A “blue moon” is called that because the phrase started as an old saying for something impossible or absurd, then shifted to mean a rare extra full Moon.

Why Is It Called a Blue Moon?

Old expression: “the Moon is blue”

In 16th‑century English, saying “the Moon is blue” was a way to mock an obviously false claim or an impossibility. You’d use it like, “They’d have you believe the Moon is blue,” meaning “they’d have you believe something ridiculous.”

Over time, that sense softened from “impossible” to “so unlikely it basically never happens,” which set the stage for today’s meaning of “once in a blue moon” as “very rarely.”

When the Moon actually looked blue

After big volcanic eruptions, the Moon really can appear bluish because of particles in the atmosphere. A famous example was the 1883 eruption of Krakatoa in Indonesia, which filled the sky with dust and ash; for a couple of years, people around the world reported oddly colored sunsets and sometimes a bluish Moon.

This real but unusual effect helped people connect the idea of “blue Moon” with rare, out‑of‑the‑ordinary events rather than literal impossibility.

From saying to calendar term

Astronomers and almanac makers later attached “blue moon” to specific “extra” full Moons in the year’s cycle.

There are two main definitions in use:

  1. Seasonal definition (older usage)
    • The year is split into four seasons (winter, spring, summer, fall), each normally with three full Moons.
 * When a season has four full Moons (because the timing drifts), the **extra** one gets called a blue Moon.
  1. Monthly definition (popular modern usage)
    • A blue Moon is the second full Moon in a single calendar month , something that happens roughly every 2–3 years.
 * This calendar‑month meaning spread widely in the 20th century after a misinterpretation in an astronomy magazine and later exposure in things like radio shows and trivia games.

Both are about an “extra” full Moon—either extra in a season or extra in a month—so they stay close to the older idea of something that doesn’t happen very often.

Does the Moon actually turn blue?

Most blue Moons look completely normal—pale white or yellowish, just like any other full Moon. The name is about rarity and calendar timing, not color.

A genuinely blue‑tinted Moon needs lots of fine particles (from big volcanic eruptions or major fires) high in the atmosphere to scatter red light and let more blue light through. That is a separate, physical phenomenon that only occasionally coincides with a “blue Moon” date in the calendar sense.

Quick FAQ style recap

  • Why is it called a blue moon?
    Because an old phrase about a “blue” Moon meaning “impossible/absurd” evolved into “rare event,” and astronomers later used it for rare extra full Moons.
  • Is a blue Moon actually blue?
    Almost never; the color is usually normal, and the term is about rarity, not hue.
  • What’s the exact definition?
    Commonly: the second full Moon in a calendar month; traditionally: the extra full Moon in a season that has four. Both point to a rare extra full Moon.

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