Natural satellite
Moons do not make their own light. We can see the Earth's moon because it acts like a mirror, and reflects the light of the Sun.  The same half of the moon faces toward Earth at all times, no matter where it moves. This effect is called tidal locking. But different parts of the moon are lit up by the Sun, so it looks different at different times of the month. This change as seen from Earth is called the phases of the moon, or lunar phases.