Question

explain why earth has a moon

Answer 1

http://arxiv.org/abs/astro-ph/0405372

Answer 2

It was an accident. A very large asteroid is believed to have hit the earth billions of years ago and that the moon was released as a molten blob by the impact. Later it cooled down to a sphere, as the Earth did.

See http://www.psi.edu/projects/moon/moon.html
for more details about the theory

Answer 3

thank you