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how did the moon form after the giant impact
Look at the Moon next to Earth: a quarter of Earth's width, huge for a moon. Captured stranger? Torn off? Grown alongside? None of those quite fit the rocks.
Apollo astronauts brought rocks home. The Moon is much less dense than Earth, so it is short of iron, yet its rock is a chemical twin of Earth's outer layers.
So here is the story. About four and a half billion years ago, a world the size of Mars, called Theia, struck the young Earth a glancing blow.
The crash flung out a ring of molten, vaporised rock from both worlds' outer layers. Theia's iron sank into Earth. Within years to a century, clumps in the ring merged into the Moon.
So the Moon is a souvenir of a collision: rock from two worlds, flung out and regathered, and still drifting away from us about three point eight centimetres a year.
Every moonrise is a glimpse of a four and a half billion year old crash, still turning slowly in the sky.