The earth moves round the sun, and the moon moves round the earth. When the part of the earth that we live turns to the sun, it is day. When the part of the earth we live turns away from the sun, it is night.
The sun is bigger than the moon. But sometimes the moon looks bigger than the sun, because the earth is far away from the sun.
The sun is extremely bright. It gives very strong light. The moon also looks quite bright, but it doesn't give any light at all.
The light of the moon comes from the sun, so the moon looks much bigger and brighter than the other stars.
But actually a majority of stars are much bigger and brighter than the moon. But they look smaller than the moon because they are farther away from the earth.