How it works
Half of the Moon is illuminated by the Sun at nearly all times. The angle among the Sun, Earth, and Moon determines whether we see a crescent, quarter, gibbous, or full phase.
What people often miss
Earth's shadow does not cause ordinary phases. It falls on the Moon only during a lunar eclipse.
Why this question comes up
The repeating month-long cycle is a change in viewing geometry, not a change in the Moon's shape.
Key points
- Moon phases occur because we see changing portions of the Moon's sunlit half as it orbits Earth.
- Half of the Moon is illuminated by the Sun at nearly all times. The angle among the Sun, Earth, and Moon determines whether we see a crescent, quarter, gibbous, or full phase.
- Earth's shadow does not cause ordinary phases. It falls on the Moon only during a lunar eclipse.
Bottom line
Moon phases occur because we see changing portions of the Moon's sunlit half as it orbits Earth.
Source and verification
This Answer was checked against NASA Science. Follow the source for the full official explanation and any later updates.