What Is a Blue Moon and When Is the Next One?
If something happens “once in a Blue Moon,” it's said to be rare. But just how rare is a Blue Moon?
![Full Moon peeking through branches. Blue and foggy background.](https://c.tadst.com/gfx/600x337/full-moon-peeking-tree-branches-fog.jpg?1)
A Blue Moon is very rarely blue.
©iStockphoto /JamesBrey
August 2024: last Blue Moon until 2026
What Is a Blue Moon?
There are two different definitions for a Blue Moon. Aseasonal Blue Moonis the thirdFull Moonof anastronomical seasonthat has four Full Moons. Amonthly Blue Moonis the second Full Moon in a calendar month with two Full Moons.
When Is the Next Blue Moon?
The nextseasonal Blue Moontakes place on August 19, 2024.
The nextmonthly Blue Moonis on May 31, 2026.
The dates may vary depending on your time zone.
Names of all the Full Moons in a year
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” meaningsomething raremay 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.’ Theunlucky status of the number 13and 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 theChristian ecclesiastical calendarfor the reason why the third Full Moon of the season is called the Blue Moon. The calendar uses thephases of the Moonto determine theexact datesfor holidays likeLentandEaster.
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 thePaschal 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.
The Moon: Our natural satellite
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, you will find the dates and times ofboth seasonal and monthly Blue Moonsin your time zone.
Sleep, crime, and menstruation: how Full Moons affect humans
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 thetime between two successive Full Moonsis approximately 29.5 days, just short of most months in theGregorian 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.
The Moon can sometimes look red
Double Blue Moon
Because of this,February,which has 28 days in aCommon Yearand 29 days in aLeap Year,can never have a monthly Blue Moon. Some years, February has no Full Moon at all, which is called aBlack Moon.A February with no Full Moon happens whenJanuaryandMarchhave 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, in2037,in many time zones.
Astronomical terms & definitions
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 in1934and the next time will be in2048.
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 in2143,while the last time was in1961.
Two seasonal Blue Moons in a year is an impossibility, as that would require 14 Full Moons in the same year.
How can Full Moon be in the daytime?
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 August 2023 takes place at 01:35UTCon August 31. Locations in time zones that are at least 1:30 hours behind UTC, includingNew York City,Rio De Janeiro,andVancouver,will see their Full Moon take place on August 30, 2023.