explain why earth has a moon
http://arxiv.org/abs/astro-ph/0405372
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
thank you
The Earth has a moon because of a process called accretion. Accretion is the gradual accumulation of mass, where smaller objects come together to form a larger object. Around 4.5 billion years ago, in the early stages of the solar system, a protoplanetary disk made up of gas and dust surrounded the young Sun. Within this disk, particles began to collide and stick together, forming larger bodies known as planetesimals.
Some of these planetesimals, ranging from the size of small asteroids to that of Mars, continued to collide and grow. Eventually, a Mars-sized object named Theia formed near Earth's orbit. Theia and Earth collided in a giant impact event called "Theia Impact."
During this impact, a massive amount of debris was ejected into space. Over time, this debris began to orbit Earth and eventually coalesced to form the Moon. This process is known as the Giant Impact Hypothesis, which is the most widely accepted explanation for the Moon's formation.
The Moon's formation also had a significant influence on Earth's evolution. It stabilized the planet's rotation, creating more stable climate conditions. The Moon's gravitational pull creates tides, which have played a role in the development of life in Earth's oceans.
The presence of the Moon has many other effects on Earth, including its impact on Earth's axial tilt and the length of Earth's day. Without the Moon, Earth would be a very different place.