Formation of the Moon. Illustration of the collision stage in the giant impact theory of the formation of the Moon. This theory says that an object about the size of Mars collided with the Earth (left) around 4.6 billion years ago, soon after they both formed. The impact was side-on, rather than a direct impact. Material from the outer layers of both bodies was thrown into orbit around the Earth, forming a ring. This then coalesced to form the Moon.