The moon does not change shape. It is a satellite moving around the earth. Just like earth other planets of the solar system have their own satellites that move around them. Planets also have their own orbits and move around the sun. Earth has two different types of movements. It moves around its own axis once every 24 hours. We measure time with the help of the movements of the earth. The continuous movement of the earth and the moon creates some visual illusions. It seems that the moon is slowly disappearing from the sky leading to the creation of new moon and then it slowly comes back to its full moon avatar.
