A “blue moon” is called that because the phrase started as a way to describe something impossible or absurd, and later shifted to mean a rare event—both in language and in astronomy.

Quick Scoop

  • The phrase “the moon is blue” appears in English from the 1500s as a way of saying “that’ll never happen.”
  • After big volcanic eruptions like Krakatoa in 1883, dust in the atmosphere sometimes made the moon actually look bluish, turning the old “impossible” idea into “strange but rare.”
  • Over time, “once in a blue moon” came to mean “once in a long while,” and astronomers/folklorists later attached that phrase to specific rare full-moon patterns.

Where the name comes from

Originally, “blue moon” wasn’t about calendars at all, but about impossibility.

  • 16th‑century English used “the Moon is blue” as a sarcastic way of saying something would never happen.
  • It sat next to other “absurd” sayings like claiming the moon was made of “green cheese.”

Then nature added a twist:

  • After the Krakatoa eruption in 1883, enough particles were thrown into the air that people reported genuinely blue‑tinted moons and odd sunsets for a couple of years.
  • That made a truly blue‑looking moon possible—but very rare—so the phrase drifted from “impossible” to “rare but real.”

So the name “blue moon” stuck to that sense of rarity.

How the modern meanings evolved

Today when people ask “why is a blue moon called a blue moon,” they’re usually thinking about special full moons, not color. There are two main astronomical/folklore uses:

  1. Seasonal (older) definition
    • Traditionally, you get 12 full moons in a year—three per season, each with a folk name.
 * When a year has 13 full moons, one season has four; that extra full moon in the season was tagged the “blue moon” to keep the other names in order.
  1. Monthly (newer) definition
    • In the 1940s, a writer misinterpreted the Maine Farmer’s Almanac and described a blue moon as the second full moon in a calendar month.
 * The idea was catchy and easy to remember, so media and trivia games popularized it, and it became the common modern meaning.

Underlying both is the same idea: you’re labeling an extra full moon that doesn’t fit the usual pattern, which matches the word’s older sense of something unusual or rare.

Does the moon ever really look blue?

Sometimes, yes—but that’s a separate (though related) story.

  • Volcanic eruptions or massive fires can throw particles into the atmosphere that scatter light in a way that makes the moon appear bluish.
  • Krakatoa is the classic example, but other big eruptions and smoke events have produced blue‑looking moons as well.

These events are rare enough that they reinforce the “once in a blue moon” vibe, even though most “blue moons” you hear about today are calendar quirks, not literally blue.

Mini timeline of the phrase

  1. 1500s: “The moon is blue” = nonsense/never.
  1. 1800s: Krakatoa and similar events make genuinely blue‑looking moons possible, but uncommon.
  1. Late 1800s–early 1900s: “Once in a blue moon” = very rare event.
  1. Early 1900s: Almanacs use “blue moon” for an extra seasonal full moon.
  1. 1940s onward: A popular misreading rebrands it as the second full moon in a month, which becomes the everyday definition.

So a “blue moon” is called that because the phrase grew from joking about an impossible blue‑colored moon, through rare real blue moons, into a handy label for rare extra full moons in our calendar and seasons.

TL;DR: It’s called a “blue moon” because English speakers once used “the moon is blue” to mean “that’ll never happen,” then rare real blue‑looking moons and odd calendar moons shifted the phrase to mean a rare extra full moon—so “blue” here is all about rarity, not normal moon color.

Information gathered from public forums or data available on the internet and portrayed here.