The Moon does not generate its own visible light; it reflects sunlight. As it orbits Earth, we see different portions of its sunlit half, creating the familiar cycle of new moon, crescent, quarter, gibbous, and full moon.

The phases are a reminder that perspective matters in astronomy. The Moon is always half-lit by the Sun, but what changes is our angle of view. A monthly sky pattern that has guided calendars and culture for millennia comes down to simple orbital geometry.

Reference:

Wikipedia: Lunar phase

https://en.wikipedia.org/wiki/Lunar_phase