Why does the moon change its shape

(as in full, half, and quarter moon)?

By astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute

We think that the moon and Earth formed at about the same time, back when our whole solar system was formed. Earth was forming from many chunks of rock and icy material. Possibly a big chunk hit the new Earth and knocked loose a big piece, which became the moon.

The bright part of the moon is the part that the sun is shining on. This is like daytime on earth. The dark part is in shadow, like night on earth. Now the moon goes around the earth once every 29 days (approximately).

At new moon, the moon and the sun are on the same side of Earth. We see the part of the moon that is in shadow, so the moon is dark. Then the moon moves around in its orbit. At first quarter, it has gone one-fourth of the way around Earth. Now we can see part of the moon that is sunlit, but part still in shadow. Note that if the sun is setting in the west, the bright part of the moon is on the side toward the sun and the dark part is away.

About a week later, the moon has moved halfway around its orbit. Now it is on the opposite side of Earth, away from the sun. Now we see only the sunlit side — that is the full moon. Note that if the sun is setting in the west, the moon is just rising in the east.

About a week later, the moon has moved now three-fourths of the way around in its circle around Earth. Once again only part of the moon is sunlit and part is dark. Now you can see the moon in the morning, and note that once again the sunlit side is on the side towards the sun, and the shadow side away. Another week and we are back to the new moon.

It's easier to demonstrate if you have a ball to represent the moon and a flashlight for the sun. Have someone stand several feet away, holding the flashlight so it shines on the ball. Hold the "moon" ball and slowly turn around, watching the moon go around you (you are Earth). Do you see the moon's phases?