Moonrise Moment

Phases glossary

What is illumination?

Illumination is the fraction of the Moon's visible disk that appears lit by direct sunlight, commonly expressed as a percentage.

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

What it means

Illumination 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 illumination.

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 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.

 Phases Lunar Phase

The changing apparent shape of the Moon's sunlit portion as the Moon moves around Earth.

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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