What causes the PHASES of the MOON | Daily Dose of Science

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The phases of the moon are the different shapes the moon appears to have in the sky over a roughly 29.5-day cycle. These phases—like full moon, new moon, crescent, and gibbous—are caused by the changing positions of the sun, Earth, and the moon. The moon doesn't produce its own light; instead, it reflects sunlight. As the moon orbits around the Earth, different portions of its sunlit side become visible to us. This changing appearance of the moon is what we call the phases.
At the beginning of the cycle, the moon is between the Earth and the sun. The side of the moon illuminated by the sun is facing away from us, so the moon appears dark or invisible which we call the new moon.
As the moon moves along its orbit, a small portion of the sunlit side becomes visible, leading to the crescent shape we see in the sky. As the moon continues its orbit, more of its sunlit side becomes visible creating a waxing gibbous. When the moon is directly opposite the sun, the entire sunlit side is facing us, and we see a full moon. After the full moon, the visible sunlit area starts to decrease. We call this a waning gibbous. Finally, the visible sunlit portion decreases until the moon becomes a thin crescent again, leading back to the new moon phase.