The Moon
The Moon didn't form gently. About 4.5 billion years ago, a planet the size of Mars collided with early Earth at an angle. The violence of that impact ejected an enormous cloud of rock and vapour into orbit. Gravity pulled it together. The Moon was born.
The giant impact
The impactor is called Theia. When it struck Earth, the collision was so violent that it partially melted both bodies. The debris — vaporised rock, molten fragments, and chunks of Earth's mantle — was flung into orbit and slowly coalesced into the Moon over thousands of years. Evidence for this comes from the Moon's composition, which closely matches Earth's mantle rather than the composition of asteroids or comets.
The Moon that stabilises us
Earth's axial tilt — currently 23.5 degrees — gives us our seasons. Without the Moon's gravitational influence, that tilt would wobble chaotically over tens of thousands of years, potentially swinging between 0 and 85 degrees. That would mean catastrophic climate swings — ice ages at the equator, tropical temperatures at the poles — making long-term evolution of complex life far less likely.
Tides and the origin of life
The Moon's gravity pulls on Earth's oceans, creating tides. When the Moon was younger and closer, tidal forces were far more extreme. These tidal cycles — regular flooding and drying of coastal rock pools — may have played a role in the chemistry that created early life, concentrating organic molecules and providing the rhythmic cycles that early biochemistry needed.
Moving away, slowing us down
The Moon is drifting away from Earth at about 3.8cm per year — the same rate your fingernails grow. The same gravitational interaction is also slowing Earth's rotation. When the Moon formed, a day on Earth was only about 6 hours long. Over 4.5 billion years, that interaction has slowed our rotation to 24 hours. Eventually, if both survive long enough, Earth's rotation will match the Moon's orbit and one side of Earth will always face the Moon.
The Apollo missions brought back 382kg of Moon rock. Analysis confirmed the giant impact hypothesis — lunar rocks have an identical oxygen isotope signature to Earth's mantle, which would only be the case if the Moon formed from ejected Earth material. The Moon also lacks a significant iron core, consistent with it forming mainly from the outer layers of both Earth and Theia rather than their cores.