Animation of the Earth and Moon early in their history. It is thought that the Moon was formed when a Mars-sized planetesimal collided with the proto-Earth some 4.5 billion years ago. This impact led to the ejection of a huge amount of vaporised rock, which condensed in orbit, forming a disc of material. This material rapidly coalesced through gravity into the Moon, which has swept a clear path through the disc here. The disc debris rained down on the Earth and Moon for millennia, heavily cratering both. Early on, the Moon was much closer to Earth than it is today, only about a tenth of its current distance.