The Moon has phases because sunlight illuminates half of it while the Moon orbits Earth. Our viewing angle changes, so the visible portion of the illuminated half shifts from new Moon to full Moon and back.
Half the Moon is lit
At every moment, sunlight illuminates one hemisphere of the Moon. The Moon does not produce its own visible light. A new Moon occurs when the lit half faces mostly away from Earth; a full Moon occurs when the lit half faces us.
The changing shape is therefore not Earth’s shadow. Earth’s shadow is involved only during a lunar eclipse.
The cycle follows the orbit
The Moon completes the phase cycle in about 29.5 days as seen from Earth. Its orbit around Earth is shorter relative to the stars, but Earth’s changing position around the Sun means the Moon must travel a little farther to repeat the same Sun-Earth-Moon geometry.
This difference explains why “month” can mean different orbital periods.
The crescent is a geometry clue
A crescent shows that most of the illuminated hemisphere is turned away from us. The faint glow on the dark portion can sometimes be earthshine: sunlight reflected from Earth onto the Moon.
Every phase is a perspective on the same lit sphere, not a change in the Moon’s shape.
Sources and evidence
- NASA Science — Top Moon questions ↗
Moon phases, illumination and orbital timing.
- NASA — Moon phases ↗
Sun-Earth-Moon geometry and phase cycle.
Sources consulted: 9 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Added original explanatory comparisons, source-linked evidence and a reader-focused example.