Up until the Apollo landings there were a number of competing theories about the origins of the moon. Study of moon rock samples brought back by the astronauts has led to the development of the impact theory. This suggests that the young earth collided with an object about the size of Mars. The debris thrown up from this collision went into orbit around the earth and formed into the moon.
Why does the moon sometimes appear to be very large in the sky ?
When you see the moon near the horizon it looks a lot bigger than when it is high up in the sky. This is due to the way the brain perceives size and distance. When the moon is close to the horizon the trees, houses etc indicate to the brain that the moon is a very long way away. When the moon is high in the sky there are no clues to the brain so it concludes that the moon is closer. Since an object's apparent distance determines its perceived size, the moon near the horizon looks twice the size compared to when it is high in the sky.
What causes the different phases of the moon ?
How does the moon cause tides on earth ?