What is the moon?

The moon is earth's only satelite, and it hes been orbiting our planet for at least 4000 million years. It is a rocky sphere about 3476 km in diameter, which is about a quarter size of the earth. scientists believe that the moon was formed when another planet about the size of Mars collided with the earth. The collision splashed a huge mass of molten rock into space. This molten rock quickly formed into a sphere, and the moon rapidly cooled into its solid form. The moon's surface is heavily pitted by collisions with debris, such as asteroids.