About 4.5 billion years ago, a Mars-sized body called Theia slammed into the young Earth. The collision was so violent it vaporized rock from both bodies. The debris formed a ring around Earth that gradually coalesced into the Moon. This is called the Giant Impact Hypothesis and explains why Moon rocks share Earth's chemical signature.

The Moon is slowly drifting away from Earth at about 3.8 centimeters per year. Billions of years ago it was much closer, making tides enormous and days far shorter. Early Earth spun so fast that a day lasted just 6 hours.