The sun does not really disappear at night. It just seems to vanish, because the part of Earth you live on has turned away from it.
Earth is slowly turning all the time. When it is morning, the part of Earth you live on is starting to face the sun. The sun seems low in the sky. As the day goes on, Earth goes on turning, and the sun appears higher in the sky. When your part of the world is directly in line with the sun, it is the time of day we call noon.
Earth continues to turn. Slowly your part of Earth moves away from the sun. That is why it gets darker and the sun seems to go down. Soon your part of Earth is completely away from the sun, and it is night.
Half of Earth is always getting light from the sun. The other half is always dark. As your part of Earth moves away from the sun, the other side is beginning to face the sun. There, the day is just beginning.
Picture Credit : Google
