Why does Earth have a moon?

Answer 1

Our best guess is that the Moon came from the Earth. People say that, maybe, billions of years ago, there was a comet or an asteroid about the size of Mars that collided with the Earth. It took out large chunks of rock from the Earth and it got trapped in our orbit, and the Moon formed from those pieces. But this isn't the only way how planets get their moons. Sometimes, some moons can get trapped in a planets orbit. But this would normally happen with a bigger target, like Jupiter or Saturn. They couldn't have had an asteroid that smashed into them, because Jupiter and Saturn don't have a surface; they are made out of gas. So, maybe a moon comes shooting towards our solar system and it gets trapped in a planet's orbit (whichever planet that would be). Jupiter and Saturn have collected a large amount of their moons, they have more than 50!

But, believe it or not, we actually have two moons, well, sort of.

About 20 years ago, there was a group of astronomers that spotted an object about 3km wide, and they called it 'Cruithne'. Then, 10 years later, they saw the same object again, and noticed it was actually sharing our orbit round the Sun; and it was actually orbiting the Earth. You'll probably know that our Moon takes 1 month to do a complete lap around Earth. But Cruithne takes 770 years to lap around us, and eventually it will escape it's orbit around Earth and fling away into Space forever.

Answer 2

The most widely accepted theory is that the Moon was formed when a planet-sized body collided with Earth early in its history, ejecting debris that eventually coalesced into the Moon. This event is known as the Giant Impact Hypothesis and explains the similarities between Earth and the Moon's composition and the Moon's relatively large size compared to Earth.