, Calendar, Months