Moonrise Moment

Phases glossary

What is a Blue Moon?

Blue Moon is a calendar nickname for an extra full Moon. It commonly means the second full Moon in one calendar month, though a seasonal definition also exists.

Eight illuminated Moon disks showing the lunar phase cycle.
A visual guide to blue moon. Original Moonrise Moment illustration.

What it means

Blue Moon belongs to the Moon's repeating cycle of illumination. Lunar phases are produced by the changing Sun-Earth-Moon geometry, not by Earth's shadow; Earth's shadow is involved only during a lunar eclipse. The same phase is visible worldwide at a given moment, although its orientation can look different by latitude and hemisphere.

For a deeper look with practical observing advice, read the Moonrise Moment guide to blue moon.

Keep exploring

Phases Crescent Moon

A phase in which less than half of the Moon's visible disk is illuminated. A crescent may be waxing or waning.

 Phases First Quarter Moon

The phase near one quarter of the way through the lunar cycle, when half of the Moon's visible disk appears illuminated and the lit portion is growing.

 Phases Full Moon

The phase when the Moon is opposite the Sun in Earth's sky and its Earth-facing hemisphere appears almost completely illuminated.

 Phases Gibbous Moon

A phase in which more than half, but not all, of the Moon's visible disk is illuminated. A gibbous Moon may be waxing or waning.

Sources and further reading

This definition was written in plain language and checked against these science references.

Moonrise Moment showing the live direction and countdown screen.

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