One of the earliest questions asked by astronomers was: how was the Moon created? There have been several origins proposed, but the current favorite in the scientific community is known as the giant impact hypothesis. This is the theory that a giant planetoid collided with the Earth billions of years ago, blasting out a cloud of debris that collected into the Moon.
This idea was first proposed by George Howard Darwin in 1898, who thought that the Moon might have been spun out of the Earth by centrifugal force when the Earth was still molten after its formation. He thought that the Earth and Moon should have been much closer and that the Moon is slowly drifting away from Earth. And in the 1960s, scientists were able to determine that the Moon is slowly moving away from Earth.
It’s now thought that a Mars-sized object collided with the Earth at an oblique angle. The core of the two planets merged together, and the outer crust and mantle sprayed outward into cloud of material orbiting the planet. Over time, this cloud of material collected together into what we now know as the Moon.
The best evidence to support this theory comes from the rocks returned by the NASA Apollo Moon missions. In analyzing these rocks, scientists have determined that their oxygen ratios exactly match the ratios we find on Earth. So the Earth and Moon were once a single object.
This isn’t the only theory developed to explain how the Moon was created. Another idea is that the Moon was formed elsewhere in the Solar System and then captured by the Earth’s gravity. It’s also possible that the Earth and the Moon just formed together at the same time, out of a small planetary disk.
We’ve also recorded an entire episode of Astronomy Cast just about this very subject. Listen here, Episode 17: Where Does the Moon Come From?