The moon is the easiest celestial object to find in the night sky — when it's there. Moon phases and the moon's orbit are a mystery to many. Because it takes 27.3 days both to rotate on its axis and to orbit Earth, the moon always shows us the same face. We see the moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the sSun.
Though a satellite of Earth, the moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger), though that viewpoint has never caught on officially. There are various theories about how the moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of Earth away.