Black moon and blue moon: definitions, dates and symbolism

Two expressions, several meanings

Astronomers use neither "blue moon" nor "black moon". These are calendar expressions used to mark a rare or curious event. Both have several overlapping meanings, which causes confusion. The starting point is a simple fact: a cycle of phases lasts 29.53 days, a little less than a calendar month. The same phase can therefore fall twice in the same month, when it occurs in the very first days.

The blue moon: two definitions

The seasonal meaning is the oldest: in the Maine Farmers' Almanac, in the United States, a season generally has three full moons; when it has four, the third is called a blue moon. This usage has been recorded in the almanac since 1937.

The calendar meaning arose from a misunderstanding: in 1946, an article in the American magazine Sky & Telescope misread the almanac's rule and defined the blue moon as the second full moon of a month. This reading spread through radio in the 1980s and then through a well-known board game. Today it is the meaning most often encountered, although both are recognised by some dictionaries.

The calendar meaning recurs about every 32 months, roughly once every two and a half to three years. Examples: 31 May 2026 and 31 December 2028 (which coincides with a total lunar eclipse). The date depends on the time zone.

A truly blue moon?

Sometimes the Moon really does look bluish. After the eruption of Krakatoa in 1883, observers reported blue, green or otherwise coloured suns and moons for months. In 1950, smoke from huge forest fires in Canada gave rise to blue moons observed in Europe. Particles of a specific size (ash, smoke) can indeed scatter red light more than blue. But the everyday meaning of "blue moon" has nothing to do with colour: the English expression "once in a blue moon" (for something very rare) is attested as early as 1833, well before the calendar definition.

In French, the same idea is expressed by the idiom tous les 36 du mois (literally "every 36th of the month"), which also means very rarely.

The black moon: even vaguer

"Black moon" is not an astronomy term either. Depending on the source, you may come across:

• the second new moon of one calendar month (by analogy with the blue moon);
• the third of four new moons in a season;
• a month with no new moon or no full moon: this can only happen in February, the shortest month;
• in astrology, the "Black Moon" or "Lilith": a calculated point on the Moon's orbit, its apogee (the point farthest from the Earth), unrelated to the phase;
• in esotericism, the dark moon: the two or three days when the Moon remains invisible around the new moon.

The Moon, of course, is never black: it is simply unlit as seen from the Earth.

Contemporary symbolism: the rare and the invisible

In practices inspired by esotericism, a blue moon is presented as a "bonus" moment, marked with a small gratitude ritual. The black moon is associated with silence, shadow and the hidden parts of oneself, before a new start. These readings are modern constructions linked to popular names: nothing indicates that these dates change anything.

They do have practical value: marking a rare date with a gentle gesture (a meal with loved ones, a note of thanks) is a pleasure without consequences, and the idea of a "shadow moon" can simply invite you to slow down.

Simple steps

• Blue moon: take a walk at moonrise, and note in a journal anything rare that has happened since the last one.
• Black moon: switch off screens for an evening, watch the starry sky, and make a "shadow inventory": what you avoid looking at, without judging yourself.
• In both cases, no promises: a moment for yourself is already worth the effort.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guide to the moon.