Will there be a Blue Moon this year? And what is a Blue Moon, exactly? Read all about Blue Moons!
What Is a Blue Moon?
Need a refresher on what a Blue Moon is? We get a lot of questions about the term, including:
- 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 an astronomical 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 August 30, 2023. That’s right: we won’t have any Blue Moons in 2022!
The Blue Moon in August 2023 will be a calendrical Blue Moon (the second full Moon in a calendar month).
After that, the next Blue Moon we’ll experience is a seasonal Blue Moon (the third of four full Moons in one astronomical season) on August 24, 2024.
How Often Does a Blue Moon Occur?
Most months have one full Moon, not two.
Since the Moon’s period of phases is 29 1/2 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.
Learn More
Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!
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The modern day definition of a Blue Moon is when there are 2 Full Moons in one month. A Full Moon occurs roughly every 29.5 days and on the rare occasion when the Full Moon falls at the very beginning of a month there is a good chance a Blue Moon will occur at the end of the month. According to this definition the next Blue Moon will occur on January 31, 2018.
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Depending on the exact time of the Blue Moon it is possible that some places in the world don't technically have a Blue Moon. For example, the Blue Moon on August 31, 2012 occurred exactly at 13:58 UT. The Blue Moon will occur on August 31st in South America, North America, Europe, Africa, Asia, India, and Australia but New Zealand will just miss out. For New Zealand the Full Moon occurs just past mid-night on September 1st. For exact times of the Full and Blue Moons see the Full Moon Calendar.
Original Blue Moon Definition
The modern definition of a Blue Moon was derived from an earlier idea of what a Blue Moon was. This earlier definition says a Blue Moon is when there are 4 Full Moons in a season rather than the usual 3. The Blue moon is the 3rd Full Moon out of the 4. This definition gets a bit complicated and its origins are murky. One school of thought has to do with the naming of the Full Moons. Many cultures named the Full Moons each month to reflect the times for planting, harvesting or seasonal conditions. When an extra Full Moon was thrown in it was referred to as a Blue Moon to keep the Full Moon names constant throughout the year.
Another origin could be from the Christian ecclesiastical calendar. This one gets even more tricky but basically has to do with the idea that there are usually 12 Full Moons in a year. The Full Moons on this calendar were important markers for determining certain dates such as Easter. When a 13th Full moon was thrown into the year it made things messy so giving it a name allowed the calendar to stay on track.
The idea of a Blue Moon being the extra full Moon in a season (or when there were 13 in a year) was widely used in 19th and early 20th center Farmers Almanacs and the more modern version seems to have come from an article written in the 1930s that misinterpreted the Farmers Almanac definition. The article was titled "Once in a Blue Moon" and from that point on the term became part of the popular culture.
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Is the moon ever the color blue?
This is a very rare event but it does happen. There are a few recorded events when forest fires or ash volcanic eruptions have given the Moon a bluish color. The Moon can also have a blueish color on very cold winter nights when ice crystals in the air form a ring around the Moon. Of course the idea of the Moon being blue is very subjective and left up to one's own interpretation.
Dark Moon
An interesting twist to the idea of a Blue Moon is the idea of Dark Moon. Sometimes the reference of a Dark Moon is given when there are 2 new Moons in a calendar month, the opposite of a Blue Moon. Other definitions say it is when there is no Full Moon in a calendar month. Neither of these definitions or the term Dark Moon is used very often. Neither has any scientific bases and is more a part of the popular culture.
To get a better idea of how Blue Moons and Dark moons all fit together, have a look at the Full Moon and New Moon calendar.