How was the Moon made?
Most scientists think the Moon formed after a planet about the size of Mars crashed into the young Earth, about 4.5 billion years ago, throwing out rock that gathered into the Moon.
The Moon has been our companion for billions of years, but how did it get there? Scientists have a favourite explanation, called the giant impact idea.
About 4.5 billion years ago, when Earth was very young, another planet, perhaps about the size of Mars, was travelling around the Sun on a path that crossed Earth's. Scientists sometimes call it Theia, after a goddess in Greek myths who was the mother of the Moon goddess.
One day, Theia smashed into Earth. It was the biggest crash Earth has ever had. The impact melted much of both planets and threw a huge cloud of hot, melted and vaporised rock into space around Earth. This cloud spread into a ring. Over time, perhaps only months to years, the rock in the ring clumped together to form the Moon.
How do scientists know? Moon rocks brought back by Apollo astronauts are very similar to rocks from Earth's mantle. The Moon has only a small iron core and very little water, which matches what scientists expect from a hot, violent birth. Computer models of the crash can make a Moon like ours.
At first, the Moon was much closer to Earth than it is today. It has been slowly moving away, about 3.8 centimetres every year, roughly as fast as your fingernails grow.