What Is a Blue Moon? Meaning, History & Rare Lunar Phenomena Explained
Science and folklore converge to explain why this lunar event is so exceptionally rare.
What Is a Blue Moon?
A blue moon is a term used to describe either the occurrence of the second full moon within a single calendar month or, traditionally, the third full moon in a season that features four full moons instead of the usual three. While the term might suggest a moon that is physically blue in color, actual blue-colored moons are extremely rare and result from atmospheric phenomena rather than lunar cycles.
Modern & Traditional Definitions of a Blue Moon
- Monthly Blue Moon: The second full moon in a calendar month. This definition is popular due to its simplicity and more frequent occurrence.
- Seasonal Blue Moon: The third full moon in an astronomical season that contains four full moons. Astronomical seasons are divided by solstices and equinoxes, so using this definition means tracking the lunar cycle according to nature’s quarters rather than the standard calendar.
Both definitions are technically correct, but the monthly version gained popularity after a 1946 magazine article’s mistake was publicized, then later cemented in the public consciousness by media and the Trivial Pursuit board game.
Summary Table: Monthly vs. Seasonal Blue Moon
| Type of Blue Moon | Definition | Frequency | Calendar System |
|---|---|---|---|
| Monthly Blue Moon | Second full moon in a calendar month | Once every 2–3 years | Gregorian Calendar |
| Seasonal Blue Moon | Third full moon in a season with four full moons | Once every 2–3 years | Based on astronomical seasons |
Why Is It Called a Blue Moon?
The phrase “blue moon” has existed for centuries, but its meaning has changed over time. Originally, it described something impossible or absurd—”the moon is blue”—before coming to represent a rare event due to atmospheric conditions that could make the moon appear bluish.
The eruption of Krakatoa in 1883 famously turned the moon blue for years, as volcanic ash in the atmosphere scattered red wavelengths of light and allowed only blue ones to pass.
How Often Does a Blue Moon Occur?
Blue moons, regardless of which definition is used, are quite rare but predictable. On average, a blue moon occurs:
- Every two to three years: This is when two full moons fall within the same calendar month or when four full moons take place in a season.
- About seven times every 19 years: The frequency is determined by the lunar cycle versus the length of our months and seasons.
- Even rarer: Occasionally, two blue moons happen in a single calendar year—a phenomenon that only occurs about four times a century.
Months with 31 days—such as January, March, May, July, August, October, and December—are more likely to host a blue moon.
Is the Moon Ever Physically Blue?
While the term usually refers to timing or frequency, the moon can actually appear blue under special atmospheric circumstances:
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- Wildfires or volcanic eruptions inject tiny dust particles into the atmosphere.
- These particles scatter red light (shorter wavelengths) and let blue light predominate, causing the moon to look blue to observers on Earth.
This visual phenomenon is rare enough to inspire the saying “once in a blue moon” as a metaphor for rarity.
The Lunar Cycle and Calendrical Oddities
The lunar cycle averages 29.5 days between full moons. Because our months are mostly 30 or 31 days long, occasionally two full moons fall in a single month. However, February, at just 28 or 29 days, never has more than one full moon and cannot host a blue moon.
Here’s an example:
- If a full moon occurs on the 1st or 2nd of a month, the next full moon can also fall within the same month (the 30th or 31st), leading to a monthly blue moon.
Origins in Folklore and Almanacs
The term “blue moon” as a calendrical event has its roots in American tradition, specifically the Maine Farmers’ Almanac of the 19th century.
- The almanac divided years by seasons (not months) and named each full moon according to seasonal agricultural activities.
- When a season had four full moons, the third was designated the blue moon—this kept the other named moons in their proper seasonal positions.
This system ensured consistency in themes like the “Easter Moon” or “Harvest Moon,” and only the unassigned moon in such an overcrowded season became the “blue moon.”
Frequently Asked Questions (FAQs) About Blue Moons
Q: Does the Blue Moon Ever Look Blue?
A: Yes, but rarely. The moon appears blue only if atmospheric conditions allow the right particles to filter light, such as after a major volcanic eruption or wildfire.
Q: How often do blue moons occur?
A: A blue moon happens about once every two to three years using either the monthly or seasonal definition.
Q: Can February ever have a blue moon?
A: No. February only has at most 29 days, which is too short for two full moons.
Q: Is there any difference between observing a “supermoon” and a blue moon?
A: These terms refer to different phenomena. A “supermoon” is when a full moon coincides with the moon’s closest approach to Earth, appearing larger and brighter. A blue moon involves timing—not size or proximity.
Q: Who first defined the modern blue moon?
A: Amateur astronomer James Hugh Pruett popularized the “second full moon in a month” definition in 1946. This definition spread through radio and games like Trivial Pursuit.
Seasonal and Monthly Blue Moon Occurrences
Below is a list of months and seasons most prone to blue moons:
- Calendar months with 31 days: January, March, May, July, August, October, December.
- Seasons with four lunar cycles: Occur every two to three years due to the mismatch between lunar and seasonal cycles.
Fun Facts & Folklore Connections
- The phrase “once in a blue moon” dates back to the 16th century, first as an expression of impossibility, later evolving to mean something rare.
- Some cultures name every full moon in a year, such as the “Wolf Moon,” “Harvest Moon,” and “Snow Moon.” The “blue moon” only enters as an extra.
- Not every location on Earth experiences blue moons in exactly the same months due to time zones and calendrical conventions.
How to Spot the Next Blue Moon
To see a blue moon, check a lunar calendar for upcoming months with two full moons. Astronomical resources and almanacs will highlight these events, providing exact dates and times for your area.
Summary Table: Blue Moon Facts
| Fact | Details |
|---|---|
| Definition | Second full moon in a month or third full moon in a season of four |
| Origin of term | 16th century, meaning “impossible”; now, “rare” event |
| Physical color change? | Only under special atmospheric conditions |
| Frequency | Every 2–3 years |
| Related phenomena | Supermoon, Harvest Moon, seasonal names |
Conclusion
The blue moon stands out in lunar lore for its rarity and its role in folklore, science, and even pop culture. Whether defined by the calendar or by the seasons, its infrequent appearance makes it a captivating event for astronomers and skywatchers alike. Though you may never see a moon turning physically blue, the legacy of volcanic eruptions, wildfires, and human imagination assures that the blue moon remains one of our most interesting celestial enigmas.
References
- https://en.wikipedia.org/wiki/Blue_moon
- https://moon.nasa.gov/news/197/super-blue-moons-your-questions-answered/
- https://www.loc.gov/everyday-mysteries/astronomy/item/what-is-a-blue-moon-is-it-ever-really-blue/
- https://www.britannica.com/science/blue-moon-astronomy
- https://www.rmg.co.uk/stories/space-astronomy/what-blue-moon-how-often-does-it-occur
- https://www.space.com/15455-blue-moon.html
- https://www.timeanddate.com/astronomy/moon/blue-moon.html
- https://www.adlerplanetarium.org/blog/what-is-a-blue-moon/
- https://www.youtube.com/watch?v=NH0hOwkft2U
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