"impact theory explained many of the things about the formation of the moon"

"...idea wasn’t given much thought until a 1974 paper by Dr. William K. Hartmann and Dr. Donald R. Davis was published in the Journal Icarus. They suggested that the early Solar System was still filled with leftover moon-sized objects which were colliding with the planets."

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http://www.universetoday.com/19718/formation-of-the-moon/