Why that is full moon and new moon appeared?

Answer 1

morality

Answer 2

The sun shines on the moon (a globe) and half is bright and half is dark in shadow. As the moon orbits around the earth taking about one month per trip, our view of the sun's shadow on the moon constantly changes (even as the earth spins on its axis once per day). When we see no shadow the moon is full and when we see almost the entire shadow the moon is new. If you stand in a room with only one light bulb at the far end and hold a globe (basketball?) at arms length, you can see how the shadow of the light on the ball changes as you rotate in a circle. Of course the moon is easier to see at night but often can be seen during the day. The word month actually means moon.