If there are rocks on earth of equivalent age as the moon rocks then that allows for the implication that the two share a common source, i.e. the moon was ejected from the earth's crust.
If it was ejected then the rocks would share more charactoristics than just age.
Pre-Apollo there were two main ideas behind the formation of the moon.
1) Separate formation and capture: Here both the Earth and the moon acreated from the disc and formed, then the moon passed close enough to the Earth that it was captured and become our moon.
2) Part of Earth: Under this theory that moon had been part of the Earth's crast, but it's rotation had forced the crust out and it had broken off and formed the moon. The belief was that the crust in the area that is the Pacific Ocean has been flung off and as now the Moon.
Sample analysis proved both wrong, with one of the most amazing discoveries being the total lack of any bonded water. On earth water is everywhere, including in the planet's core. Rocks all have it bonded into them on a chemical level, you can't get away from it. The moon however has none. That means that the rocks the moon is made of much have been hot at the time the moon formed, and all volitiles, such as water, were lost before it started cooling. Neither the separate accresion nor the part of Earth theory work for this, so the Impactor Theory come into being.
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