blue moon: meaning, definition, pronunciation and examples
B2Informal, colloquial, occasionally literary.
Quick answer
What does “blue moon” mean?
An astronomical event where the moon appears full twice in a single calendar month, or the third full moon in a season containing four full moons.
Audio
Pronunciation
Definition
Meaning and Definition
An astronomical event where the moon appears full twice in a single calendar month, or the third full moon in a season containing four full moons.
A very rare event; something that happens infrequently, used primarily in the phrase 'once in a blue moon'.
Dialectal Variation
British vs American Usage
Differences
No significant differences in definition or core usage. Both varieties use the idiom identically.
Connotations
Conveys a sense of whimsy or gentle exaggeration about rarity in both cultures.
Frequency
Equally common in both UK and US English.
Grammar
How to Use “blue moon” in a Sentence
[Subject] + happens + [once in a blue moon].Vocabulary
Collocations
Examples
Examples of “blue moon” in a Sentence
adjective
British English
- It was a blue-moon opportunity they couldn't miss.
American English
- He made a blue-moon visit to his hometown.
Usage
Meaning in Context
Business
"Such a favourable market alignment occurs once in a blue moon."
Academic
"Statistical outliers of this magnitude appear only once in a blue moon."
Everyday
"We only see our old neighbours once in a blue moon."
Technical
"The astronomical blue moon is based on the tropical year, not the calendar month."
Vocabulary
Synonyms of “blue moon”
Strong
Neutral
Weak
Vocabulary
Antonyms of “blue moon”
Watch out
Common Mistakes When Using “blue moon”
- Using 'a blue moon' to mean 'often' (semantic reversal).
- Writing 'once in a blue moon' without the article 'a'.
- Using it as a standalone noun (*"It was a blue moon."*) instead of within the fixed idiom.
FAQ
Frequently Asked Questions
Rarely. The term's origin is idiomatic for rarity. The moon can appear bluish if atmospheric conditions scatter red light, e.g., after a volcanic eruption, but this is unrelated to the calendar-based 'blue moon' definition.
The older 'seasonal' definition is the third full moon in an astronomical season with four. The modern 'monthly' definition is the second full moon in a calendar month. The latter is now more common in popular usage.
It's unusual. While you can refer to the astronomical event directly ("The next blue moon is in July"), using it adjectivally ("a blue-moon chance") is creative and less common.
By the modern calendar definition, about once every 2 to 3 years, making the idiom 'once in a blue moon' a slight exaggeration for emphasis.
An astronomical event where the moon appears full twice in a single calendar month, or the third full moon in a season containing four full moons.
Blue moon is usually informal, colloquial, occasionally literary. in register.
Blue moon: in British English it is pronounced /ˌbluː ˈmuːn/, and in American English it is pronounced /ˌblu ˈmun/. Tap the audio buttons above to hear it.
Phrases
Idioms & Phrases
- “once in a blue moon”
Learning
Memory Aids
Mnemonic
Think of a calendar month with two full moons. The second one is so rare and unexpected you could colour it blue.
Conceptual Metaphor
RARITY IS A CELESTIAL/UNNATURAL EVENT.
Practice
Quiz
What does 'once in a blue moon' mean?