What Is a Blue Moon?

Our Moon is a mystical place to those of us on Earth. Dating back centuries and millennia, ancient humans have given names to the Moon to represent different times of the year or spiritual happenings.

Of the terms we use today to describe our Moon, a blue moon is one of the most popular. You may have heard this term yourself, but there is more to the term than meets the eye. In order to truly understand what a blue moon is, we first need to better understand a full moon.

What Is A Blue Moon

What Is a Full Moon?

As the Moon travels around the Earth, it traverses eight different phases. These phases are dependent on the Moon’s location relative to both the Earth and the Sun. When the Moon is the biggest and brightest in the nighttime sky, we are looking at a Full Moon.

luminous full moon
A luminous full moon.

How Many Full Moons per Year?

Our calendar follows the amount of time it takes for the Earth to travel completely around the Sun one time. This event happens once every 365.25 days.

It takes the Moon just under one month, at 29.5 days, to go through all eight phases and return to a Full Moon. If you do the math, that averages to about one full moon every month, or 12 in a calendar year.

What Constitutes a Blue Moon?

To make things as difficult as possible, there are actually three different definitions of the term “Blue Moon.” They really don’t have anything to do with each other, other than the fact that they are both Full Moons.

A Seasonal Blue Moon

The traditional definition of a Blue Moon dates back at least one hundred years. A seasonal blue moon is the third full moon in an astronomical season containing four full moons. Come again?

To clarify, astronomical seasons represent the amount of time between solstices and equinoxes. Still unclear? Solstices begin on the first day of summer and winter, and equinoxes occur on the first day of spring and fall.

Each of our seasons lasts around 90 days. In the Northern Hemisphere, summer begins around June 21st and goes until approximately September 21st.

With an orbit of Earth taking 29.5 days, the Moon makes just over three orbits during that time. Since the Moon begins its fourth cycle around the Earth before the season ends, it will occasionally reach a fourth full moon during that time.

Seasonal Blue Moon
Not a plain Blue Moon, but a seasonal one. (Image credit: “Seasonal Blue Moon” by bossco on Flickr CC BY-SA 2.0)

This event is actually quite rare and only happens about once every two and a half years.

It’s worth noting that the Moon doesn’t look any different in the sky, and only in incredibly rare circumstances would it have a blue color. You won’t even know this phenomenon is happening unless you follow the Moon’s phases or hear about it happening from somewhere.

A Monthly Blue Moon

If you’ve heard about a blue moon in the past, it’s likely this is the definition you’re familiar with. Believe it or not, what we know of as a monthly blue moon was actually a misinterpretation of the 1937 Farmer’s Almanac.

This misinterpretation was written in a 1946 article for Sky & Telescope magazine. In the article, it mentioned that “Seven times in 19 years there were – and still are – 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”

Surprisingly, this mistake of a blue moon representing the second full moon in a calendar month is the most common explanation in use today!

As with a seasonal blue moon, there is no difference between a monthly blue moon and any other full moon. It’s simply a rare event that takes place approximately once every two and a half years.

Interestingly, because of time zones, it is possible for a blue moon to occur in one part of the world but not another.

A Literal Blue Moon

There are actually times where the Moon can appear a bluish color in the sky, following extenuating circumstances.

blue moon waxing
Blue Moon of 2007. (Image credit: “blue moon waxing” by Paul Moody on Flickr CC BY-NC 2.0)

This effect occurs when smoke or dust particles of a specific size enter the atmosphere. These particles are usually the result of a large fire or volcanic eruption and have to be wider than the wavelength of red light. As a result, these particles diffuse red light from the sky, making the Moon appear blue instead.

Such a blue moon is an incredibly rare occurrence, but after forest fires or volcanic eruptions, the Moon can appear blue near the site of the disaster for months or even years.

The Origin of the Term “Blue Moon”

The origin of the term “blue moon” isn’t entirely clear. It does appear in literature from as far back as the 1600’s as a way of indicating how absurd something happened to be. Hearing “I believe that just as the moon is blue” suggests the person does not believe whatever it is you are saying.

Furthermore, the phrase was used to indicate the likelihood of something happening. Someone saying, “I’ll do something when the Moon is blue” was clearly indicating the event was not going to happen. Even today, we use the expression “once in a blue moon” to demonstrate how rare we believe something to be.

The scientists at NASA believe the phrase came into light during the massive eruption of the Krakatoa volcano in 1883. The volcano kicked up so much ash into the atmosphere that the Moon appeared with a bluish hue around the world for nearly two years!

What a Blue Moon Really is and How the Expression “Once in a Blue Moon” Came About

February Gets Left Out

Due to February’s unique status as the shortest month of the year, it misses out entirely on all these phenomena. Even during a leap year, when February has 29 days, the month is still shorter than the 29.5 days it takes the Moon to travel through its cycle. February, unfortunately, can never have a blue moon.

On the flip side, being 28 days in length, there are also years where February doesn’t even have one full moon. This event occurs only once every 19 years. Due to the length of the month and the Moon’s orbit, this means both January and March will each have a monthly blue moon.

The Black Moon

The Black Moon
The Black Moon.

On the other side of the lunar spectrum, a black moon is all about the frequency of the new moon. A new moon appears completely invisible to us on Earth, as all of the Sun’s rays are reaching the side of the Moon that faces away from the Earth.

Similar to a blue moon, a black moon is the third new moon in an astronomical season of four new moons. The definition can also be used to describe the second new moon in the same calendar month.

This is the exact opposite occurrence to a blue moon, where an extra full moon is visible. Like a blue moon, a black moon happens only once every two and a half years. With the moon invisible to us in the sky, at least the name makes more sense!


The term “blue moon” is quite a bit more complex than it first appears, and the most popular definition today has nothing to do with the color of the Moon. Even so, each definition does seem to paint the Moon in a shade of wonder. The next blue moon may not be for a few years, but mark your calendars now!

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.