The Moon (or Earth I) is the only natural satellite of Earth, and the fifth-largest moon in the Solar System at just under 3,500 kilometers in diameter. Because it has no atmosphere, the side of the moon facing the Sun gets very hot.
Scienists beleive that the Moon was formed when a planet roughly the size of Mars (referred to as Theia) collided wih the still-developing Earth. The collision would have cause massive chunks of rocks and magma to spray out in the area. Over time, the remainer of Theia would have started orbiting Earth, whilst the remaining rocks and debris were slowly pulled into either Earth or Theia (now the Moon).