Four and a half billion years ago, a Mars-sized body struck the young Earth with enough violence to vaporize rock and fling debris into orbit — debris that never scattered, but gathered into the moon we still see tonight. Scientists call this the giant-impact hypothesis, and decades of chemical analysis, geological sediment records, and NASA simulations have steadily confirmed it as the most coherent answer to a longstanding puzzle: why Earth's moon is so improbably large. The same forces that built the moon continue reshaping our world, slowing our days and carrying the moon incrementally awa