Why the Moon has phases

The Sun always lights half the Moon; as it orbits Earth we see more or less of that lit half.

1. We watch the Moon from Earth as it orbits us. 2. The Moon makes no light of its own; moonlight is reflected sunlight. 3. The Sun always lights half of the Moon: its day side. 4. The Moon's cycle of phases repeats about every 29.5 days. 5. As the Moon orbits Earth, we see more or less of its sunlit half: the phases.

Sources: NASA, “Moon Phases” (Public domain (US Government work)); Wikipedia, “Lunar phase (Wikipedia)” (CC BY-SA 4.0). Illustrative, not to scale; the Sun is far off to one side.

Source: Moon Phases - NASA Science