Backyard Observatory

Everything You Need to Know About Blue Moons & Why They Happen so Infrequently

Though special, they don't look any different than a regular full moon.

what is a blue moon
Nicolas Economou/NurPhoto via Getty Images

You're only going to see it come around once in a blue moon. 

As the saying implies, a blue moon is not a frequent occurrence. (Though, the phrase "blue moon" has been around for more than 400 years and originally meant the speaker was referring to something absurd, according to folklorist Philip Hiscock.) Defining what it is astronomically, however, is a little complicated. It's an appearance of an additional full moon during a certain period of time, but there are a couple of definitions. 

What is a blue moon?

Language is always changing, and this newer definition arrives because of a strange combination of a misunderstanding in a 1946 issue of Sky & Telescope, a radio host in the 80s, and the game "Trivial Pursuit." Despite that weird series of dominos, this recent definition is the more widely used of the two definitions. 

Under this description, a blue moon occurs anytime there are two full moons in a given month. The second full moon in that month is a blue moon. It's rare. A full moon arrives about once every 29.5 days. So, a full moon must land on the first day or so of a month to get a blue moon at the end of the month. This happens about once every two-and-a-half years, according to NASA.

Because of that 29.5-day gap between any two full moons, you'll never see a blue moon in February. Though, February does give rise to an exception to the approximately two-and-a-half-year gap between blue moons. It's possible that February lands just right and doesn't have a full moon at all. When that happens, that year will have two blue moons. This won't happen until 2037, when there's a blue moon on January 31 and March 31. 

What is a seasonal blue moon?

The other definition of a blue moon is the third full moon in a season that has four full moons, per NASA.

Every year contains 12 full moons, each of which has a name like "harvest moon" or "hunter moon." With 12 per year, we get three full moons per season. With the 29.5-day gap between full moons, we sometimes have 13 full moons in a year. The bonus full moon over the course of a year means that somewhere along the way, we're getting four full moons in a single season.

Is a blue moon actually blue?

It's not. The name of a blue moon only refers to when it appears and has nothing to do with the color of the full moon. 

Though, NASA says it's believed that the term "blue moon" meaning a rare occurrence arrived in 1883 after the eruption of Krakatoa. The volcano put so much dust in the air that its light began to look blue. It was such a rare sight, the story goes, that "once in a blue moon" arrived as a phrase. 

When is the next blue moon?

The below list of upcoming blue moons, which take us from this year's Halloween blue moon through the next Halloween blue moon in 2039, is courtesy of Bruce McClure at EarthSky, a great resource for stargazers. 

1. October 31, 2020
2. August 31, 2023
3. May 31, 2026
4. December 31, 2028
5. September 30, 2031
6. July 31, 2034
7. January 31, 2037
8. March 31, 2037
9. October 31, 2039

Ready to go stargazing?

Here are all the best stargazing events that you can get out and see this month or you could stay in a stream the northern lights from home. If you're just getting started, check out our guide to astronomy for beginners or easy stargazing road trips from big US cities

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Dustin Nelson is a Senior Staff Writer on the news team at Thrillist. Follow him @dlukenelson.