How many theories exist about how the moon formed?
Up until about the 1990s, there were at least 3 major theories as to how the moon formed. All of them turned out to be wrong!
When the Apollo missions in the 1970s brought back several hundred pounds of moon rocks, the theories regarding the formation of the Moon started to change in a new direction.
Most scientists are now quite confident that "the big splat" or "big thwack" is probably the most likely one. As the solar system was forming the Earth started to gather matter and did the other planets. But there were also other proto-planets that were forming. One such proto-planet, called Theia, was thought to be forming close to the Earth's orbit. However, if planetary orbits, no two planets can occupy the same orbit, so when the smaller proto-planet struck Earth some kind of a blow, the Earth absorbed most of this earlier planet and spat out the remains which became our moon.
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