Blue Moon

Also known as: Seasonal Blue Moon (traditional), Monthly Blue Moon (modern)

A Blue Moon refers either to the second Full Moon within a single calendar month (modern usage) or the third Full Moon in a season containing four (older definition).


The Blue Moon is not a color — it is a counting irregularity. A surplus within the expected pattern. Astronomically ordinary, calendrically rare.

Here you will find articles unpacking the history of the term, the confusion that shaped its modern meaning, atmospheric conditions that can tint the Moon blue, and the symbolic appeal of rarity. The sky does not change — but our naming does.

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