The moon is very large and stabilizes the Earth to make our climate very consistent. No one knows for sure how it was all arranged by accident in our solar system. The chemicals and isotopes on both bodies are extremely similar which mean they are probably part of the same original body and somehow were split up.-http://www.evolutionnews.org/2013/12/our_moon_still079861.html- The ID folks, of course, point out the possibilty of design.