An observer on Earth sees the phase of moon because

We can see the phases of the moon because when it revolves around the Earth the sun shadows certain parts of the moon depending on its placement around the Earth, tis cycle goes on for about a month and the reason we can see the moon is because of the way the reflects of off the sun.

Explanation:

Cause of the Sun.

Explanation:

Because the earth orbits around the sun and the moon orbits around earth when the sun hits the moon by LIGHT not literally it cause the other side of the moon facing earth that isn't getting light to phase or be black.